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

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