Introduction to Protein

Protein makes up approximately 20 percent of the human body and is present in every single cell. The word protein is a Greek word, meaning "of utmost importance." Proteins are called the workhorses of life as they provide the body with structure and perform a vast array of functions. You can stand, walk, run, skate, swim, and more because of your protein-rich muscles. Protein is necessary for proper immune system function, digestion, and hair and nail growth, and is involved in numerous other body functions. In fact, it is estimated that more than one hundred thousand different proteins exist within the human body. In this lesson you will learn about the structure of protein, the important roles that protein serves within the body, how the body uses protein, the risks and consequences associated with too much or too little protein, and where to find healthy sources of it in your diet.

Topics Covered in this Unit

1. Protein Structure

2. Protein Functions

3. Protein in Foods and Dietary Recommendations

4. Protein Digestion and Absorption

5. Health Consequences of Too Little and Too Much Dietary Protein

6. Environmental Consequences of Protein Choices

Course Learning Outcomes Covered

1. Define and classify the six classes of nutrients.

2. Identify where the six classes of nutrients are found in foods.

3. Explain how the six classes of nutrients are digested, absorbed, metabolized, and utilized.

4. Distinguish between adequate nutrient intake, deficiencies, and toxicities and how these levels impact body systems and health outcomes.

5. Acknowledge the importance of a moderate approach when it comes to nutrition and weight management, recognizing all foods can fit into a healthful diet.

References:

1. "Defining Protein", section 6.1 from the book An Introduction to Nutrition (v. 1.0)

Image Credits:

1. Yoga image by Dave Rosenblum, https://flic.kr.p/fMQ8FW, CC-BY-2.0

To get started, scroll down and click, "next page" or click, "Struc" in the navigation above.

Note: this unit was authored by Alice Callahan, PhD and Tamberly Powell, MS, RD

CC BY-NC-SA 4.0

Protein Structure

What Is Protein?

Proteins are macromolecules composed of amino acids. For this reason, amino acids are commonly called the building blocks of protein. There are 20 different amino acids, and we require all of them to make the many different proteins found throughout the body. Proteins are crucial for the nourishment, renewal, and continuance of life.

Just like carbohydrates and fats, proteins contain the elements carbon, hydrogen, and oxygen, but proteins are the only macronutrient that also contain nitrogen. In each amino acid, the elements are arranged into a specific conformation, consisting of a central carbon bound to the following four components:

• A hydrogen

• A nitrogen-containing amino group

• A carboxylic acid group (hence the name "amino acid")

• A side chain

The first three of those components are the same for all amino acids. The side chain -- represented by an "R" in the diagram below -- is what makes each amino acid unique.

Figure 1.1 Amino Acid Structure.

Amino acid side chains vary tremendously in their size and can be as simple as one hydrogen (as in glycine, shown in Figure 1.1) or as complex as multiple carbon rings (as in tryptophan). They also differ in their chemical properties, thus impacting the way amino acids act in their environment and with other molecules. Because of their side chains, some amino acids are polar, making them hydrophilic and water-soluble, whereas others are nonpolar, making them hydrophobic or water-repelling. Some amino acids carry a negative charge and are acidic, while others carry a positive charge and are basic. Some carry no charge. Some examples are given below. For this class, you don't need to memorize amino acid structures or names, but you should appreciate the diversity of amino acids and understand that it is the side chain that makes each different.

Fig. 1.2 - Amino acids have different structures and chemical properties, determined by their side chains.

Essential and Nonessential Amino Acids

We also classify amino acids based on their nutritional aspects (Table 1.1 "Essential and Nonessential Amino Acids"):

• Nonessential amino acids are not required in the diet, because the body can synthesize them. They're still vital to protein synthesis, and they're still present in food, but because the body can make them, we don't have to worry about nutritional requirements. There are 11 nonessential amino acids.

• Essential amino acids can't be synthesized by the body either at all or in sufficient amounts. They must be obtained in the diet in adequate amounts. There are 9 essential amino acids.

Table 1.1 Essential and Nonessential Amino Acids

Sometimes during infancy, growth, and in diseased states, the body cannot synthesize enough of some of the nonessential amino acids and more of them are required in the diet. These types of amino acids are called conditionally essential amino acids.

The nutritional value of a protein is dependent on what amino acids it contains and in what quantities. As we'll discuss later, a food that contains all of the essential amino acids in adequate amounts is called a complete protein source, whereas one that does not is called an incomplete protein source.

The Many Different Types of Proteins

There are over 100,000 different proteins in the human body. Proteins are similar to carbohydrates and lipids in that they are polymers (simple repeating units); however, proteins are much more structurally complex. In contrast to carbohydrates, which have identical repeating units, proteins are made up of amino acids that are different from one another. Different proteins are produced because there are 20 types of naturally occurring amino acids that are combined in unique sequences.

Additionally, proteins come in many different sizes. The hormone insulin, which regulates blood glucose, is composed of only 51 amino acids. On the other hand, collagen, a protein that acts like glue between cells, consists of more than 1,000 amino acids. Titin is the largest known protein. It accounts for the elasticity of muscles and consists of more than 25,000 amino acids!

The huge diversity of proteins is also due to the unending number of amino acid sequences that can be formed. To understand how so many different proteins can be made from only 20 amino acids, think about music. All of the music that exists in the world has been derived from a basic set of seven notes C, D, E, F, G, A, B (with the addition of sharps and flats), and there is a vast array of music all composed of specific sequences from these basic musical notes. Similarly, the 20 amino acids can be linked together in an extraordinary number of sequences. For example, if an amino acid sequence for a protein is 104 amino acids long, the possible combinations of amino acid sequences is equal to 20104, which is 2 followed by 135 zeros!

Building Proteins with Amino Acids

The decoding of genetic information to synthesize a protein is the central foundation of modern biology. The building of a protein consists of a complex series of chemical reactions that can be summarized into three basic steps: transcription, translation, and protein folding.

Figure 1.3 - Overview of protein synthesis. Protein folding happens after translation.

1. Transcription - In the nucleus of the cell, the genetic information in double-stranded deoxyribonucleic acid (DNA) is transcribed or copied into the single-stranded messenger ribonucleic acid (mRNA).

2. Translation - At the ribosomes in the cell's cytosol, amino acids are linked together in the specific order dictated by the mRNA. Each amino acid is connected to the next amino acid by a special chemical bond called a peptide bond (Figure 1.4). The peptide bond forms between the carboxylic acid group of one amino acid and the amino group of another, releasing a molecule of water. As amino acids are linked sequentially by peptide bonds, following the specific order dictated by the mRNA, the protein chain, also known as a polypeptide chain, is built (Figure 1.5).

Figure 1.4 - Peptide bond formation

Figure 1.5 - A polypeptide chain

Watch these videos to learn more about transcription and translation:

DNA Transcription - https://youtu.be/5MfSYnItYvg

mRNA Translation - https://youtu.be/8dsTvBaUMvw

3. Protein folding - The polypeptide chain folds into specific three-dimensional shapes, as described in the next section.

Protein Organization

Protein's structure enables it to perform a variety of functions. There are four different structural levels of proteins (Figure 1.5):

1. Primary structure - This is the one-dimensional polypeptide chain of amino acids, held together by peptide bonds.

2. Secondary structure - The polypeptide chain folds into simple coils (also called helices) and sheets, determined by the chemical interactions between amino acids.

3. Tertiary structure - This is the unique three-dimensional shape of a protein, formed as the different side chains of amino acids chemically interact, either repelling or attracting each other. Thus, the sequence of amino acids in a protein directs the protein to fold into a specific, organized shape.

4. Quaternary structure - In some proteins, multiple folded polypeptides called subunits combine to make one larger functional protein. This is called quaternary protein structure. The protein hemoglobin is an example of a protein that has quaternary structure. It is composed of four polypeptides that bond together to form a functional oxygen carrier.

Figure 1.5 - A protein has four different structural levels.

Watch the animation, "What is a Protein," for an overview of the structure of amino acids, the four different structural levels of protein, and examples of different types of proteins in the body.

A protein's structure also influences its nutritional quality. Large fibrous protein structures are more difficult to digest than smaller proteins and some, such as keratin, are indigestible. Because digestion of some fibrous proteins is incomplete, not all of the amino acids are absorbed and available for the body to utilize, thereby decreasing their nutritional value.

The specific three-dimensional structure of proteins can be disrupted by changes in their physical environment, causing them to unfold. This is called denaturation, and it results in loss of both structure and function of proteins. Changes in pH (acidic or basic conditions) and exposure to heavy metals, alcohol, and heat can all cause protein denaturation. The proteins in cooked foods are at least partially denatured from the heat of cooking, and denaturation in the stomach is an important part of protein digestion, as we'll discuss later in this unit. We can see everyday examples of denaturation in cooking techniques, like how egg whites become solid and opaque with cooking, and cream becomes fluffy when it's whipped. Both of these are examples of denaturation leading to physical changes in protein structure, and because protein structure determines function, denaturation also causes proteins to lose their function.

You can learn more about denaturation in this video animation: Heat Changes Protein Structure (http://www.sumanasinc.com/webcontent/animations/content/proteinstructure.html)

Shape Determines Function

An important concept with proteins is that SHAPE determines FUNCTION. A change in the amino acid sequence will cause a change in protein shape. Each protein in the human body differs in its amino acid sequence and consequently, its shape. The synthesized protein is structured to perform a particular function in a cell. A protein made with an incorrectly placed amino acid may not function properly, and this can sometimes cause disease. An example of this is sickle cell anemia, a genetic disorder.  Below is a picture of hemoglobin, a protein with a globular three-dimensional structure. When packed in red blood cells to deliver oxygen, this structure gives red blood cells a donut shape.

Figure 1.6 - Structure of hemoglobin

In people with sickle cell anemia, DNA gives cells the incorrect message when bonding amino acids together to make hemoglobin. The result is crescent-shaped red blood cells that are sticky and do not transport oxygen like normal red blood cells, as illustrated in the figure below.

Figure 1.7 - Difference in blood cells and blood flow between normal red blood cells and sickle shaped blood cells.

Self Check:

References:

1. Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR

2. "Defining Protein", section 6.1 from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

Image Credits:

1. Amino acid structure from "Defining Protein", section 6.1 from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

2. Amino acids diagram from Section 3.4 of Biology by OpenStax, CC BY 4.0, Download for free at http://cnx.org/contents/db3ce6d5-01bd-4ac5-a641-de285fdac0f1@8

3. Protein synthesis diagram from "Intro to gene expression (central dogma)" by Khan Academy, CC BY-NC-SA 4.0, https://www.khanacademy.org/science/biology/gene-expression-central-dogma/central-dogma-transcription/a/intro-to-gene-expression-central-dogma

4. Polypeptide chain, http://www.genome.gov/Glossary/index.cfm?id=149

5. Peptide bond formation, http://en.wikipedia.org/wiki/File:Peptidformationball.svg

6. Structural levels of protein from "Defining Protein", section 6.1 from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

7. Hemoglobin,https://commons.wikimedia.org/wiki/File:1GZX_Haemoglobin.png#/media/File:1GZX_Haemoglobin.png

8. Blood flow and red blood cell shape of normal and sickle cell shaped hemoglobin, https://commons.wikimedia.org/wiki/File:Sickle_cell_01.jpg#/media/File:Sickle_cell_01.jpg

Protein Functions

Proteins are the "workhorses" of the body and participate in many bodily functions. As we've already discussed, proteins come in all sizes and shapes, and each is specifically structured for its particular function. This page describes some of the important functions of proteins. As you read through them, keep in mind that synthesis of all of these different proteins requires adequate amounts of amino acids. As you can imagine, consuming a diet that is deficient in protein and essential amino acids can impair many of the body's functions. (More on that later in the unit.)

Figure 2.1 - Examples of proteins with different functions, sizes, and shapes.

Major types and functions of proteins are summarized in the table below, and the subsequent sections of this page give more detail on each of them.

Structure

More than one hundred different structural proteins have been discovered in the human body, but the most abundant by far is collagen, which makes up about 6 percent of total body weight. Collagen makes up 30 percent of bone tissue and comprises large amounts of tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein made up of mostly glycine and proline amino acids. Within its quaternary structure, three protein strands twist around each other like a rope and then these collagen ropes overlap with others.

Figure 2.2 Triple-helix structure of collagen

This highly ordered structure is even stronger than steel fibers of the same size. Collagen makes bones strong but flexible. Collagen fibers in the skin's dermis provide it with structure, and the accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and then let go; the collagen and elastin proteins in skin allow it to go back to its original shape. Smooth-muscle cells that secrete collagen and elastin proteins surround blood vessels, providing the vessels with structure and the ability to stretch back after blood is pumped through them. Another strong, fibrous protein is keratin, an important component of skin, hair, and nails.

Enzymes

Enzymes are proteins that conduct specific chemical reactions. An enzyme's job is to provide a site for a chemical reaction and to lower the amount of energy and time it takes for that chemical reaction to happen (this is known as "catalysis"). On average, more than 100 chemical reactions occur in cells every single second, and most of them require enzymes. The liver alone contains over 1,000 enzyme systems. Enzymes are specific and will use only particular substrates that fit into their active site, similar to the way a lock can be opened only with a specific key. Fortunately, an enzyme can fulfill its role as a catalyst over and over again, although eventually it is destroyed and rebuilt. All bodily functions, including the breakdown of nutrients in the stomach and small intestine, the transformation of nutrients into molecules a cell can use, and building all macromolecules, including protein itself, involve enzymes.

Figure 2.3 Enzymes are proteins. An enzyme's job is to provide a site for substances to chemically react and form a product, and decrease the amount of energy and time it takes for this to happen.

The following video demonstrates the action of enzymes:  https://youtu.be/V4OPO6JQLOE.

Hormones

Proteins are responsible for hormone synthesis. Hormones are the chemical messengers produced by the endocrine glands. When an endocrine gland is stimulated, it releases a hormone. The hormone is then transported in the blood to its target cell, where it communicates a message to initiate a specific reaction or cellular process. For instance, after you eat a meal, your blood glucose levels rise. In response to the increased blood glucose, the pancreas releases the hormone insulin. Insulin tells the cells of the body that glucose is available and to take it up from the blood and store it or use it for making energy or building macromolecules. A major function of hormones is to turn enzymes on and off, so some proteins can even regulate the actions of other proteins. While not all hormones are made from proteins, many of them are.

Fluid and Acid-Base Balance

Adequate protein intake enables the basic biological processes of the body to maintain homeostasis (constant or stable conditions) in a changing environment. One aspect of this is fluid balance, keeping water distributed properly in the different compartments of the body. If too much water suddenly moves from the blood into a tissue, the results are swelling and, potentially, cell death. Water always flows from an area of high concentration to an area of low concentration. As a result, water moves toward areas that have higher concentrations of other solutes, such as proteins and glucose. To keep the water evenly distributed between blood and cells, proteins continuously circulate at high concentrations in the blood. The most abundant protein in blood is the butterfly-shaped protein known as albumin. The presence of albumin in the blood makes the protein concentration in the blood similar to that in cells. Therefore, fluid exchange between the blood and cells is not in the extreme, but rather is minimized to preserve homeostasis.

Figure 2.4 The butterfly-shaped protein, albumin, has many functions in the body including maintaining fluid and acid-base balance and transporting molecules.

Protein is also essential in maintaining proper pH balance (the measure of how acidic or basic a substance is) in the blood. Blood pH is maintained between 7.35 and 7.45, which is slightly basic. Even a slight change in blood pH can affect body functions. The body has several systems that hold the blood pH within the normal range to prevent this from happening. One of these is the circulating albumin. Albumin is slightly acidic, and because it is negatively charged it balances the many positively charged molecules circulating in the blood,, such as hydrogen protons (H+), calcium, potassium, and magnesium. Albumin acts as a buffer against abrupt changes in the concentrations of these molecules, thereby balancing blood pH and maintaining homeostasis. The protein hemoglobin also participates in acid-base balance by binding hydrogen protons.

Transport

Proteins also play vital roles in transporting substances around the body. For example, albumin chemically binds to hormones, fatty acids, some vitamins, essential minerals, and drugs, and transports them throughout the circulatory system. Each red blood cell contains millions of hemoglobin molecules that bind oxygen in the lungs and transport it to all the tissues in the body. A cell's plasma membrane is usually not permeable to large polar molecules, so to get the required nutrients and molecules into the cell, many transport proteins exist in the cell membrane. Some of these proteins are channels that allow particular molecules to move in and out of cells. Others act as one-way taxis and require energy to function.

Figure 2.5 Molecules move in and out of cells through transport proteins, which are either channels or carriers.

This two minute tutorial describes how the sodium-potassium pump uses active transport to move sodium ions (Na+) out of a cell, and potassium ions (K+) into a cell: https://youtu.be/_bPFKDdWlCg.

Immunity

Proteins also play important roles in the body's immune system. The strong collagen fibers in skin provide it with structure and support, but it also serves as a barricade against harmful substances. The immune system's attack and destroy functions are dependent on enzymes and antibodies, which are also proteins. For example, an enzyme called lysozyme is secreted in the saliva and attacks the walls of bacteria, causing them to rupture. Certain proteins circulating in the blood can be directed to build a molecular knife that stabs the cellular membranes of foreign invaders. The antibodies secreted by white blood cells survey the entire circulatory system, looking for harmful bacteria and viruses to surround and destroy. Antibodies also trigger other factors in the immune system to seek and destroy unwanted intruders.

Watch this video to observe how antibodies protect against foreign intruders: https://youtu.be/Ys_V6FcYD5I.

Energy Production

Some of the amino acids in proteins can be disassembled and used to make energy. Only about 10 percent of dietary proteins are catabolized each day to make cellular energy. The liver is able to break down amino acids to the carbon skeleton, which can then be fed into the citric acid or Krebs cycle. This is similar to the way that glucose is used to make ATP. If a person's diet does not contain enough carbohydrates and fats, their body will use more amino acids to make energy, which can compromise the synthesis of new proteins and destroy muscle proteins if calorie intake is also low.

Not only can amino acids be used for energy directly, but they can also be used to synthesize glucose through gluconeogenesis. Alternatively, if a person is consuming a high protein diet and eating more calories than their body needs, the extra amino acids will be broken down and transformed into fat. Unlike carbohydrate and fat, protein does not have a specialized storage system to be used later for energy.

Self Check:

References:

1. "Protein Functions", section 6.4 from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

Image Credits:

1. Enzyme , antibody, and hormone images from "Protein Functions", section 6.4 from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

2. Collagen image from https://commons.wikimedia.org/wiki/File:Collagentriplehelix.png#/media/File:Collagentriplehelix.png

3. Enzyme activity image from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

4. Albumin image from https://commons.wikimedia.org/wiki/File:PDB_1ao6_EBI.jpg#/media/File:PDB_1ao6_EBI.jpg

5. Protein carriers in cell membranes image by LadyofHats, Mariana Ruiz Villarreal, public domain, https://en.wikipedia.org/wiki/Membrane_transport_protein#/media/File:Scheme_facilitated_diffusion_in_cell_membrane-en.svg

Protein in Foods and Dietary Recommendations

In this section, we'll discuss how to determine how  much protein you need and your many choices in designing an optimal diet with high-quality protein sources.

How Much Dietary Protein Does a Person Need?

Because our bodies are so efficient at recycling amino acids, protein needs are not as high as carbohydrate and fat needs. The Recommended Daily Allowance (RDA) for a sedentary adult is 0.8 g per kg body weight per day. This would mean that a 165 pound man, and a 143 pound woman would need 60 g and 52 g of protein per day, respectively. The Acceptable Macronutrient Distribution Range (AMDR) for protein for adults is 10% to 35% of total energy intake. A Tolerable Upper Intake Limit for protein has not been set, but it is recommended that you not exceed the upper end of the AMDR.

Protein needs are higher for the following populations:

• growing children and adolescents

• women who are pregnant (they're using protein to help grow a fetus)

• lactating women (breast milk has protein in it for the baby's nutrition, so mothers need more protein to  synthesize that milk)

• athletes

The Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine recommend 1.2 to 2.0 grams of protein per kilogram of body weight per day for athletes, depending on the type of training. Higher intakes may be needed for short periods during intensified training or with reduced energy intake. We will discuss protein needs for athletes more in later weeks when we discuss nutrition and physical activity.

Nitrogen Balance to Determine Protein Needs

The appropriate amount of protein in a person's diet is that which maintains a balance between what is taken in and what is used. The RDAs for protein were determined by assessing nitrogen balance. Nitrogen is one of the four basic elements contained in all amino acids. When  amino acids are broken down, nitrogen is released. Most nitrogen is excreted as urea in urine, but some urea is also contained  in feces. Nitrogen is also lost in sweat and as hair and nails grow. The RDA, therefore, is the amount of protein a person should consume in their diet to balance the amount of protein used by the body, measured as the amount of nitrogen lost from the body. The Institute Of Medicine used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the RDA for protein.

• Nitrogen Balance- A person is said to be in nitrogen balance when the nitrogen consumed equals the amount of nitrogen excreted. Most healthy adults are in nitrogen balance. If they consume more protein, they excrete the extra nitrogen to keep nitrogen in balance. The lowest amount of protein a person can consume and still remain in nitrogen balance represents that person's minimum protein requirement.

Figure 3.1 - People are in nitrogen balance when they excrete as much nitrogen as they consume.

• Negative Nitrogen Balance- A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, meaning that the body is breaking down more protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy. Someone who is eating a low-protein diet may also be in negative nitrogen balance as they are taking in less protein than they actually need.

Figure 3.2 - People are in negative nitrogen balance when they excrete more nitrogen than they consume, usually because they are not eating enough protein to meet their needs.

• Positive Nitrogen Balance- A person is in positive nitrogen balance when a person excretes less nitrogen than what is taken in by the diet, such as during pregnancy or  growth in childhood. At these times the body requires more protein to build new tissues, so more of what gets consumed gets used up and less nitrogen is excreted. A person healing from a severe wound may also be in positive nitrogen balance because protein is being used up to repair tissues.

Figure 3.3 - People are in positive nitrogen balance when they excrete less nitrogen than they consume, because they are using protein to actively build new tissue.

Dietary Sources of Protein

Although meat is the typical food that comes to mind when thinking of protein, many other foods are rich in protein as well, including dairy products, eggs, beans, whole grains, and nuts. Tabel 3.1 lists the grams of protein in a standard serving for a variety of animal and plant foods.  

Table 3.1 Protein in Common Foods

Notice in the table above that whole foods contain more protein than refined foods. When foods are refined -- for example going from a whole almond to almond milk  or whole grain to refined grain -- protein is lost in that processing. Very refined foods like oil and sugar contain no protein.

The USDA provides some tips for choosing your dietary protein sources. The overall suggestion is to eat a variety of protein-rich foods to benefit health. Examples include:

• Lean meats, such as round steaks, top sirloin, extra lean ground beef, pork loin, and skinless chicken.

• 8 ounces of cooked seafood every week (typically as two 4-ounce servings).

• Choosing to eat beans, peas, or soy products as a main dish. For example, chili with kidney and pinto beans, hummus on pita bread, and black bean enchiladas.

• Enjoy nuts in a variety of ways. Put them on a salad, in a stir-fry, or use them as a topping for steamed vegetables in place of meat or cheese.

Protein Quality

While protein is contained in a wide variety of foods, it differs in quality. High-quality complete proteins contain all nine essential amino acids. Lower-quality incomplete proteins do not contain all nine essential amino acids in proportions needed to support growth and health.

Foods that are complete protein sources include animal foods such as milk, cheese, eggs, fish, poultry, and meat. A few plant foods also are complete proteins, such as soy (soybeans, soy milk, tofu, tempeh) and quinoa.

Most plant-based foods are deficient in at least one essential amino acid and therefore are incomplete protein sources. For example, grains are usually deficient in the amino acid lysine, and legumes are low in  methionine and tryptophan. Because grains and legumes are not deficient in the same amino acids, they can complement each other in a diet. When consumed in tandem, they contain all nine essential amino acids at adequate levels, so they are called complementary proteins. Some examples of complementary protein foods are given in Table 3.2. Mutual supplementation is another term used when combining two or more incomplete protein sources to make a complete protein. Complementary protein sources do not have to be consumed at the same time—as long as they are consumed within the same day, you will meet your protein needs. Most people eat complementary proteins without thinking about it, because they go well together. Think of a peanut butter sandwich and beans and rice; these are examples of complementary proteins. So long as you eat a variety of foods, you don't need to worry much about incomplete protein foods. They may be called "lower quality" in terms of protein, but they're still great choices, as long as they're not the only foods you eat!

Table 3.2 Complementary Protein Sources

The second component of protein quality is digestibility, as not all protein sources are equally digested. In general, animal-based proteins are more fully digested than plant-based proteins, because some proteins are contained in the plant's fibrous cell walls and these pass through the digestive tract unabsorbed by the body. Animal proteins tend to be 95 percent or more digestible; soy is estimated at 91 percent; and many grains are around 85 to 88 percent digestible.

Self Check:

References:

1. "Proteins, Diet, and Personal Choice", section 6.4 from the book An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

2. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of Dietitians of Canada, the Academy of Nutrition and Dietetics and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501-528.

3. USDA National Nutrient Database for Standard Reference, December 2018.

4. Tome, D. Criteria and markers for protein quality assessment – a review. British Journal of Nutrition 108, S222–S229 (2012).

Image Credits:

1. Meat! picture by Chris Suderman, CC BY-NC-ND 2.0, http://flic.kr/p/4ukmdH

2. Mexican-Rice-and-Beans-2 picture by Meg H, CC BY 2.0, http://flic.kr/p/Wdk5b3

3. Dancing Exercises picture by Forum Danca, CC BY-NC 2.0, http://flic.kr/p/bZLZgo

4. Nitrogen balance image by Tamberly Powell,  CC BY-NC-SA 4.0

5. Indian Prisoners of War picture by Chris Turner, CC BY 2.0, https://flic.kr/p/96L1fu

6. Negative nitrogen balance image by Tamberly Powell, CC BY-NC-SA 4.0

7. Child at Seoul picture by Philippe Teuwen, CC BY-SA 2.0, https://flic.kr/p/Jh58q

8. Positive nitrogen balance image by Tamberly Powell,  CC BY-NC-SA 4.0

Protein Digestion and Absorption

When you eat food, the body's digestive system breaks down dietary protein into individual amino acids, which are absorbed and used by cells to build other proteins and a few other macromolecules, such as DNA. Let's follow the path that proteins take down the gastrointestinal tract and into the circulatory system.

Eggs are a good dietary source of protein and will be used as our example as we discuss the processes of digestion and absorption of protein. One egg, whether raw, hard-boiled, scrambled, or fried, supplies about six grams of protein.

In the image below, follow the numbers to see what happens to the protein in our egg at each site of digestion.

Fig. 4.1 - Protein digestion in the human GI tract.

1 - Protein digestion in the mouth

Unless you are eating it raw, the first step in digesting an egg (or any other solid food) is chewing. The teeth begin the mechanical breakdown of large egg pieces into smaller pieces that can be swallowed. The salivary glands secrete saliva to aid swallowing and the passage of the partially mashed egg through the esophagus.

2 - Protein digestion in the stomach

The mashed egg pieces enter the stomach from the esophagus. As illustrated in the image below, both mechanical and chemical digestion take place in the stomach. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which initiate the breakdown of protein. Muscular contractions, called peristalsis, also aid in digestion. The powerful stomach contractions churn the partially digested protein into a more uniform mixture, which is called chyme.

Fig. 4.2 - Protein digestion in the stomach

Because of the hydrochloric acid in the stomach, it has a very low pH of 1.5-3.5. The acidity of the stomach causes food proteins to denature, unfolding their three-dimensional structure to reveal just the polypeptide chain. Recall that the three-dimensional structure of a protein is essential to its function, so denaturation in the stomach also destroys protein function. (This is why a protein such as insulin can't be taken as an oral medication. Its function is destroyed in the digestive tract, first by denaturation and then further by enzymatic digestion. Instead, it has to be injected so that it is absorbed intact into the bloodstream.)

Fig. 4.3 - In the stomach, proteins are denatured because of the acidity of hydrochloric acid.

Once proteins are denatured in the stomach, the peptide bonds linking amino acids together are more accessible for enzymatic digestion. That process is started by pepsin, an enzyme that is secreted by the cells that line the stomach and is activated by hydrochloric acid. Pepsin begins breaking peptide bonds, creating shorter polypeptides.

Fig. 4.4 - Enzymatic digestion of proteins begins in the stomach with the action of the enzyme pepsin.

Proteins are large globular molecules, and their chemical breakdown requires time and mixing. Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Food remains in the stomach longer, making you feel full longer.

3 - Protein digestion and absorption in the small intestine

The chyme leaves the stomach and enters the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juices into the small intestine, and these contain more enzymes to further break down polypeptides.

The two major pancreatic enzymes that digest proteins in the small intestine are chymotrypsin and trypsin. Trypsin activates other protein-digesting enzymes called proteases, and together, these enzymes break proteins down to tripeptides, dipeptides, and individual amino acids. The cells that line the small intestine release additional enzymes that also contribute to the enzymatic digestion of polypeptides.

Tripeptides, dipeptides, and single amino acids enter the enterocytes of the small intestine using active transport systems, which require ATP. Once inside, the tripeptides and dipeptides are all broken down to single amino acids, which are absorbed into the bloodstream. There are several different types of transport systems to accommodate different types of amino acids. Amino acids with structural similarities end up competing to use these transporters. That's not a problem if your protein is coming from food, because it naturally contains a mix of amino acids. However, if you take high doses of amino acid supplements, those could theoretically interfere with absorption of other amino acids.

Fig. 4.5 - Summary of protein digestion. Note that the lines representing polypeptide chains in the stomach consist of strings of amino acids connected by peptide bonds, even though the individual amino acids aren't shown in this simplified representation.

Proteins that aren't fully digested in the small intestine pass into the large intestine and are eventually excreted in the feces. Recall from the last page that plant-based proteins are a bit less digestible than animal proteins, because some proteins are bound in plant cell walls.

What happens to absorbed amino acids?

Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Dietary amino acids then become part of the body's amino acid pool.

Assuming the body has enough glucose and other sources of energy, those amino acids will be used in one of the following ways:

• Protein synthesis in cells around the body

• Making nonessential amino acids needed for protein synthesis

• Making other nitrogen-containing compounds

• Rearranged and stored as fat (there is no storage form of protein)

If there is not enough glucose or energy available, amino acids can also be used in one of these ways:

• Rearranged into glucose for fuel for the brain and red blood cells

• Metabolized as fuel, for an immediate source of ATP

In order to use amino acids to make ATP, glucose, or fat, the nitrogen first has to be removed in a process called deamination, which occurs in the liver and kidneys. The nitrogen is initially released as ammonia, and because ammonia is toxic, the liver transforms it into urea. Urea is then transported to the kidneys and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it ideal for transporting excess nitrogen out of the body.

Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.

Self-Check:

References:

1. Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR

2. "Protein Digestion and Absorption," section 6.3 from An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

Image Credits:

1. Digestive system image by Mariana Ruiz, public domain, edited by Alice Callahan

2. Protein digestion in the stomach from "Protein Digestion and Absorption," section 6.3 from An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

3. Process of denaturation image by Scurran, CC BY-SA 4.0, https://commons.wikimedia.org/wiki/File:Process_of_Denaturation.svg, edited by Alice Callahan, CC BY-SA 4.0.

4. Process of denaturation image by Scurran, CC BY-SA 4.0, https://commons.wikimedia.org/wiki/File:Process_of_Denaturation.svg, edited by Alice Callahan, CC BY-SA 4.0.

5. Again uses the protein image from (3) and (4) within a larger diagram by Alice Callahan, CC BY-SA 4.0.

Health Consequences of Too Little and Too Much Dietary Protein

A healthy diet incorporates all nutrients in moderation, meaning that there's neither too little nor too much. As with all nutrients, having too little or too much protein can have health consequences.

The AMDR for protein for adults is between 10 and 35 percent of kilocalories. That's a fairly wide range, and it encompasses typical protein intakes of many traditional human cultures.

Fig. 5.1 - Diverse human cultures have survived on different levels of dietary protein. The photo on the left shows Inuit families sharing frozen, aged walrus meat. Their traditional diet is very dependent on meat and high in both protein and fat. On the right is a traditional vegetarian meal in India, high in carbohydrates but still providing adequate levels of protein.

Protein intake below the RDA is inadequate to support the body's needs for synthesis of structural and functional proteins. On the other hand, there are some concerns that high protein intake is associated with chronic disease. However, as we'll discuss, it's not just the quantity of protein that matters, but also the nutritional package that it comes in.

According to a 2018 study published in the American Journal of Clinical Nutrition, most Americans get enough protein, averaging about 88 grams per day and 14 to 16 percent of caloric intake. The study also found that diets with protein above 35 percent of caloric intake, the upper end of the AMDR, were extremely rare.

Health Consequences of Protein Deficiency

Although severe protein deficiency is rare in the developed world, it is a leading cause of death in children in many poor, underdeveloped countries. There are two main syndromes associated with protein deficiencies: Kwashiorkor and Marasmus.

Kwashiorkor affects millions of children worldwide. When it was first described in 1935, more than 90 percent of children with Kwashiorkor died. Although the associated mortality is slightly lower now, most children still die after the initiation of treatment.

The name Kwashiorkor comes from a language in Ghana and means, "rejected one." The syndrome was named because it occurs most commonly in children recently weaned from breastfeeding, usually because the mother had a new baby, and the older child is switched to a diet of watery porridge made from low-protein grains. The child may be consuming enough calories, but not enough protein.

Kwashiorkor is characterized by swelling (edema) of the feet and abdomen, poor skin health, poor growth, low muscle mass, and liver malfunction. Recall that one of the roles of protein in the body is fluid balance. Diets extremely low in protein do not provide enough amino acids for the synthesis of the protein albumin. One of the functions of albumin is to hold water in the blood vessels, so having lower concentrations of blood albumin results in water moving out of the blood vessels and into tissues, causing swelling. The primary symptoms of Kwashiorkor include not only swelling, but also diarrhea, fatigue, peeling skin, and irritability. Severe protein deficiency in addition to other micronutrient deficiencies, such as folate, iodine, iron, and vitamin C all contribute to the many health manifestations of this syndrome.

Children and adults with marasmus are protein deficient, but at the same time, they're also not taking in enough calories. Body weights of children with Marasmus may be up to 80 percent less than that of a healthy child of the same age. Marasmus is a Greek word meaning "starvation." The syndrome affects more than fifty million children under age five worldwide. It is characterized by an extreme emaciated appearance, poor skin health, poor growth, and increased risk of infection. The symptoms are acute fatigue, hunger, and diarrhea.

Figure 5.2. The photo on the left shows a child suffering from kwashiorkor (note the swollen belly) in the late 1960s in a Nigerian relief camp during the Nigerian-Biafran War. The photo on the right shows an Indian child suffering from marasmus.

Kwashiorkor and marasmus often coexist as a combined syndrome termed marasmic kwashiorkor. Children with the combined syndrome have variable amounts of edema and the characterizations and symptoms of marasmus. Although organ system function is compromised by undernutrition, the ultimate cause of death is usually infection. Undernutrition is intricately linked with suppression of the immune system at multiple levels, so undernourished children commonly die from severe diarrhea and/or pneumonia resulting from bacterial or viral infection. According to the United Nations Children's Fund (UNICEF), nearly half of all deaths of children under age five are related to malnutrition. That translates to about 3 million child deaths each year.

While severe protein deficiency is rare in the U.S., there are several groups at risk of low protein intake. A 2018 study found that 23 percent of U.S. adolescent girls (aged 14 to 18 years old) and 11 percent of adolescent boys were consuming below the RDA for protein, which may compromise their growth and development. This is thought to be related to the growing independence in food choices and the high prevalence of dieting in this group.

Low protein intake is also a concern for the elderly in the U.S. The same 2018 study found that among those 71 years and older, 19 percent of women and 13 percent of men consume less protein than the RDA. This is a particular concern in this age group, as loss of muscle is accelerated with aging, and that can lead to greater frailty, loss of balance, and greater risk of falls. Some researchers argue that older adults actually need more protein than recommended by the RDA in order to maintain muscle mass and function.

Health Consequences of Too Much Protein in the Diet

When the Food and Nutrition Board of the Institute of Medicine wrote the DRI for macronutrients, published in 2005, they concluded that there wasn't enough evidence to establish an Upper Limit for protein. The high end of the AMDR, 35 percent of kilocalories for protein, was set in order to allow the total diet to be well-balanced with carbohydrate and fat. Higher levels of protein intake haven't been well-studied, but over the years, there have been many concerns with high protein diets. However, current evidence indicates it's large amounts of animal protein (particularly from red meat or processed meats) that can be problematic, not high amounts of protein per se.

For example, a diet containing lots of steak, bacon, and sausage would be high in protein, but it also might be high in saturated fat, cholesterol, salt, and nitrates. Eating more red meat and processed red meat is linked to an increased risk of heart disease, stroke, and cancer (especially colorectal, stomach, pancreatic, prostate, and breast cancers). This link doesn't seem to be caused by the protein but rather the nutritional package that it comes in. In addition, the link to cancer may be related to the carcinogens that can form when meat is cooked at high temperatures, particularly when it's charred by grilling.

On the other hand, studies show that when protein comes from lean meat and plant sources, risk of chronic diseases may be reduced. For example, a 2015 study found that frequent consumption of red meat in adolescence was associated with a higher risk of breast cancer later in life, whereas consuming poultry, fish, legumes, and nuts instead lowered risk. Other studies have shown that higher protein diets can reduce the risk of heart disease, provided the protein comes from healthier sources.

Fig. 5.3. Compare several different "protein packages." The steak and bacon provide protein but also large amounts of saturated fat and sodium. Salmon provides as much protein as the steak but with less saturated fat and more polyunsaturated fats. Lentils are a good source of protein, are low in fat, and are a great source of fiber.

Several other concerns about high protein diets haven't turned out to be problematic after all:

• Osteoporosis - High protein diets were once thought to increase risk of osteoporosis, because researchers noticed that urinary calcium excretion increases when people consume high amounts of protein. However, a 2017 systematic review and meta-analysis from the National Osteoporosis Foundation found that this is not a concern, and some studies even show that higher protein intake is associated with greater bone mineral density.

• Kidney function -  Another concern was that high protein diets would strain the kidneys because of the increased need to filter and excrete nitrogen. A 2018 meta-analysis concluded that this is not a concern. However, people who already have chronic kidney disease should avoid high protein diets and maintain protein intake around the RDA of 0.8 g/kg, a lower intake than many Americans are consuming. A 2009 study tested a very low protein diet of 0.6 g/kg protein per day in people with chronic kidney disease and found that it did not prevent the development of kidney failure but did increase the risk of death, underscoring the importance of adequate protein even in people with kidney disease.

From all of this research, there's little evidence that a high protein diet is inherently harmful, so long as the protein doesn't come packaged with a lot of saturated fat and red meat consumption is limited. Still, there's little research directly testing the health effects of very high protein diets, including those achieved using protein supplements of purified protein, so it's probably wise to keep protein balanced with the other macronutrients, focusing on whole foods from all the food groups.

Self-Check:

References:

1. Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR

2. "Diseases Involving Proteins," section 6.5 from An Introduction to Nutrition (v. 1.0), CC by-nc-sa 3.0

3. Food and Nutrition Board & Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. 1359 pp. (The National Academies Press, 2005).

4. Berryman, C. E., Lieberman, H. R., Fulgoni, V. L. & Pasiakos, S. M. Protein intake trends and conformity with the Dietary Reference Intakes in the United States: analysis of the National Health and Nutrition Examination Survey, 2001-2014. Am. J. Clin. Nutr. 108, 405–413 (2018).

5. UNICEF, "Malnutrition in Children - UNICEF Data," May 2018, https://data.unicef.org/topic/nutrition/malnutrition/, last accessed November 29, 2018.

6. Traylor, D. A., Gorissen, S. H. M. & Phillips, S. M. Perspective: Protein Requirements and Optimal Intakes in Aging: Are We Ready to Recommend More Than the Recommended Daily Allowance? Adv Nutr 9, 171–182 (2018).

7. Harvard T.H. Chan School of Public Health. Protein. https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/protein/#protein-research, last accessed November 30, 2018.

8. Farvid, M. S., Cho, E., Chen, W. Y., Eliassen, A. H. & Willett, W. C. Adolescent meat intake and breast cancer risk. Int. J. Cancer 136, 1909–1920 (2015).

9. Shams-White, M. M. et al. Dietary protein and bone health: a systematic review and meta-analysis from the National Osteoporosis Foundation. Am. J. Clin. Nutr. 105, 1528–1543 (2017).

10. Devries, M. C. et al. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis. J Nutr 148, 1760–1775 (2018).

11. Menon, V. et al. Effect of a very low-protein diet on outcomes: long-term follow-up of the Modification of Diet in Renal Disease (MDRD) Study. Am. J. Kidney Dis. 53, 208–217 (2009).

Image Credits:

1. Photo of Inuit families sharing walrus meat, by Ansgar Walk, https://commons.wikimedia.org/wiki/File:Walrus_meat_1_1999-04-01.jpg, CC BY-2.0

2. Photo of traditional Indian vegetarian meal, by GracinhaMarco Abundo, https://en.wikipedia.org/wiki/Indian_cuisine#/media/File:Vegetarian_Curry.jpeg, CC BY-2.0

3. Photo of child with kwashiorkor, Centers for Disease Control and Prevention, public domain, https://phil.cdc.gov/details.aspx?pid=6901

4. Photo of child with marasmus, Centers for Disease Control and Prevention, public domain, https://phil.cdc.gov/Details.aspx?pid=1702

5. Photo of sirloin steak by Steven Depolo, CC BY-2.0, https://flic.kr/p/9rc1i2

6. Photo of bacon by Bradley Gordon, CC BY-2.0, https://flic.kr/p/676bHJ

7. Photo of salmon by Ketzirah Lesser and Art Drau, CC BY-SA 2.0, https://flic.kr/p/5UQ3B6

8. Photo of lentils by Lucas Falcao, CC BY-NC-ND 2.0, https://flic.kr/p/G4yt11

Protein Food Choices and Sustainability

Before it gets to our plates, every food has a story. Maybe it started as a seed, planted in soil and nurtured to maturity with water, sunlight, and fertilizer. Or maybe it came from an animal, one raised for its meat or to produce milk or eggs. We choose foods based on their taste, price, convenience, and nutritional value, but it's also worth considering their backstories. This is particularly true for protein foods, because animal protein production generally consumes more resources and is less sustainable than plant protein sources. Agricultural animals need care, feeding, housing, disposal of their waste, and sometimes medication use throughout their lives. It's worth considering where our protein comes from and how our choices affect the planet, especially since most of us consume more protein than we need.

Animal Agriculture and Resource Use

The World Resources Institute, a global research non-profit organization with a mission "to move human society to live in ways that protect Earth's environment" has compiled data on the environmental impact of protein choices. In the graphic below, you'll see that in terms of greenhouse gas emissions, protein sources from plants have a much lower impact than protein sources from animals. Most plant proteins, with the exception of nuts, are also less expensive. Beef, lamb, and goat meat come at a higher cost to the environment and your wallet.

Fig. 6.1 - Protein Scorecard from the World Resources Institute. Source: https://www.wri.org/resources/data-visualizations/protein-scorecard

Beef is among the most resource-intensive sources of protein. A 2014 study published in the Proceedings of the National Academy of Sciences calculated that beef production uses 28 times more land and 11 times more irrigation water than the average of dairy, poultry, pork, and egg production.  

It's important to point out that animal agriculture does fill some important roles that plants can't. For example, much of the world's pasture land is on steep terrain that wouldn't work well for growing food crops. And animal waste – in the form of manure – is an important fertilizer, including in organic food systems. So, animal agriculture and eating meat aren't inherently bad for the environment – but it would probably be good for the planet if we ate less meat, as shown in the graphic below.

Fig. 6.2 - Shifting High Consumers' Diets Can Greatly Reduce Per Person Land Use and GHG Emissions from the World Resources Institute. Source: https://www.wri.org/resources/charts-graphs/animal-based-foods-are-more-resource-intensive-plant-based-foods

In terms of environmental impact, making small shifts can have a significant impact. Consider the following approaches:

• If you eat meat every day, try adding a "meatless Monday" into your week and experiment with some vegetarian recipes. Once you've adapted to that, try adding another day.

• Replace some of your beef meals with dishes featuring chicken, pork, eggs, fish, or legumes.

• Eat smaller portions of meat and add more plant foods to your plate. For example, if you enjoy spaghetti and meat sauce, try using less meat in your sauce and adding in vegetables like mushrooms, bell peppers, and carrots. Your meal will be more nutrient-dense and maybe even more flavorful.

When you consider that moderate shifts like these would not only be good for the planet but also good for our health, then they don't seem like much of a sacrifice. A 2016 study published in the Proceedings of the National Academy of Sciences concluded that just following standard dietary guidelines (which recommend a variety of protein sources, including plant proteins, and eating more whole grains, fruits and vegetables) could reduce mortality by 6 to 10 percent and cut greenhouse gas emissions by 29 to 70 percent.

This page from the World Resources Institute provides more information: Sustainable Diets: What You Need to Know in 12 Charts, by Janet Ranganathan and Richard Waite, April 20, 2016.

Animal Agriculture and Antibiotic Resistance

One of the biggest current threats to public health is antibiotic resistance. Antibiotics are life-saving drugs, but over time, bacteria can develop resistance to them. This means that the antibiotics no longer work to kill the bacteria causing infections, leaving people with more severe illnesses and fewer treatment options, often needing to try different antibiotics that have more side effects. There are now some bacterial infections for which we have no working antibiotics to treat them. According to the CDC, more than 2 million people are infected with antibiotic-resistant bacteria each year in the U.S, and these infections are thought to kill at least 23,000 people annually. Addressing this problem will require us to be more careful about how we use antibiotics, invest in research to develop new ones, and to develop other ways of preventing bacterial disease, such as new vaccines.

Antibiotics are important to both human and animal medicine. When we're sick with a bacterial illness, we may need antibiotics to treat it, and the same is true of animals, whether they're raised for agriculture or part of our families as our pets. The problem is that the more we use antibiotics, the more chances bacteria have to evolve resistance to them, and the less effective those antibiotics become.

Fig. 6.3 - Scanning electron micrograph of methicillin-resistant Staphylococcus aureus (MRSA, brown) surrounded by cellular debris. MRSA resists treatment with many antibiotics. Credit: NIAID

Watch how quickly bacteria can develop resistance to antibiotics when they're exposed to them and how resistant populations can grow in this video: Watch Antibiotic Resistance Evolve, Science News, September 8, 2016, https://youtu.be/yybsSqcB7mE

The overuse of antibiotics in both human and animal medicine has contributed to the growth of antibiotic-resistant bacteria. For example, taking antibiotics for an illness caused by a virus, such as the common cold or the flu, won't make you better and just gives harmful bacteria chances to evolve resistance. Historically, antibiotics were also routinely given to food production animals to make them fatter, and that allowed for the growth of antibiotic resistance. As of 2017, the FDA ruled that antibiotics can no longer be used for growth promotion in animal agriculture, a significant step in reducing the overuse of these drugs that are important to both humans and animals. Data released by the FDA in December 2018 show that antibiotic sales for farm animals have dropped significantly after this rules change.

Antibiotics can still be used to treat sick animals or stop the spread of disease, and in addition, they can be used to prevent disease in animals that might become sick. Many experts argue that allowing antibiotics to be used for disease prevention leaves a loophole for large amounts of antibiotics to continue to be used, especially in farming systems where animals are crowded and diseases can spread quickly. The World Health Organization has called for this practice to stop, reserving antibiotics only for use in animals that are already sick, not healthy animals.

When antibiotics are used in food animals and bacteria evolve resistance to those antibiotics, the bacteria can be present in your meat, and it can spread in the environment from animal feces, including into the water used to irrigate fruits and vegetables. Humans exposed to these bacteria by handling or eating contaminated food can then become sick with infections that are resistant to antibiotic treatment, as shown in this infographic from the CDC:

Fig. 6.4 - Antibiotic Resistance from Farm to Table infographic from the CDC. Source: https://www.cdc.gov/foodsafety/challenges/from-farm-to-table.html

This video from PBS Newshour explains the link between agriculture and development of antibiotic-resistant bacteria: How industrial farming techniques can breed superbugs, PBS NewsHour, August 9, 2017, https://youtu.be/F9b4KTJM8ps

What can you do to prevent yourself and your family from getting sick with antibiotic-resistant infections? The CDC offers these tips:

• Take antibiotics only when needed.

• Follow simple Food Safety Tips:

• COOK. Use a food thermometer to ensure that foods are cooked to a safe internal temperature: 145°F for whole beef, pork, lamb, and veal (allowing the meat to rest for 3 minutes before carving or consuming), 160°F for ground meats, and 165°F for all poultry, including ground chicken and turkey.

• CLEAN. Wash your hands after touching raw meat, poultry, and seafood. Also wash your work surfaces, cutting boards, utensils, and grill before and after cooking.

• CHILL. Keep your refrigerator below 40°F and refrigerate foods within 2 hours of cooking (1 hour during the summer heat).

• SEPARATE. Germs from raw meat, poultry, seafood, and eggs can spread to produce and ready-to-eat foods unless you keep them separate. Use different cutting boards to prepare raw meats and any food that will be eaten without cooking.

• Wash your hands after contact with poop, animals, or animal environments.

• Report suspected outbreaks of illness from food to your local health department.

• Review CDC's Traveler's Health recommendations when preparing to travel to a foreign country.

When you shop for meat, you'll see lots of different types of labels making claims about how the animals were raised. Does any of this matter when it comes to antibiotic resistance? First, it's important to note that no meat, milk, or eggs should ever contain antibiotics. This is required by federal law, and farmers have practices like stopping antibiotic treatment for a certain amount of time before slaughter to ensure this is the case. However, if animals were routinely treated with antibiotics earlier in their lives, those practices have likely contributed to the growing problem of antibiotic resistance. Choosing meat and eggs that are certified organic ensures that antibiotics weren't used in their production. You'll also see labels stating "raised without antibiotics," and buying these products helps to support farmers and companies that have committed to reducing antibiotic use in their production systems. However, antibiotic resistant bacteria may still be present in products that are labeled certified organic or "raised without antibiotics" (they could have spread to these animals from somewhere else), so follow the food safety rules no matter where your meat comes from.

Figure 6.5 - These eggs are certified organic, so you can be confident that antibiotics weren't used in their production. The sausage is not organic, but it is made from chickens raised without antibiotics, so its production is unlikely to have contributed to the problem of antibiotic resistance.

More and more companies are also recognizing how the overuse of antibiotics can contribute to antibiotic resistance, and they're changing their practices. In December 2018, a large consortium of companies and industry groups, including Walmart, McDonald's, and Tyson Foods, committed to a framework for more responsible use of antibiotics.

Additional reading:

How Drug-Resistant Bacteria Travel from the Farm to Your Table (https://www.scientificamerican.com/article/how-drug-resistant-bacteria-travel-from-the-farm-to-your-table/)

By Melinda Wenner Moyer, Scientific American, 12/1/16

Issues of Fish Sustainability

Fish are a good source of protein and healthful polyunsaturated fats, as well as micronutrients like vitamin D, so they're often mentioned as a good choice. From the charts at the top of this page, you can also see that fish are a relatively sustainable source of protein in terms of using little land and freshwater and producing low levels of greenhouse gases.

However, the oceans have been overfished, and global supplies of wild-caught fish are dwindling. Aquaculture, or fish farming, has also created new environmental challenges. Both of these issues are being solved with good management, like careful limits on wild-caught fishing and new management practices for fish farming. You can encourage these practices by purchasing sustainably-sourced seafood. The Monterey Bay Aquarium Seafood Watch program can help with this. You can download their app to help with buying decisions in the grocery store and find more information on their website: Monterey Bay Aquarium Seafood Watch.

Fig. 6.6 - A screenshot from the Seafood Watch app, showing how it can help you make sense of seafood buying options.

Learn more about problems with fish sustainability and solutions by watching this video: Can the Oceans Keep Up with the Hunt? from the Monterey Bay Aquarium, https://youtu.be/QCs1H1dBIYU

You may have also heard that fish can contain dangerous levels of mercury. This is true of some large species of fish that are higher up the food chain, because they accumulate mercury from their smaller prey and then we get a big dose when we eat them. Fish that have dangerous levels of mercury include king mackerel, marlin, orange roughy, shark, swordfish, tilefish, and bigeye tuna. Pregnant women and growing children in particular should take care to avoid these types of fish, because mercury can interfere with brain development. However, the same groups also stand to benefit from healthful omega-3 fatty acids, like DHA and EPA, which are helpful for brain development. Thus, it's good for pregnant women and children to eat fish, so long as they avoid the ones with high levels of mercury. Most common types of fish have lower levels of mercury and can be eaten at least once per week, if not two or three times per week. Learn more at this FDA page: Eating Fish: What Pregnant Women and Parents Should Know.

Self-Check:

References:

1. Eshel, G., Shepon, A., Makov, T. & Milo, R. Land, irrigation water, greenhouse gas, and reactive nitrogen burdens of meat, eggs, and dairy production in the United States. PNAS 111, 11996–12001 (2014), http://www.pnas.org/content/111/33/11996.short

2. Springmann, M., Godfray, H. C. J., Rayner, M. & Scarborough, P. Analysis and valuation of the health and climate change cobenefits of dietary change. PNAS 113, 4146–4151 (2016).

3. Aleksandrowicz, L., Green, R., Joy, E. J. M., Smith, P. & Haines, A. The Impacts of Dietary Change on Greenhouse Gas Emissions, Land Use, Water Use, and Health: A Systematic Review. PLOS ONE 11, e0165797 (2016).

4. Harvard T.H. Chan School of Public Health. Sustainability. https://www.hsph.harvard.edu/nutritionsource/sustainability/#plate-and-planet (Accessed: December 4, 2018)

5. CDC. Drug Resistance & Food. Centers for Disease Control and Prevention (2018). Available at: https://www.cdc.gov/features/antibiotic-resistance-food/index.html. (Accessed: December 4, 2018)

6. United States Department of Agriculture. Organic Livestock Requirements. (2103). Available at: https://www.ams.usda.gov/sites/default/files/media/Organic%20Livestock%20Requirements.pdf (Accessed: December 4, 2018)

7. Smith, T. C. What does 'meat raised without antibiotics' mean — and why is it important? Washington Post, October 28, 2015, https://www.washingtonpost.com/news/speaking-of-science/wp/2015/10/28/what-does-raised-without-antibiotics-mean-and-why-is-it-important/?noredirect=on&utm_term=.50a91959b167 (Accessed: December 4, 2018)

8. Dall, Chris. FDA reports major drop in antibiotics for food animals. Center for Infectious Disease Research and Policy, December 19, 2018, http://www.cidrap.umn.edu/news-perspective/2018/12/fda-reports-major-drop-antibiotics-food-animals (Accessed: December 20, 2018)

9. Pew Charitable Trusts, Groups Issue Framework for Antibiotic Stewardship in Food Animal Production, December 18, 2018, https://www.pewtrusts.org/en/about/news-room/press-releases-and-statements/2018/12/18/groups-issue-framework-for-antibiotic-stewardship-in-food-animal-production (Accessed: December 20, 2018)

10. Ocean Issues. Seafood Watch Program at the Monterey Bay Aquarium Available at: http://www.seafoodwatch.org/ocean-issues. (Accessed: 4th December 2018)

11. Consumers - Eating Fish: What Pregnant Women and Parents Should Know. Center for Food Safety and Applied Nutrition Available at: https://www.fda.gov/Food/ResourcesForYou/Consumers/ucm393070.htm. (Accessed: 4th December 2018)

Image Credits:

1. Protein Scorecard from the World Resources Institute. CC BY-4.0, Source: https://www.wri.org/resources/data-visualizations/protein-scorecard

2. Shifting High Consumers' Diets Can Greatly Reduce Per Person Land Use and GHG Emissions from the World Resources Institute. CC BY-4.0, Source: https://www.wri.org/resources/charts-graphs/animal-based-foods-are-more-resource-intensive-plant-based-foods

3. Scanning electron micrograph of methicillin-resistant Staphylococcus aureus (MRSA, brown) surrounded by cellular debris. MRSA resists treatment with many antibiotics. Credit: NIAID, CC BY 2.0, https://flic.kr/p/9y4sDM

4. Antibiotic Resistance from Farm to Table infographic from the CDC, public domain, https://www.cdc.gov/foodsafety/challenges/from-farm-to-table.html

5. Photos of egg and sausage labels by Alice Callahan, CC BY-4.0