Nutrition FN 225

Introduction to Carbohydrates

If someone says to you, "I love carbohydrates, and I eat them all day long!" what would you assume they're eating?

Do you picture this?

close-up photograph of potato chips Photo of pile of M&M candies close-up photo of a pastry dusted with powdered sugar

Fig. 1.1. Examples of carbohydrate-rich snack foods.

And this?

photo of a pile of bread, as in a bakery photo of a serving of spaghetti with a little tomato sauce, on a plate bowl of white rice

Fig. 1.2. Examples of grain-based foods.

When we ask this question in class, most students describe foods like the ones above. However, carbohydrates are found not just in grains, or in sweets and processed foods, but in every food group. In fact, carbohydrates are the most abundant nutrient (except water) in the diets of most humans around the world. Since the dawn of agriculture, human cultures have relied on staple grains, such as corn, rice, and wheat, as the foundation of their diets, and these foods are rich in carbohydrates. But fruits and vegetables, dairy products, legumes, and nuts also have naturally-occurring carbohydrates. And of course, carbohydrates are a key ingredient in desserts, sugar-sweetened beverages like sodas, and many of the packaged snack foods that are readily available and -- let's face it -- can be hard to stop eating. In other words, if someone says they eat a high carbohydrate diet, that could mean many different things. They very well could be talking about a balanced diet focused on whole foods, like this:

close-up of a bowl of fresh fruit, including apples, oranges, and peaches Photo showing several bowls and piles of uncooked beans and rice photo showing cubes of several kinds of cheese

Fig. 2.3. Examples of whole foods containing carbohydrates, including fresh fruit, legumes and grains, and cheese.

The diet industry likes to sell us simple messages about "good" and "bad" foods, and these days, we tend to hear that carbohydrates are in the "bad" group. But given that carbohydrates are in so many different types of foods, that's obviously an oversimplified message -- and it's not fair to all of the awesome sources of carbohydrates in the world of food. Not all carbohydrate-rich foods are the same. In this unit, you'll learn to appreciate the nutrient-dense carbohydrate foods, identify which don't offer as valuable a nutritional package, and understand how a balanced diet can include all of them.

Topics Covered in this Unit

  1. Types of carbohydrates

  2. Food sources of carbohydrates and guidelines for intake

  3. Digestion and absorption of carbohydrates

  4. Glucose utilization and regulation in the body

  5. Fiber: Benefits, recommendations, food sources, and whole vs. refined grains

  6. Sugar: Food sources, health Implications, intakes, and label-reading to identify sugar

  7. Sugar substitutes

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.

  6. Critically evaluate and compare nutrition labels and determine the nutrient density of each food.

Image Credits

  1. Potato chips picture by Kate Ter Haar, CC BY 2.0,; M&Ms picture by Wade Brooks, CC BY-NC 2.0,; Pecan pastry picture by Artizone, CC BY-NC-ND 2.0,

  2. Bread picture by David Stewart, CC BY 2.0,; Pasta picture by Yasumari SASAKI, CC BY 2.0,; Rice photo by Francesca Nocella, CC BY-SA 2.0,

  3. Fruit picture by Allen Gottfried, CC BY-SA 2.0,;  Beans and grain picture by Evans E, CC BY 2.0,; Cheese picture by Finite Focus, CC BY-NC 2.0,

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



Types of Carbohydrates

On this page, we'll get acquainted with the chemical structure of different types of carbohydrates and learn where we find them in foods.

First, all carbohydrates are made up of the same chemical elements:

For this reason, you may see carbohydrates abbreviated as "CHO" in our class.

Carbohydrates can be divided into two main types: simple and complex. Simple carbohydrates are made up of just one or two sugar units, whereas complex carbohydrates are made up of many sugar units. We'll look at each of these in turn. This figure gives you an overview of the types of carbohydrates that we'll cover.

The figure outlines the major types of carbohydrates, organized as simple and complex. Under simple carbohydrates, the 3 monosaccharides (glucose, fructose, and galactose) and 3 disaccharides (maltose, sucrose, lactose) are listed. Under complex carbohydrates, starch, glycogen, and fiber are listed.

Fig. 2.1. Carbohydrates can be divided into two main types: simple (including monosaccharides and disaccharides) and complex.

Simple carbohydrates

Simple carbohydrates are sometimes called "sugars" or "simple sugars." There are 2 types of simple carbohydrates: monosaccharides and disaccharides.

Monosaccharides contain just one sugar unit, so they're the smallest of the carbohydrates. (The prefix "mono-" means "one.") The small size of monosaccharides gives them a special role in digestion and metabolism. Food carbohydrates have to be broken down to monosaccharides before they can be absorbed in the gastrointestinal tract, and they also circulate in blood in monosaccharide form.

There are 3 monosaccharides:

  1. Glucose

  2. Fructose

  3. Galactose

Note that all three have the same chemical formula (C6H12O6); the atoms are just arranged a bit differently.

1 - Glucose

Here's the chemical structure of glucose:

figure shows Hayworth projection of glucose chemical structure

In this class, we'll sometimes use a simpler green hexagon to represent glucose:

You're already familiar with glucose, because it's the main product of photosynthesis. Plants make glucose as a way of storing the sun's energy in a form that it can use for growth and reproduction.

In humans, glucose is one of the most important nutrients for fueling the body. It's especially important for the brain and nervous system, which aren't very good at using other fuel sources. Muscles, on the other hand, can use fat as an energy source. (In practice, your muscles are usually using some combination of fat and glucose for energy, which we'll learn more about later.)

Food sources of glucose: Glucose is found in fruits and vegetables, as well as honey, corn syrup, and high fructose corn syrup. (All plants make glucose, but much of the glucose is used to make starch, fiber, and other nutrients. The foods listed here have glucose in its monosaccharide form.)

2 - Fructose

Here's the chemical structure of fructose:

figure shows Hayworth projection of fructose chemical structure

In this class, we'll sometimes use a simpler purple pentagon to represent fructose:

Fructose is special because it is the sweetest carbohydrate. Plants make a lot of fructose as a way of attracting insects and animals, which help plants to reproduce. For example, plants make nectar, which is high in fructose and very sweet, to attract insects that will pollinate it. Plants also put fructose into fruit to make it tastier. Animals eat the fruit, wander away, and later poop out the seeds from the fruit, thereby sowing the seeds of the next generation. Animal gets a meal, and the plant gets to reproduce: win-win!

photo showing a bee at a lavender flower Photo shows a jar of golden honey, with a spoon raised above it and honey drizzling out of it photo shows kiwifruit sliced in half

Fig. 2.2. Fructose in nature: A bee collects sweet nectar from a flower, in the process spreading pollen from flower to flower and helping plants to reproduce. Bees use nectar to make honey, which humans harvest for use as a sweetener. (Honey contains a mix of sucrose, fructose, and glucose). A kiwi is sweetened in part by fructose. Animals enjoy the sweet fruit and then later poop out the seeds, sowing them for a new generation of kiwi trees.

Food sources of fructose: Fruits, vegetables, honey, high fructose corn syrup

3 - Galactose

Here is the chemical structure of galactose:

figure shows Hayworth projection of galactose chemical structure

In this class, we'll sometimes use a blue hexagon to represent galactose:

Food sources of galactose: Galactose is found in milk (and dairy products made from milk), but it's almost always linked to glucose to form a disaccharide (more on that in a minute). We rarely find it in our food supply in monosaccharide form.

The second type of simple carbohydrates is disaccharides. They contain two sugar units bonded together.

There are 3 disaccharides:

  1. Maltose (glucose + glucose)

  2. Sucrose (glucose + fructose)

  3. Lactose (glucose + galactose)

1 - Maltose

Maltose is made of two glucose molecules bonded together. It doesn't occur naturally in any appreciable amount in foods, with one exception: sprouted grains. Grains contain a lot of starch, which is made of long chains of glucose (more on this in a minute), and when the seed of a grain starts to sprout, it begins to break down that starch, creating maltose. If bread is made from those sprouted grains, that bread will have some maltose. Sprouted grain bread is usually a little heavier and sweeter than bread made from regular flour.

Maltose also plays a role in the production of beer and liquor, because this process involves the fermentation of grains or other carbohydrate sources. Maltose is formed during the breakdown of those carbohydrates, but there is very little remaining once the fermentation process is complete.

You can taste the sweetness of maltose if you hold a starchy food in your mouth for a minute or so. Try this with a simple food like a soda cracker. Starch is not sweet, but as the starch in the cracker begins to break down with the action of salivary amylase, maltose will form, and you'll taste the sweetness!

2 - Sucrose

Sucrose is made of a glucose molecule bonded to a fructose molecule. It's made by plants for the same reason as fructose -- to attract animals to eat it and thereby spread the seeds.

Sucrose is naturally-occurring in fruits and vegetables. (Most fruits and vegetables contain a mixture of glucose, fructose, and sucrose.) But humans have also figured out how to concentrate the sucrose in plants (usually sugar cane or sugar beets) to make refined table sugar. We also find sucrose in maple syrup and honey.

The sucrose found in sweet potato is chemically identical to the sucrose found in table sugar. Likewise, the fructose found in a fig is chemically identical to the fructose found in high fructose corn syrup. As we'll discuss more later, what's different is the package the sugars come in. When you eat a sweet potato or a fig, you also get lots of fiber, vitamins, and minerals in that package, whereas sugar and high fructose corn syrup only provide sugar, nothing else. It's not a bad thing to eat sugar. After all, it's a vital fuel for our brain and nervous system. But paying attention to the package it comes in can help us make good overall choices for health.  

3 - Lactose

Lactose is made of a glucose molecule bonded to a galactose molecule. It is sometimes called "milk sugar" as it is found in dairy products like milk, yogurt, and cheese. These are the only animal foods that have significant amounts of carbohydrate. Most of our carbohydrates come from plant foods.



Complex carbohydrates

Complex carbohydrates are also called polysaccharides, because they contain many sugars. (The prefix "poly-" means "many.") There are 3 main polysaccharides:

  1. Starch

  2. Glycogen

  3. Fiber

All three of these polysaccharides are made up of many glucose molecules bonded together, but they differ in their structure and the type of bonds.

1 - Starch

Starch is made up of long chains of glucose. If these chains are straight, they're called amylose; if they're branched, they're called amylopectin.

Here is an amylose segment containing 3 glucose units.

figure shows chemical structure of a segment of amylose (a type of starch) with 3 glucose units

The next figure shows an amylopectin segment containing 4 glucose units. The chemical structure is represented differently, but can you spot the place where it branches?

figure shows chemical structure of a segment of amylopectin (a type of starch) with 4 glucose units linked together, including one branch point

Using our green hexagon to represent glucose, you can picture starch as something like this:

The figure shows simple schematics of two types of starch: amylose and amylopectin. Amylose is depicted as a chain of green hexagons (each representing glucose) linked together. Amylopectin is depicted as a chain of green hexagons with several branch points in it.

Humans have digestive enzymes to break down both types of starch, which we'll discuss on the next page.

Starch is the storage form of carbohydrate in plants. Plants make starch in order to store glucose. For example, starch is in seeds to give the seedling energy to sprout, and we eat those seeds in the form of grains, legumes (soybeans, lentils, pinto and kidney beans, for example), nuts, and seeds. Starch is also stored in roots and tubers to provide stored energy for the plant to grow and reproduce, and we eat these in the form of potatoes, sweet potatoes, carrots, beets, and turnips.

When we eat plant foods with starch, we can break it down into glucose to provide fuel for our body's cells. In addition, starch from whole plant foods comes packaged with other valuable nutrients. We also find refined starch - such as corn starch - as an ingredient in many processed foods, because it serves as a good thickener.

2 - Glycogen

Glycogen is structurally similar to amylopectin, but it's the storage form of carbohydrate in animals, humans included. It's made up of highly branched chains of glucose, and it's stored in the liver and skeletal muscle. The branched structure of glycogen makes it easier to break down quickly to release glucose to serve as fuel when needed on short notice.

Liver glycogen is broken down to glucose, which is released into the bloodstream and can be used by cells around the body. Muscle glycogen provides energy only for muscle, to fuel activity. That can come in handy if you're being chased by a lion, or sprinting to make your bus!

Even though glycogen is stored in the liver and muscles of animals, we don't find it in meat, because it's broken down soon after slaughter. Thus, glycogen is not found in our food. Instead, we have to make it in our liver and muscle from glucose.

Here's a beautiful depiction of glycogen.

The image is an illustration depicting glycogen, showing a three-dimensional protein at the center (looks like colorful, curled ribbons), and radiating from it are long, branching chains of glucose.

Fig. 2.3 - Glycogen is made from long, branching chains of glucose, radiating around a central protein.

3 - Fiber

Fiber includes carbohydrates and other structural substances in plants that are indigestible to human enzymes. Fiber is made by plants to provide protection and structural support. Think about thick stems that help a plant stand upright, tough seed husks, and fruit skin that protect what's growing inside. These are full of fiber.

close-up photo of golden tops of wheat plants in a field, against a backdrop of a blue sky a photo of a broccoli plant, showing broccoli florets surrounded by large, dark green leaves photo of two apples, colored green and red, growing on a tree branch

Fig. 2.4 - Examples of food plants high in fiber, including wheat, broccoli, and apples.

In our food, we find fiber in whole plant foods like whole grains, seeds, nuts, fruits, vegetables, and legumes.

One of the most common types of fiber is cellulose, the main component in plant cell walls. The chemical structure of cellulose is shown in the figure below, with our simplified depiction next to it. You can see that cellulose has long chains of glucose, similar to starch, but they're stacked up, and there are hydrogen bonds linking the stacks.

image shows chemical structure of cellulose, with a total of 16 glucose units arranged in rows of 4 each, with hydrogen bonds linking them vertically, as in a grid.              image is a schematic showing multiple green hexagons, each representing a glucose molecule, arranged in rows with lines linking them both vertically and horizontally, as in a grid

When we eat fiber, it passes through the small intestine intact, because we don't have digestive enzymes to break it down. Then, in the large intestine, our friendly microbiota -- the bacteria that live in our colons -- go to work on the fiber. Some fiber can be fermented by those bacteria. We'll discuss fiber more later in the unit.








  1. Levin, R.J. Carbohydrates. In: Modern Nutrition in Health and Disease, 9th Ed., Baltimore, MD, Lippincott Williams and Wilkins, 1999

  2. US Department of Agriculture (USDA), Agricultural Research Service, Nutrient Data Laboratory. USDA National Nutrient Database for Standard Reference, Legacy. Version Current: April 2018. Internet:

Image Credits

  1. Fig. 2.1 - Types of carbohydrates diagram by Alice Callahan made with Microsoft SmartArt, CC BY-SA 4.0

  2. Chemical structures of glucose, fructose, galactose, amylose, amylopectin are public domain, accessed from Wikipedia

  1. Simple carbohydrate diagrams (with hexagons, pentagon) by Alice Callahan, CC BY-SA 4.0

  2. Fig. 2.2 - Flower with bee image by pontla,; Honey image by sunny mama,; Kiwi image by ereta ekarafi,, all CC BY-NC-ND 2.0

  3. Fig. 2.3 - Glycogen image by Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain.

  4. Fig. 2.4 - Wheat image by Bernat Caser,; Broccoli image by albedo20,; Apple - Fiona Shields,, all CC BY-NC-ND 2.0

  5. Chemical structure of cellulose by laghi.l, CC BY-SA 3.0



Carbohydrate Food Sources and Guidelines for Intake

Where do we find carbohydrates in foods?

Looking at the food groups represented in MyPlate below, which food groups do you think contain carbohydrates? If you answered, all of them, you're correct! This section will review which food groups contain the different types of carbohydrates. One of the goals of this course is to learn more about the different nutrients in foods and to understand the importance of eating a wide variety of foods from the different food groups.

The MyPlate image that illustrates the USDA food groups, and what proportions they should contribute; half the plate is fruit and vegetables and the other half grains and protein. There is also a side of dairy.

Figure 3.1 - Choose MyPlate graphic illustrating the USDA food groups: fruits, vegetables, grains, protein and dairy.

Fruits- Fruits are sweet, so we know they must contain sugar. Fruits contain sucrose, glucose, and fructose. This sugar is naturally-occurring and comes packaged with other great nutrients, like Vitamin C and potassium. Whole fruit also contains fiber, since fiber is found in all whole plant foods. Juice has little to no fiber, even high pulp orange juice.

Vegetables- Some vegetables are sweet and also contain sugar, although much less than fruit. Similar to fruits, some vegetables (like carrots and green beans) contain small amounts of sucrose, glucose, and fructose. Starchy vegetables (corn, peas, and potatoes, for example) primarily contain starch but some are also sweet and contain sucrose, glucose, and fructose (sweet potatoes and sweet corn, for example). Just like whole fruit, any whole vegetable also contains fiber.  

Dairy- This is the one animal food that contains carbohydrate. Milk, cheese, and yogurt contain naturally-occurring lactose. If dairy (like yogurt) is sweetened, then it will also contain added sugar like sucrose (white cane sugar) or fructose and glucose (honey and/or HFCS).

Grains- Grains naturally contain starch and fiber. Sprouted grains also contain maltose. If grains are sweetened (sugar is added), they might contain  sucrose (white cane sugar) or fructose and glucose (honey and/or HFCS).

Protein- Meats do not contain carbohydrate, but many plant foods that fall into the protein group, like beans and nuts, contain starch and fiber.  

Fats- Concentrated fats like butter and oil do not contain carbohydrate.

This information is summarized in the table below:


Table 3.1 USDA food groups with examples of foods and type of carbohydrate present within each food group.

Food Group

Example of Food

Type of Carbohydrate Present


Apple, orange, banana

Orange juice

Sucrose, glucose, fructose, and fiber

Sucrose, glucose, fructose


Non-starchy veggies

Starchy veggies (corn, potatoes, sweet potatoes, peas)

Sucrose, glucose, fructose, and fiber

Starch and fiber, with varying amount of sucrose, glucose, and fructose


Milk, plain yogurt, cheese



Rice, oatmeal, barley

Sprouted grains

Starch and fiber

Starch, fiber, and maltose



Beans and nuts


Starch and fiber


Oils, Butter



Looking at all the foods that contain carbohydrates, you might be able to guess why eliminating carbohydrates from the diet can lead to weight loss. It drastically reduces the variety of choices one has, leaving you primarily with low carbohydrate veggies and meats. Not surprisingly, people usually consume less calories with this way of eating.  However, for most people, this is not a sustainable or enjoyable way of eating, and it can also be hard to consume a nutritionally balanced diet with so many foods off-limits.

Carbohydrate Guidelines for Intake

Total Carbohydrate Intake

The recommended dietary allowance (RDA) for total carbohydrate intake is 130 grams. This is the minimum amount of glucose utilized by the brain, so if you consume less than this, you will probably go into ketosis. In order to meet the body's high energy demand for glucose, the acceptable macronutrient distribution range (AMDR) for an adult is 45%-65% of total calories. This is about 225 grams to 325 grams of carbohydrate per day if eating a 2,000 Calorie diet. (REMEMBER: 1 gram of carbohydrate contains 4 Calories.)

Fiber Intake

The Adequate Intake (AI) for fiber is 14 grams of fiber for every 1,000 Calories consumed. This is about 28 grams for an adult female (19-30 years old) and 38 grams for an adult male (19-30 years old). Most people in the United States only get half the amount of fiber they need in a day -- about 12 to 18 grams.

Added Sugar Intake

The 2015 dietary guidelines recommend that less than 10% of total Calories come from added sugars because of its link to obesity and chronic disease. This means that someone eating a 2,000 Calorie diet would want to limit their added sugar intake to about 12 teaspoons per day. To put that in perspective, a 12 oz can of soda has about 10 teaspoons of sugar. We will discuss added sugar in more detail later in the lesson.

Below is a chart summarizing the above recommendations.


Table 3.2 Dietary Recommendations for Carbohydrates


RDA for Total Carbohydrate

130 grams

AMDR for Total Carbohydrate

45% - 65% of total Calories

AI for Fiber

14 grams for every 1,000 Calories consumed

Dietary guidelines for added sugar

Less than 10% of total Calories


Self Check





  1. US Department of Health and Human Services and U.S. Department of Agriculture. 2015. Dietary Guidelines for Americans.

  2. Institute of Medicine, Food and Nutrition Board, 2005. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, DC; The National Academy of Sciences.

Image Credits

  1. MyPlate image from, public domain.

Digestion and Absorption of Carbohydrates

A woman smiles after taking a bite of a piece of pizza topped with cheese and basil leaves.

Imagine taking a bite of pizza. It tastes amazing, but it's also full of fuel for your body, much of it in the form of carbohydrates.

What types of carbohydrates would you find in that bite?

In order to use these food carbohydrates in your body, you first need to digest them. Last week, we explored the gastrointestinal system and the basic process of digestion. Now that you know about the different types of carbohydrates, we'll take a closer look at how these molecules are digested as they travel through the GI system.

In the image below, follow the numbers to see what happens to carbohydrates at each site of digestion.

Illustration of the digestive system, with major sites of digestion numbered: oral cavity (1); stomach (2); small intestine (3); and large intestine or colon (4).

Fig. 4.1. The digestive system

1 - Mouth or Oral Cavity

As you chew your bite of pizza, you're using mechanical digestion to begin to break it into smaller pieces and mix it with saliva, produced by several salivary glands in the oral cavity.

Some enzymatic digestion of starch occurs in the mouth, due to the action of the enzyme salivary amylase. This enzyme starts to break the long glucose chains of starch into shorter chains, some as small as maltose. (The other carbohydrates in the bread don't undergo any enzymatic digestion in the mouth.)

Illustration showing that the enzyme salivary amylase breaks starch into smaller polysaccharides and maltose. The image shows a long chain of starch (shown as green hexagons) that is then broken into shorter lengths, including maltose, by salivary amylase.

Fig. 4.2. The enzyme salivary amylase breaks starch into smaller polysaccharides and maltose.

2 - Stomach

The low pH in the stomach inactivates salivary amylase, so it no longer works once it arrives at the stomach. Although there's more mechanical digestion in the stomach, there's little chemical digestion of carbohydrates here.

3 - Small intestine

Most carbohydrate digestion occurs in the small intestine, thanks to a suite of enzymes. Pancreatic amylase is secreted from the pancreas into the small intestine, and like salivary amylase, it breaks starch down to small oligosaccharides (containing 3 to 10 glucose molecules) and maltose.

Illustration showing that the enzyme pancreatic amylase breaks starch into smaller polysaccharides and maltose. The image shows a long chain of starch (shown as green hexagons) that is then broken into shorter lengths, including maltose, by pancreatic amylase.

Fig. 4.3. The enzyme pancreatic amylase breaks starch into smaller polysaccharides and maltose.

The rest of the work of carbohydrate digestion is done by enzymes produced by the enterocytes, the cells lining the small intestine. When it comes to digesting your slice of pizza, these enzymes will break down the maltase formed in the process of starch digestion, the lactose from the cheese, and the sucrose present in the sauce.

Illustration showing maltose (represented by two green hexagons linked together) being broken into two glucose molecules by the enzyme maltase.

Illustration showing lactose (represented by a green hexagon linked to a blue hexagon) being broken into one glucose molecule and one galactose molecule by the enzyme lactase.

Illustration showing sucrose (represented by a green hexagon linked to a purple pentagon) being broken into one glucose molecule and one fructose molecule by the enzyme sucrase.

Fig. 4.4. Action of the enzymes maltase, lactase, and sucrase.

(Recall that if a person is lactose intolerant, they don't make enough lactase enzyme to digest lactose adequately. Therefore, lactose passes to the large intestine. There it draws water in by osmosis and is fermented by bacteria, causing symptoms such as flatulence, bloating, and diarrhea.)

By the end of this process of enzymatic digestion, we're left with three monosaccharides: glucose, fructose, and galactose. These can now be absorbed across the enterocytes of the small intestine and into the bloodstream to be transported to the liver.

Digestion and absorption of carbohydrates in the small intestine are depicted in a very simplified schematic below. (Remember that the inner wall of the small intestine is actually composed of large circular folds, lined with many villi, the surface of which are made up of microvilli. All of this gives the small intestine a huge surface area for absorption.)

Cartoon illustration showing major processes involved in digestion and absorption of carbohydrates in the small intestine. The figure shows starch and polysaccharides being digested down to maltose by pancreatic amylase; maltose digested to two glucose molecules; sucrose digested to one glucose and one fructose; and lactose digested to one glucose and one galactose. Monosaccharides are then absorbed into the bloodstream and travel to the liver.

Fig. 4.5. Digestion and absorption of carbohydrates in the small intestine.

Fructose and galactose are converted to glucose in the liver. Once absorbed carbohydrates pass through the liver, glucose is the main form of carbohydrate circulating in the bloodstream.

4 - Large Intestine or Colon

Any carbohydrates that weren't digested in the small intestine -- mainly fiber -- pass into the large intestine, but there's no enzymatic digestion of these carbohydrates here. Instead, bacteria living in the large intestine, sometimes called our gut microbiota, ferment these carbohydrates to feed themselves. Fermentation causes gas production, and that's why we may experience bloating and flatulence after a particularly fibrous meal. Fermentation also produces short-chain fatty acids, which our large intestine cells can use as an energy source. Over the last decade or so, more and more research has shown that our gut microbiota are incredibly important to our health, playing important roles in the function of our immune response, nutrition, and risk of disease. A diet high in whole food sources of fiber helps to maintain a population of healthy gut microbes.

Summary of Carbohydrate Digestion:

The primary goal of carbohydrate digestion is to break polysaccharides and disaccharides into monosaccharides, which can be absorbed into the bloodstream.   

1. After eating, nothing needs to happen in the digestive tract to the monosaccharides in a food like grapes, because they are already small enough to be absorbed as is.  

2. Disaccharides in that grape or in a food like milk are broken down (enzymatically digested) in the digestive tract to monosaccharides (glucose, galactose, and fructose).   

3. Starch in food is broken down (enzymatically digested) in the digestive tract to glucose molecules.  

4. Fiber in food is not enzymatically digested in the digestive tract, because humans don't have enzymes to do this. However, some dietary fiber is fermented in the large intestine by gut microbes.

Table 4.1. Summary of Enzymatic Digestion of Carbohydrate

Macronutrients in Food

Is this Macronutrient Enzymatically Digested?

(enzyme name)

What Is Absorbed Into the Villi After Digestion?







Fructose. It is then transported to the liver where it is converted to glucose.



Galactose. It is then transported to the liver where it is converted to glucose.



Yes (maltase)



Yes (sucrase)

Glucose, Fructose


Yes (lactase)

Glucose, Galactose




(amylase, maltase)



No (Humans don't have the digestive enzymes to break down fiber, but some is fermented by gut microbes in the large intestine.)


This video reviews the process of carbohydrate digestion:

This video will help you identify carbohydrates in foods, what carbohydrates need to be enzymatically digested, and what is absorbed:


Self Check







  1. Klein, S., Cohn, S.M., Alpers, D.H., The Alimentary Tract in Nutrition, In: Modern Nutrition in Health and Disease, 9th Ed., Baltimore, MD, Lippincott Williams and Wilkins, 1999

  2. Harvard T. H. Chan School of Public Health, "The Microbiome,", accessed September 10, 2018

Image Credits

  1. Eating pizza photo by @mark-c from, CC0

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

  1. Carbohydrate digestion schematics by Alice Callahan, CC BY-NC-SA 4.0

  2. Carbohydrate absorption image Alice Callahan, CC BY-NC-SA 4.0

  3. Carbohydrate and digestion summary chart by Tamberly Powell, CC BY-NC-SA 4.0



Glucose Regulation and Utilization in the Body

On the last page, we traced the process of digesting the carbohydrates in a slice of pizza through the gastrointestinal tract, ending up with the absorption of monosaccharides across the cells of the small intestine and into the bloodstream. From there, they travel to the liver, where fructose and galactose are converted to glucose.

After any meal containing carbohydrates, you experience a rise in blood glucose that can serve as fuel for cells around the body. But during the periods between meals, including while you're sleeping and exercising, your body needs fuel, too. To ensure that you have enough glucose in your blood at any given time, your body has a finely-tuned system to regulate your blood glucose concentration. This system allows you to store glucose when you have excess available (when your blood glucose is high) and to pull glucose out from your stores when needed (when your blood supply gets low).

Your body's ability to maintain equilibrium or a steady state in your blood glucose concentration is called homeostasis. It's a critical part of normal physiology, because if your blood glucose gets too low (called hypoglycemia), cellular function starts to fail, especially in the brain. If blood glucose gets too high (called hyperglycemia), it can cause damage to cells.

Hormones Involved in Blood Glucose Regulation

Central to maintaining blood glucose homeostasis are two hormones, insulin and glucagon, both produced by the pancreas and released into the bloodstream in response to changes in blood glucose.


The image below depicts a mouse islet of Langerhans, a cluster of endocrine cells in the pancreas. The beta-cells of the islet produce insulin, and the alpha-cells produce glucagon.

A mouse pancreatic islet is seen as a cluster of cells, sitting next to a blood vessel. Cell nuclei both inside and outside the islet are stained blue so are seen as circular shapes throughout the pancreatic section. Insulin is stained red so makes the entire islet appear red. Blood vessels are stained green.

Fig. 5.1. A mouse islet of Langerhans, visualized with immunofluorescent microscopy. In this image, cell nuclei are stained blue, insulin is stained red, and blood vessels are stained green. You can see that this islet is packed with insulin and sits right next to a blood vessel, so that it can secrete the two hormones, insulin and glucagon, into the blood. Glucagon is not stained in this image, but it's there!

In the figure below, you can see blood glucose and insulin throughout a 24-hour period, including three meals. You can see that when glucose rises, it is followed immediately by a rise in insulin, and glucose soon drops again. The figure also shows the difference between consuming a sucrose-rich food and a starch-rich food. The sucrose-rich food results in a greater spike in both glucose and insulin. Because more insulin is required to handle that spike, it also causes a more precipitous decline in blood glucose. This is why eating a lot of sugar all at once may increase energy in the short-term, but soon after may make you feel like taking a nap!

The figure shows a line graph, with time over 24 hours on the x-axis and with blood glucose concentrations on the left y-axis and blood insulin concentrations on the right y-axis. The graph shows 3 peaks during the day for meals, with insulin level closely matched to glucose level. The effects of a sugar-rich meal shows a higher glucose and insulin peak and a more precipitous decline in glucose in response.

Fig. 5.2. Typical pattern of blood glucose and insulin during a 24-hour period, showing peaks for each of 3 meals and highlighting the effects of consumption of sugar-rich foods.

Let's look a little closer at how insulin works, illustrated in the figure below. Insulin is released by the pancreas into the bloodstream. Cells around the body have receptors for insulin on their cell membranes. Insulin fits into its receptors (labeled as step 1 in the figure), kind of like a key in a lock, and through a series of reactions (step 2), triggers glucose transporters to open on the surface of the cell (step 3). Now glucose can enter the cell, making it available for the cell to use and at the same time lowering the concentration of glucose in the blood.

Schematic showing insulin binding to its receptors on the cell membrane, triggering GLUT-4 glucose transporters to open on the membrane. This allows glucose to enter the cell, where it can be used in several ways.

Fig. 5.3. Insulin binds to its receptors on the cell membrane, triggering GLUT-4 glucose transporters to open on the membrane. This allows glucose to enter the cell, where it can be used in several ways.

The figure also shows several different ways glucose can be used once it enters the cell.

In addition to its role in glucose uptake into cells, insulin also stimulates glycogen and fat synthesis as described above. It also stimulates protein synthesis. You can think of its role as signaling to the body that there's lots of energy around, and it's time to use it and store it in other forms.


On the other hand, when blood glucose falls, several things happen to restore homeostasis.

  1. You receive messages from your brain and nervous system that you should eat. If that doesn't work, or doesn't work fast enough….

  2. Glucagon is released from the pancreas into the bloodstream. In liver cells, it stimulates the breakdown of glycogen, releasing glucose into the blood.

  3. In addition, glucagon stimulates a process called gluconeogenesis, in which new glucose is made from amino acids (building blocks of protein) in the liver and kidneys, also contributing to raising blood glucose.

How Glucose Provides Energy

Now let's zoom in on how exactly glucose provides energy to the cell. We can trace this process in the figure below.

Schematic showing an overview of glucose metabolism in the fed state, when there is adequate glucose available. Glucose can be used to generate ATP for energy (going through glycolysis, Krebs cycle, and the electron transport chain), or it can be stored in the form of glycogen or converted to fat for storage in adipose tissue.

Fig. 5.4. Overview of glucose metabolism in the fed state, when there is adequate glucose available. Glucose can be used to generate ATP for energy, or it can be stored in the form of glycogen or converted to fat for storage in adipose tissue.

  1. Glucose, a 6-carbon molecule, is broken down to two 3-carbon molecules called pyruvate through a process called glycolysis.

  2. Pyruvate enters a mitochondrion of the cell, where it is converted to a molecule called acetyl CoA.

  3. Acetyl CoA goes through a series of reactions called the Krebs cycle. This cycle requires oxygen and produces carbon dioxide. It also produces several important high energy electron carriers called NADH2 and FADH2.

  4. These high energy electron carriers go through the electron transport chain to produce ATP -- energy for the cell!

  5. Note that the figure also shows that glucose can be used to synthesize glycogen or fat, if the cell already has enough energy.

What Happens When There Isn't Enough Glucose?

We've already talked about what happens when blood glucose falls: glucagon is released, and that stimulates the breakdown of glycogen as well as the process of gluconeogenesis from amino acids. These are important mechanisms for maintaining blood glucose levels to fuel the brain when carbohydrate is limited. Hypoglycemia (low blood glucose) can cause you to feel confused, shaky, and irritable, because your brain doesn't have enough glucose. If it persists, it can cause seizures and eventually coma, so it's good we have these normal mechanisms to maintain blood glucose homeostasis!

What happens if your carbohydrate supply is limited for a long time? This might happen if a person is starving or consuming a very low carbohydrate diet. In this case, your glycogen supplies will become depleted. How will you get enough glucose (especially for the brain) and energy? You'll have to use the other two macronutrients in the following ways:

  1. Protein -- You'll continue to use some amino acids to make glucose through gluconeogenesis and others as a source of energy through acetyl CoA. However, if a person is starving, they also won't have extra dietary protein. Therefore, they start breaking down body proteins, which will cause muscle wasting.

  1. Fat -- You can break down fat as a source of energy, but you can't use it to make glucose. Fatty acids can be broken down to acetyl CoA in the liver, but acetyl CoA can't be converted to pyruvate and go through gluconeogenesis. It can go through the Krebs cycle to produce ATP, but if carbohydrate is limited, the Krebs cycle gets overwhelmed. In this case, acetyl CoA is converted to compounds called ketones or ketone bodies. These can then be exported to other cells in the body, especially brain and muscle cells.

These pathways are shown in the figure below:

Schematic showing that during starvation or when consuming a low-carbohydrate diet, protein (amino acids) can be used to make glucose by gluconeogenesis, and fats can be used to make ketones in the liver. The brain can adapt to using ketones as an energy source in order to conserve protein and prevent muscle wasting.

Fig. 5.5. During starvation or when consuming a low-carbohydrate diet, protein (amino acids) can be used to make glucose by gluconeogenesis, and fats can be used to make ketones in the liver. The brain can adapt to using ketones as an energy source in order to conserve protein and prevent muscle wasting.

Ketone production is important, because ketones can be used by tissues of the body as a source of energy during starvation or a low carbohydrate diet. Even the brain can adapt to using ketones as a source of fuel after about three days of starvation or very low carbohydrate diet. This also helps to preserve the protein in the muscle.

Ketones can be excreted in urine, but if ketone production is very high, they begin to accumulate in the blood, a condition called ketosis. Symptoms of ketosis include sweet-smelling breath, dry mouth, and reduced appetite. People consuming a very low carbohydrate diet may be in ketosis, and in fact, this is a goal of the currently popular ketogenic diet. (Ketones are acidic, so severe ketosis can cause the blood to become too acidic, a condition called ketoacidosis. This mainly happens with uncontrolled diabetes.)

Is following a ketogenic diet an effective way to lose weight? It can be, but the same can be said of any diet that severely restricts the types of foods that you're allowed to eat. Following a ketogenic diet means eating a high fat diet with very little carbohydrate and moderate protein. This means eating lots of meat, fish, eggs, cheese, butter, oils, and low carbohydrate vegetables, and eliminating grain products, beans, and even fruit. With so many fewer choices, you're likely to spend more time planning meals and less time mindlessly snacking. Being in ketosis also seems to reduce appetite, and it causes you to lose a lot of water weight initially. However, studies show that being in ketosis doesn't seem to increase fat-burning or metabolic rate. There are also concerns that the high levels of saturated fat in most ketogenic diets could increase risk of heart disease in the long term.  Finally, it's a very difficult diet to maintain for most people, and reverting back to your previous dietary patterns usually means the weight will come back. The ketogenic diet is also very similar to the Atkins diet that was all the rage in the 1990's, and we tend to be skeptical of such fad diets, preferring to focus instead on balance, moderation, and enjoyment of a wide variety of foods.

The following video reviews the concepts just covered: Glucose Regulation and Utilization in the Body


Diabetes is a chronic disease in which your normal system of regulating blood glucose doesn't work. There are three main types of diabetes: type 1, type 2, and gestational diabetes.

Type 1 Diabetes:

This is an autoimmune disease in which the beta-cells of the pancreas are destroyed by your own immune system. Without the beta-cells, you can't make enough insulin, so in type 1 diabetes, you simply don't have enough insulin to regulate your blood glucose levels. Remember how we said insulin is like the key that lets glucose into the body's cells? In type 1 diabetes, you're missing the key, so glucose stays in the blood and can't get into cells.

Schematic showing that in type 1 diabetes, the pancreas does not make enough insulin, so glucose transporters (GLUT-4) do not open on the cell membrane, and glucose is stuck outside the cell.

Fig. 5.6. In type 1 diabetes, the pancreas does not make enough insulin, so glucose transporters (GLUT-4) do not open on the cell membrane, and glucose is stuck outside the cell.

Common symptoms include weight loss and fatigue, because the body's cells are starved of glucose. Excess glucose from the blood is also excreted in the urine, increasing urination and thirst.

Once diagnosed, type 1 diabetics have to take insulin in order to regulate their blood glucose. Traditionally, this has required insulin injections timed with meals. New devices like continuous glucose monitors and automatic insulin pumps can track glucose levels and provide the right amount of insulin, making managing type 1 diabetes a little easier. Figuring out the right amount of insulin is important, because chronically elevated blood glucose levels can cause damage to tissues around the body. However, too much insulin will cause hypoglycemia, which can be very dangerous.

Type 1 diabetes is most commonly diagnosed in childhood, but it has been known to develop at any age. It's much less common than type 2 diabetes, accounting for 5-10% of cases of diabetes.

The following video, from the charity Diabetes UK, provides a nice review of type 1 diabetes:

Type 2 Diabetes:

Development of type 2 diabetes begins with a condition called insulin resistance. At least initially, the pancreas is producing enough insulin, but the body's cells don't respond appropriately. It's as if you still have the insulin key but can't find the keyhole to unlock the doors and let the glucose in.

Schematic showing that in type 2 diabetes, the cell does not respond appropriately to insulin, so glucose is stuck outside the cell.

Fig. 5.7. In type 2 diabetes, the cell does not respond appropriately to insulin, so glucose is stuck outside the cell.

The result is the same: high blood glucose. At this point, you may be diagnosed with a condition called prediabetes. The pancreas tries to compensate by making more insulin, but over time, it becomes exhausted and eventually produces less insulin, leading to full-blown type 2 diabetes. According to the CDC, 100 million Americans are living with diabetes (30.3 million) or prediabetes (84.1 million).

Although people of all shapes and sizes can get Type 2 diabetes, it is strongly associated with abdominal obesity. In the past, it was mainly diagnosed in older adults, but it is becoming more and more common in children and adolescents as well, as obesity has increased in all age groups. In the maps below, you can see that as obesity has increased in states around the country, so has diabetes.   

The figure shows maps of the U.S. from 1994, 2000, and 2015, showing that the prevalence of both obesity and type 2 diabetes have increased over time. In general, states with greater obesity also have greater incidence of type 2 diabetes.

Fig. 5.8. Data from the CDC show the increasingly prevalence of both obesity and type 2 diabetes between 1994 and 2015.

The complications of type 2 diabetes result from long-term exposure to high blood glucose, or hyperglycemia. This causes damage to the heart, blood vessels, kidneys, eyes, and nerves, increasing the risk of heart disease and stroke, kidney failure, blindness, and nerve dysfunction. People with uncontrolled Type 2 diabetes can also end up with foot infections and ulcers because of impaired nerve function and wound healing. If left untreated, this results in amputation.

This video, also from Diabetes UK, reviews the causes, complications, and treatments for type 2 diabetes:

The link between obesity and type 2 diabetes is explored in this video, part of the Weight of the Nation series by HBO:

Gestational diabetes:

Gestational diabetes is diabetes that develops during pregnancy in women that did not previously have diabetes. It affects approximately 1 to 2 percent of pregnancies in the U.S. It can cause pregnancy complications, mostly associated with excess fetal growth because of high blood glucose. Although it usually goes away once the baby is born, women who have gestational diabetes are more likely to develop type 2 diabetes later in life, so it is a warning sign for them.

This video from Kahn Academy does a nice job of explaining the causes of the different types of diabetes:

Diabetes Management:

All of the following have been shown to help manage diabetes and reduce complications. Diabetes management, as well as prevention (particularly if you've been diagnosed with prediabetes), starts with lifestyle choices.

Self Check


  1. Salway, J.G., Metabolism at a Glance, Blackwell Publishing, 2004.

  2. Smolin, L. and Grosvenor, M. 2016. Nutrition Science and Applications. John Wiley and Sons.

  3. Gibson, A.A. et al. 2015. Do ketogenic diets really suppress appetite? A systematic review and meta-analysis, Obesity Reviews, 16(1):64-76,

  4. Hall, K.D. et al. 2016. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Am. J. Clin. Nutr. 104(2):324-33.

  5. Abassi, J. 2018. Interest in Ketogenic Diet Grows for Weight Loss and Type 2 Diabetes, JAMA 319(3):215-217.

  6. Belluz, J. 2018. The Keto Diet, Explained.,

  7. CDC, Diabetes Basics,

Image Credits

  1. Fig. 4.1. Mouse islet image by Jakob Suckale, Wikipedia, CC BY-SA 3.0

  2. Fig. 4.2. Glucose/insulin patterns in 24-hours figure by Jakob Suckale and Michele Solimena, CC BY 3.0, https///

  3. Fig. 4.3. Insulin action figure by Meiquer, Wikipedia, CC BY-SA 3.0, with additions by Alice Callahan, CC BY-NC-SA 4.0

  4. Fig. 4.4. Glucose metabolism figure by Alice Callahan, CC BY-NC-SA 4.0, with ATP star by Anastasia Latysheva from the Noun Project

  5. Fig. 4.5. Gluconeogenesis and ketogenesis figure by Alice Callahan, CC BY-NC-SA 4.0, with brain by monstara and liver by maritacovarrubias, both from Open Clip Art, public domain

  6. Figs. 4.6 and 4.7 by Brian Lindshield, from "Kansas State University Human Nutrition (FNDH 400) Flexbook" (2018). NPP eBooks. 19.

  7. Fig. 4.8 from CDC, public domain,



Fiber - Types, Food Sources, Health Benefits, and Whole vs. Refined Grains

Dietary fiber is defined by the Institute of Medicine's Food and Nutrition Board as "nondigestible carbohydrates and lignin that are intrinsic and intact in plants." Fiber plays an important role in giving plants structure and protection, and it also plays an important role in the human diet.

Cellulose is one type of fiber. The chemical structure of cellulose is shown in the figure below, with our simplified depiction next to it. You can see that cellulose has long chains of glucose, similar to starch, but they're stacked up, and there are hydrogen bonds linking the stacks. The special bonds between these glucose units in fiber are not enzymatically digested in the digestive tract, and therefore, fiber passes undigested to the colon or large intestine.

image shows chemical structure of cellulose, with a total of 16 glucose units arranged in rows of 4 each, with hydrogen bonds linking them vertically, as in a grid.              image is a schematic showing multiple green hexagons, each representing a glucose molecule, arranged in rows with lines linking them both vertically and horizontally, as in a grid

Figure 6.1 - The chemical structure of cellulose, and a simplified illustration of cellulose.

You might be wondering how fiber has any benefit to us if we can't digest it. However, it doesn't just pass through the digestive tract as a waste product. Instead, it serves many functions on its journey, and these contribute to our health. Let's explore the different types of fiber, where we find them in foods, and what benefits they provide!

Types of Fiber

Whole plant foods contain many different types of molecules that fit within the definition of fiber. One of the ways that types of fiber are classified is by their solubility in water. Whole plant foods contain a mix of both soluble and insoluble fiber, but some are better sources of one than the other.

  1. Soluble Fiber - These fibers dissolve in water, forming a viscous gel in the GI tract, which helps to slow digestion and the absorption of glucose. This means that including soluble fiber in a meal helps to prevent sharp blood sugar spikes, instead making for a more gradual rise in blood glucose. Consuming a diet high in soluble fiber can also help to lower blood cholesterol levels, because soluble fiber binds cholesterol and bile acids (which contain cholesterol) in the GI tract.  Soluble fiber is also highly fermentable, so it is easily digested by bacteria in the large intestine. Pectins and gums are common types of soluble fibers, and good food sources include oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables. (Psyllium fiber supplements like Metamucil are composed mainly of soluble fiber, so if you've ever stirred a spoonful of this into a glass of water, you've seen the viscous consistency characteristic of soluble fiber.)

  1. Insoluble Fiber - These fibers typically do not dissolve in water and are nonviscous. Some are fermentable by bacteria in the large intestine but to a lesser degree than soluble fibers. Insoluble fibers help prevent constipation, as they create a softer, bulkier stool that is easier to eliminate. Lignin, cellulose, and hemicellulose are common types of insoluble fibers, and food sources include wheat bran, vegetables, fruits, and whole grains.

Food Sources

Since fiber provides structure to plants, fiber can be found in all whole plant foods, including whole grains (like oatmeal, barley, rice and wheat), beans, nuts, seeds, and whole fruits and vegetables.

An example of a fiber packed meal. A bowl of oatmeal topped with blueberries and sunflower seeds.

Figure 6.2 - A bowl of oatmeal topped with blueberries and sunflower seeds.

This meal is packed with fiber from the oatmeal, blueberries, and sunflower seeds.

When foods are refined, parts of the plant are removed, and during this process, fiber and other nutrients are lost. For example, fiber is lost when going from a whole fresh orange to orange juice. A whole orange contains about 3 grams of fiber, whereas a glass of orange juice has little to no fiber. Fiber is also lost when grains are refined. We will discuss this more a little later.

Take a look at the list of foods below to see the variety of foods which provide dietary fiber.

Table 6.1 Common foods listed with standard portion size, and calories and fiber in a standard portion.


Standard Portion Size

Calories in Standard Portion

Dietary Fiber in Standard Portion (g)

Shredded wheat ready-to-eat cereal (various)

1-1 ¼cup



Wheat bran flakes ready-to-eat cereal (various)

¾ cup



Lentils, cooked

½ cup



Black beans, cooked

½ cup



Refried beans, canned

½ cup




½ cup



Pear, raw

1 medium



Pear, dried

¼ cup



Apple, with skin

1 medium




½ cup



Mixed vegetables, cooked from frozen

½ cup



Potato, baked, with skin

1 medium



Pumpkin seeds, whole, roasted

1 ounce



Chia seeds, dried

1 Tbsp



Sunflower seed kernels, dry roasted

1 ounce




1 ounce



Plain rye wafer crackers

2 wafers



Bulgur, cooked

½ cup



Popcorn, air-popped

3 cups



Whole wheat spaghetti, cooked

½ cup



Quinoa, cooked

½ cup



Although you can get fiber from supplements, whole foods are are a better source, because the fiber comes packaged with other essential nutrients and phytonutrients.

Health Benefits of Dietary Fiber

A high-fiber diet has many benefits, which include:

Whole vs. Refined Grains

Figure 6.3 - Wheat growing in a field.


Before they are harvested, all grains  are whole grains. They contain the entire seed (or kernel) of the plant. This seed is made up of three edible parts: the bran, the germ, and the endosperm. The seed is also covered by an inedible husk that protects the seed.

The anatomy of a wheat kernel is illustrated showing that the largest part of the wheat kernel is the endosperm located in the middle of the seed. The germ is a smaller part at the bottom of the seed, and the bran is the outer covering.

Figure 6.4 - The anatomy of a wheat kernel which includes the bran, endosperm, and germ.

  1. The bran is the outer skin of the seed. It contains antioxidants, B vitamins and fiber.

  2. The endosperm is by far the largest part of the seed and provides energy in the form of starch to support reproduction. It also contains protein and small amounts of vitamins and minerals.

  3. The germ is the embryo of the seed -- the part that can sprout into a new plant. It contains B vitamins, protein, minerals like zinc and magnesium, and healthy fats.

The Dietary Guidelines for Americans define whole grains and refined grains in the following way:

"Whole Grains—Grains and grain products made from the entire grain seed, usually called the kernel, which consists of the bran, germ, and endosperm. If the kernel has been cracked, crushed, or flaked, it must retain the same relative proportions of bran, germ, and endosperm as the original grain in order to be called whole grain. Many, but not all, whole grains are also sources of dietary fiber."

Whole grains include foods like barley, corn (whole cornmeal and popcorn), oats (including oatmeal), rye, and wheat. (For a more complete list of whole grains, check out the Whole Grain Council.)

"Refined Grains—Grains and grain products with the bran and germ removed; any grain product that is not a whole-grain product. Many refined grains are low in fiber but enriched with thiamin, riboflavin, niacin, and iron, and fortified with folic acid."

Refined grains include foods like white rice and white flour. According to the Whole Grain Council, "Refining a grain removes about a quarter of the protein in a grain, and half to two thirds or more of a score of nutrients, leaving the grain a mere shadow of its original self."

Refined grains are often enriched with vitamins and minerals, meaning that some of the nutrients lost during the refining process are added back in after processing. However, many vitamins and minerals are not added back, and neither are the fiber, protein, and healthy fats found in whole grains. In the chart below you can see the differences in essential nutrients between whole wheat flour, refined wheat flour, and enriched wheat flour.

This figure is illustrating the nutrient content of refined wheat and enriched wheat as compared to whole wheat flour. Refined wheat flour has 8% of vitamin E, 11% of vitamin B6, 16% of magnesium, 24% of thiamin, 24% of riboflavin, 25% of niacin, 25% of fiber, 29% of potassium, 33% of iron, 59% of folate, and 78% of protein as compared to whole wheat flour which has 100% of all these nutrients. Enriched wheat flour has 156% thiamin, 299% riboflavin, 119% niacin, 129% iron, and 661% folate compared to whole wheat flour which has 100% of all these nutrients.

Figure 6.5 - The nutrient content of refined wheat and enriched wheat as compared to whole wheat flour.

Because whole grains offer greater nutrient density, MyPlate and the Dietary Guidelines recommend that at least half of our grains are whole grains. Yet current data show that while most Americans are eating enough grains overall, they're eating too many refined grains and not enough whole grains, as shown in this graphic from the Dietary Guidelines:

This figure shows that average intakes of whole grains are far below recommended levels across all age-sex groups, and average intakes of refined grains are well above recommended limits for most age-sex groups.

Figure 6.6 - Average Whole & Refined Grain Intakes in Ounce-Equivalents per Day by Age-Sex Groups, Compared to Ranges of Recommended Daily Intake for Whole Grains & Limits for Refined Grains.


Looking for whole grain products at the grocery store can be tricky, since the front-of-package labeling is about marketing and selling products. Words like "made with whole grain" and "multigrain" on the front of the package make it appear like a product is whole grain, when in fact there may be very few whole grains present.

The color of a bread can be deceiving too. Refined grain products can have added caramel color to make them appear more like whole grains.

To determine if a product is a good source of whole grain, the best place to look is the ingredient list on the Nutrition Facts panel. The ingredients should list a whole grain as the first ingredient (ex.100%  whole wheat), and it should not be followed by a bunch of refined grains (like enriched wheat flour).

Getting familiar with the name of whole grains will help you identify them. Common varieties include wheat, barley, brown rice, buckwheat, corn, rye, oats, and wild rice. Less known varieties are: teff, amaranth, millet, quinoa, black rice, black barley, and spelt.

Most, but not all, whole grains are a good source of fiber, and that is one of the benefits of choosing whole grains. Keep in mind that some products add extra fiber as a separate ingredient, like wheat bran, inulin, or cellulose. These boost the grams of fiber on the Nutrition Facts label and may make the product a good source of fiber, but it doesn't mean it's a good source of whole grains. In fact, it may be a product made mostly of refined grains, so it would still be missing the other nutrients that come packaged in whole grains and may not have the same health benefits.  Therefore, just looking at fiber on the Nutrition Facts label is not a good indicator of whether or not the product is made with whole grains.

Also, some products that are 100% whole wheat but do not appear to be a good source of fiber, because the serving size is small. The bread label below is an example of this. The first ingredient is "stone ground whole wheat flour" with no refined flours listed, but it still has only 2g of fiber and 9% DV. But of course, that still contributes to your fiber intake for the day, and if you made yourself a sandwich with two slices of bread, that would provide 18% of the DV.

This is an example of a product that is 100% whole wheat, but does not appear to be a good source of fiber, because the serving size is small. The nutrition facts lists 2g of fiber and 9%DV for fiber (for 1 slice or 26g), therefore this whole wheat bread is not a good source of fiber.