Chapter 10: Carbohydrates

An Introduction to Carbohydrates

Throughout history, carbohydrates have been staples in human diets. Wheat, corn, potatoes, rice, taro, and breadfruit are common foods filled with starches; the sweetness of sugars in fruit, honey, and milk encourage humans to find and eat them.

Digestible carbohydrates such as starch and sugars provide quick energy and provide energy to all cells. Most cells can use other molecules for energy if glucose is not available. Muscle cells, for example, can use fatty acids if glucose is scarce. Some cells, such as red blood cells, require glucose in order to function. The cells of the brain prefer to use glucose if it is available but can use alternate fuels in an emergency.

Carbohydrates also include indigestible types that human enzymes cannot break down. Although it seems unusual to eat something that we cannot digest, these carbohydrates, which include fiber, provide several health benefits.

Plants synthesize glucose from water and carbon dioxide in the air using the sun’s energy, capturing the radiant energy of sunlight as chemical energy. Plants use glucose as building blocks of sugars and the complex carbohydrates starch and fiber. The fiber cellulose is one of the most common molecules on earth. When we eat plants, we harvest the energy of glucose to support life’s processes.

Carbohydrates are a group of organic compounds containing the ratio of one carbon atom to two hydrogen atoms to one oxygen atom. The word “carbo” means carbon and “hydrate” means water. Glucose, the most abundant carbohydrate in the human body, has six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. The chemical formula for glucose is C₆H₁₂O₆. The simplest unit of a carbohydrate is a monosaccharide. “Saccharide” comes from the Latin word for sugar.

Carbohydrates are classified into two subgroups, simple (“fast-releasing”) and complex (“slow-releasing”). Simple carbohydrates include monosaccharides and disaccharides. Complex carbohydrates are polysaccharides, long chains of monosaccharides.

Simple/Fast-Releasing Carbohydrates

The simple carbohydrates, sugars, include monosaccharides and disaccharides. Important dietary monosaccharides include glucose, fructose, and galactose, and the disaccharides include lactose, maltose, and sucrose.

Simple carbohydrates stimulate the sweet taste sensation, which is the most sensitive of all taste sensations. Humans can taste even extremely low concentrations of sugar in food. Sugars vary in sweetness; fructose is the sweetest.

Monosaccharides

Glucose is the preferred fuel source for many organisms and most animal cells. The brain depends on glucose as its energy source except during starvation conditions. The monosaccharide galactose also has the chemical formula C₆H₁₂O₆ and differs from glucose only in a hydroxyl (−OH) group facing a different direction on the number four carbon (Figure 10.2). This structural alteration causes galactose to be metabolized differently from glucose; the liver rapidly converts it to glucose. Galactose is one component of the milk sugar lactose but is not found in many other foods.

Fructose also has the same chemical formula as glucose but differs in its chemical structure, as the ring structure contains only five carbons and not six. Fructose, in contrast to glucose, is not an energy source for cells in the body. Mostly found in fruits, honey, and sugarcane, fructose is one of the most common monosaccharides in nature. It is an important component of high fructose corn syrup, a sweetener used in many popular foods, including soft drinks, cereals, and desserts.

The dietary sugars in Figure 10.2 have six carbons and are called hexoses. Pentoses, sugars with five carbons, are less common in foods but are abundant in the nucleic acids RNA and DNA, and also as components of fiber.

Lastly, there are the sugar alcohols, which are industrially synthesized derivatives of monosaccharides. Some examples of sugar alcohols are sorbitol, xylitol, and glycerol. Xylitol is similar in sweetness as table sugar. Sugar alcohols are used in food manufacturing to sweeten foods as they are incompletely digested and absorbed, and therefore less caloric. The bacteria in your mouth do not digest them and produce tooth-damaging acids, so sugar alcohols do not cause tooth decay. Interestingly, the sensation of “coolness” that occurs when chewing gum that contains sugar alcohols comes from them dissolving in the mouth, a chemical reaction that requires heat from the inside of the mouth.

Disaccharides

Disaccharides are composed of pairs of monosaccharides linked together. Disaccharides include sucrose, lactose, and maltose (see Figure 10.3). All of the disaccharides contain at least one glucose molecule.

Sucrose, which contains a glucose linked to a fructose, is known as table sugar. Sucrose is also found in many fruits and vegetables, and at high concentrations in sugar beets and sugarcane, which are used to make table sugar. Lactose, milk sugar, is composed of one glucose and one galactose. Lactose is prevalent in dairy products such as milk, yogurt, and cheese. Maltose consists of two glucose molecules bonded together. It is a common breakdown product of plant starches and is rarely found in foods as a disaccharide.

Complex/Slow-Releasing Carbohydrates

Complex carbohydrates are polysaccharides, long chains of monosaccharides that may be branched or un-branched. There are two main groups of polysaccharides: starches and fibers.

Starches

Starch molecules are abundant in grains, legumes, and root vegetables, such as potatoes. Amylose, a plant starch, is a linear chain containing hundreds of glucose units. Amylopectin, another plant starch, is a branched chain containing thousands of glucose units. These large starch molecules form crystals and are the energy-storing molecules of plants. Both are found in foods; amylopectin is more abundant. Raw foods containing starches provide less energy than cooked foods as cooking breaks down the crystal structure of starches and makes them easier to digest. Some types of starch do not break down in the digestive tract; these are called resistant starches. Bacteria in the gut can break some of these down and may benefit gastrointestinal health. Isolated and modified starches are used widely in the food industry as food thickeners.

Humans and animals store glucose energy from starches in the form of the very large molecule, glycogen. It has many branches that allow it to break down quickly when energy is needed by cells in the body. It is predominantly found in liver and muscle tissue in animals.

Dietary Fibers

Dietary fibers are polysaccharides that are highly branched and cross-linked. Some dietary fibers are pectin, gums, cellulose, hemicellulose, and lignin. Lignin, however, is not composed of carbohydrate units. Humans do not produce enzymes that can break down dietary fiber; however, bacteria in the intestines do. Dietary fibers are very beneficial to our health, such as reducing the risk for obesity and diabetes, which are primary risk factors for cardiovascular disease.

Dietary fiber is categorized as water-soluble or insoluble. Some examples of soluble fibers are inulin, pectin, and guar gum, and they are found in peas, beans, oats, barley, and rye. Cellulose and lignin are insoluble fibers and a few dietary sources of them are whole-grain foods, flax, cauliflower, and avocados. Cellulose is the most abundant fiber in plants, making up the cell walls and providing structure. Soluble fibers are more easily accessible to bacterial enzymes in the large intestine so they can be broken down to a greater extent than insoluble fibers.

Some foods contain functional fiber. Functional fiber may be extracted from plants or made synthetically. Functional fibers are added to foods with the goal of providing health benefits. An example of a functional fiber is psyllium-seed husk. Scientific studies show that consuming psyllium-seed husk reduces blood-cholesterol levels; this health claim has been approved by the FDA. Total dietary fiber intake is the sum of dietary fiber and functional fiber consumed.

Digestion and Absorption of Carbohydrates

From the Mouth to the Stomach

The mechanical and chemical digestion of carbohydrates begins in the mouth. Chewing, also known as mastication, crumbles foods into smaller and smaller pieces. The salivary glands in the oral cavity secrete saliva that contains the enzyme salivary amylase. This enzyme breaks the bonds between the monomeric sugar units of starches, breaking amylose and amylopectin into smaller chains of glucose, called dextrins, and maltose. The increased concentration of maltose in the mouth that results from the mechanical and chemical breakdown of starches in whole grains is what makes us experience them as sweet. Only about five percent of starches are broken down in the mouth. (This is a good thing as more glucose in the mouth would lead to more tooth decay.) When carbohydrates reach the stomach no further chemical breakdown occurs because the amylase enzyme does not function in the acidic conditions of the stomach. But mechanical breakdown is ongoing—the strong peristaltic contractions of the stomach mix the carbohydrates into chyme.

From the Stomach to the Small Intestine

The chyme is gradually expelled into the upper part of the small intestine. Upon entry of the chyme into the small intestine, the pancreas releases pancreatic juice through a duct. Pancreatic juice contains the enzyme, pancreatic amylase, which breaks remaining starches and dextrins into shorter and shorter carbohydrate chains. Additionally, enzymes are secreted by the intestinal cells that line the villi. These enzymes include the disaccharides sucrase, maltase, and lactase. Sucrase breaks sucrose into glucose and fructose molecules. Maltase breaks the bond between the two glucose units of maltose, and lactase breaks the bond between galactose and glucose in lactose. Once carbohydrates are broken into monosaccharides they are absorbed into the intestinal cells and into the bloodstream.

When people do not have enough of the enzyme lactase, lactose is not digested in the small intestine. Undigested lactose in the large intestine is met by bacteria which are able to digest it, producing gas and causing diarrhea, bloating, and abdominal cramps. This is called lactose intolerance, which is a common condition in human adults. The National Institute of Diabetes and Digestive and Kidney Diseases states that African Americans, Hispanic/Latino Americans, American Indians, and Asian Americans have high incidences of lactose intolerance while those of northern European descent have the least.^[1] Most people with lactose intolerance can tolerate some dairy products in their diet. The severity of the symptoms depends on how much lactose is consumed and the degree of lactase deficiency.

Absorption: Into the Blood Stream

The cells in the small intestine have membranes with transport proteins to move monosaccharides and other nutrients into the blood. The bloodstream carries them to the rest of the body. The first organ to receive glucose, fructose, and galactose is the liver. The liver converts galactose and fructose to glucose, breaks fructose into smaller carbon-containing units, and either stores glucose as glycogen or exports it back to the blood. How much glucose the liver exports to the blood is under hormonal control.

Maintaining Blood Glucose Levels: The Pancreas and Liver

Glucose levels in the blood are tightly controlled, as having either too much or too little glucose in the blood has health consequences. Glucose regulates its levels in the blood via a process called negative feedback. An example of negative feedback is an oven thermostat. When you set the temperature to cook a delicious homemade noodle casserole at 375°F the thermostat sends an electrical signal to turn the elements on to heat up the oven. When the temperature reaches 375°F the thermostat senses the temperature and sends a signal to turn the element off. Similarly, your body senses blood glucose levels and maintains the glucose concentration in the target range. The glucose sensor is in the cells of the pancreas.

A meal containing carbohydrates causes glucose levels in the blood to rise. When certain cells in the pancreas sense that glucose levels are high, they release the hormone insulin into the blood. Insulin allows body cells to take glucose in, which removes glucose from the bloodstream. The cells use glucose to make energy. Muscle tissue and liver store glucose in the form of glycogen when insulin is present. As glucose is transported into the cells around the body, the blood glucose levels decrease.

When glucose levels are low, cells in the pancreas release a different hormone called glucagon. Glucagon signals the liver to break down glycogen and release the stored glucose into the blood. The rise and fall of these two hormones throughout the day keep glucose levels within a healthy range and ensure that cells get the fuel needed to function properly.

Leftover Carbohydrates: The Large Intestine

Most carbohydrates are efficiently digested and absorbed into the body in the small intestine. Some indigestible carbohydrates, including dietary fiber and resistant starches, remain to pass into the large intestine, where they are broken down by bacteria. The products of bacterial digestion of these slow-releasing carbohydrates are short-chain fatty acids (SCFAs) and some gases. SCFAs can be used by bacteria, eliminated in the feces, or absorbed into large intestine cells, with a small amount being transported to the liver. Human cells lining the large intestine use the short-chain fatty acids to support some of their functions. The liver can also metabolize the short-chain fatty acids into cellular energy. Circulating SCFAs in the bloodstream have beneficial effects and protect the circulatory system. The yield of energy from dietary fiber is about 2 kilocalories per gram for humans, but is highly dependent upon the fiber type, with soluble fibers and resistant starches yielding more energy than insoluble fibers. Fiber-rich foods (including whole grains, legumes, and vegetables) are digested more slowly than simpler sugars and starches, so the rise in blood glucose after eating them is less, and slower. These foods are linked to a reduced risk of obesity and chronic diseases, such as Type 2 diabetes and cardiovascular disease.

Glycemic Index

A measure of the effect of carbohydrate-containing food on blood-glucose levels is the glycemic index (GI). This numerical value compares the impact of a food on the blood glucose compared to a white bread, a reference food. Foods with a low GI raise blood-glucose levels more slowly than those with a higher GI. A diet of low-GI foods has been shown in some epidemiological and clinical trial studies to reduce the risk of obesity, Type 2 diabetes, and cardiovascular disease.^{[2, 3]}

Here is a list of foods’ glycemic indices compared to glucose:

• Low GI Foods (< 55)
  • Apple, raw 36
  • Orange, raw 43
  • Banana, raw 51
  • Mango, raw 51
  • Carrots, boiled 39
  • Taro, boiled 53
  • Corn tortilla 46
  • Spaghetti (whole wheat) 37
  • Baked beans 48
  • Soy milk 34
  • Skim milk 37
  • Whole milk 39
  • Yogurt, fruit 41
  • Yogurt, plain 14
  • Ice cream 51
• Medium GI Foods (56–69)
  • Pineapple, raw 59
  • Cantaloupe 65
  • Mashed potatoes 70
  • Whole-wheat bread 69
  • Brown rice 55
  • Cheese pizza 60
  • Sweet potato, boiled 63
  • Macaroni and cheese 64
  • Popcorn 65
• High GI Foods (70 and higher)
  • Banana (over-ripe) 82
  • Corn chips 72
  • Pretzels 83
  • White bread 70
  • White rice 72
  • Bagel 72
  • Rice milk 86
  • Cheerios 74
  • Raisin Bran 73
  • Fruit roll-up 99
  • Gatorade 78

This list (http://www.mendosa.com/gilists.htm) contains more foods listed by category and also by low, medium, or high glycemic index.

Fat and fiber in foods increase digestion time, delay gastric emptying, and ultimately reduce GI. Processing and cooking can raise a food’s GI by increasing digestibility. Advancements in food processing and the high consumer demand for convenient, precooked foods in the United States have created foods that are digested and absorbed rapidly. Breakfast cereal, bread, pasta, and other processed foods have a high GI. In contrast, most raw foods have a low GI.

Most meals are combinations of foods, which limits the usefulness of GI in meal planning. Snack decisions, however, might be guided by GI. It is also good to remember that some nutrient-dense foods have higher GIs than less nutritious food. For example, oatmeal has a higher GI than chocolate because chocolate contains fat. Meats and fats do not have a GI since they do not contain carbohydrates.

References

1. “Definition and Facts for Lactose Intolerance.” National Institute of Diabetes and Digestive and Kidney Diseases. Updated February 2018. Accessed February 21, 2025.
2. Brand-Miller J, et al. Dietary Glycemic Index: Health Implications. J Am Coll Nutr. 2009; 28(4), 446S–49S. https://www.ncbi.nlm.nih.gov/pubmed/20234031
3. Schwingshackl L, Hoffmann G. Long-term effects of low glycemic index/load vs. high glycemic index/load diets on parameters of obesity and obesity-associated risks: a systematic review and meta-analysis. Nutr Metab Cardiovasc Dis. 2013 Aug;23(8):699-706. doi: 10.1016/j.numecd.2013.04.008. Epub 2013 Jun 17. PMID: 23786819.

Functions of Carbohydrates in the Body

There are five primary functions of carbohydrates in the human body. They are energy production, energy storage, building macromolecules, sparing protein, and assisting in lipid metabolism.

Energy Production

The primary role of carbohydrates is as fuel for production of energy in body cells. Many cells prefer glucose as a source of energy versus other compounds like fatty acids. Some cells, such as red blood cells, are only able to produce cellular energy from glucose. The brain is highly sensitive to low blood-glucose levels. There is an alternative fuel (ketone bodies) that the brain can use under conditions of starvation, but it is uses glucose to function under normal conditions. About 70 percent of the glucose from digestion is redistributed by the liver back into the blood for use by other tissues. Cells that require energy remove the glucose from the blood with a transport protein in their membranes. The energy from glucose comes from the chemical bonds between the carbon atoms. Sunlight energy was required to produce these high-energy bonds in the process of photosynthesis. Cells in our bodies break these bonds and use the energy to perform cellular respiration. Cellular respiration (see Chapter 14) is controlled burning of glucose: a series of many chemical reactions that leads to the release of energy from glucose and other molecules.

Energy Storage

If the body has enough energy to support its functions, the excess glucose is stored as glycogen (the majority of which is stored in the muscles and liver). A molecule of glycogen may contain in excess of fifty thousand single glucose units and is highly branched, allowing for the rapid dissemination of glucose when it is needed to make cellular energy.

The amount of glycogen in the body at any one time is equivalent to about 4,000 kilocalories—3,000 in muscle tissue and 1,000 in the liver. Prolonged muscle use (such as exercise for longer than a few hours) can deplete the glycogen energy reserve, leading to fatigue and a decrease in exercise performance. Prolonged exercise depletes muscle glycogen and requires muscles to rely on different energy sources. Athletes increase their glycogen reserves by reducing training intensity and increasing their carbohydrate intake to between 60 and 70 percent of total calories three to five days prior to an event. People who are not hardcore training and choose to run a 5-kilometer race for fun do not need to consume a big plate of pasta prior to a race since the adaptation of increased muscle glycogen requires longer-term training.

The liver stores glucose as glycogen, but in contrast to muscle tissue it will sacrifice its stored glucose energy to other tissues in the body when blood glucose is low. Approximately one-quarter of total body glycogen content is in the liver. This is approximately equivalent to a four-hour supply of glucose, depending on activity level. The liver uses this glycogen reserve to keep blood-glucose levels within a narrow range between meals. When the liver’s glycogen supply is exhausted, the body activates different pathways to make glucose from stored lipids and proteins.

Building Macromolecules

Although most absorbed glucose is used to make energy, glucose can be used to build ribose and deoxyribose, two components of the important macromolecules RNA, DNA, ATP and NADPH. NADPH protects against oxidative stress and is used in many chemical reactions in the body. When these needs are met, the body can be used to make fat. A diet high in carbohydrates can lead to obesity; dietary fat is not the only candidate for weight gain!

Sparing Protein

If there is not enough glucose to meet the body’s needs, glucose is synthesized from amino acids. Because there is no storage molecule of amino acids, this process requires the destruction of proteins, primarily from muscle tissue. The presence of adequate glucose spares the breakdown of proteins from being used to make glucose needed by the body.

Lipid Metabolism

When blood glucose levels are high, the use of lipids as an energy source is inhibited. Glucose has a “fat-sparing” effect because the increase in insulin causes cells to use glucose (instead of lipids) to make energy. When glucose levels are low for an extended period, the liver uses fatty acids to produce an alternative fuel called ketone bodies. Ketone bodies are acidic and high levels in the blood can cause the blood to become acidic. The resulting condition, called ketosis, can be dangerous for groups of people such as alcoholics, people who are malnourished, and individuals who have Type 1 diabetes. The minimum daily carbohydrate intake to prevent ketosis in adults is 50 grams per day.

High-Carbohydrate Diets

Can America blame its obesity epidemic on the higher consumption of added sugars and refined grains? This is a hotly debated topic by both the scientific community and the general public. In this section, we will give a brief overview of the scientific evidence.

Added Sugars

The Food and Nutrition Board of the Institute of Medicine (IOM) defines added sugars as “sugars and syrups that are added to foods during processing or preparation…Major sources of added sugars include soft drinks, sports drinks, cakes, cookies, pies, fruitades, fruit punch, dairy desserts, and candy.” Processed foods, even microwaveable dinners and other foods that don’t seem sweet, often contain added sugars. Added sugars do not include sugars that occur naturally in whole foods (such as an apple), but do include natural sugars such as brown sugar, corn syrup, dextrose, fructose, fruit juice concentrates, maple syrup, sucrose, and raw sugar added in processing. A 2008 survey of forty-two thousand Americans reports the average intake of added sugars is 15 percent of total calories, a drop from 18 percent of total calories in 2000.^[1]

This is still above the recommended intake of less than 10 percent of total calories. The US Department of Agriculture (USDA) reports that sugar consumption in the American diet in 2008 was, on average, 28 teaspoons per day (Figure 10.12).

 

Obesity, Diabetes, and Heart Disease: Linked to Sugar and Refined Carbohydrate?

To understand the magnitude of the health problem in the United States consider this—in the United States approximately 130 million adults are overweight, and 30 percent of them are considered obese. The obesity epidemic has reached young adults and children and will affect their health in adulthood. Health consequences linked to being overweight or obese include type 2 diabetes, cardiovascular disease, arthritis, depression, and some cancers. Overeating sugary foods and refined grains likely contributes this, as do the consumption of high-calorie foods of all types, saturated fat, and a sedentary lifestyle. Some studies have shown a positive correlation between diets high in added sugars/refined grains and weight gain and disease risk, but others do not. Studies show that eating a higher percentage of carbohydrates as whole grains has health benefits.

A major source of added sugars in the American diet is soft drinks. There is consistent scientific evidence that consuming sugary soft drinks increases weight gain and disease risk.^[2] A study at the Harvard School of Public Health linked the consumption of sugary soft drinks to an increased risk for heart disease.^[3]

The American Heart Association (AHA) recommends that intake of sugar be limited to 9 teaspoons per day for men and 6 teaspoons for women. After its 2010 scientific conference on added sugars, the AHA made the following related dietary recommendations.^[4]

• Know the number of total calories you should eat each day.
• Consume an overall healthy diet of nutrient-dense foods; using foods high in added sugars as discretionary calories (those left over after getting all recommended nutrients subtracted from the calories used).
• Lower sugar intake by limiting sugary drinks, candies, cakes, and cookies.

Fructose

Before high-fructose corn syrup (HFCS) was marketed as the best food and beverage sweetener, sucrose (table sugar) was the number-one sweetener in America. Sucrose is a disaccharide of one glucose unit and one fructose unit. HFCS also contains the simple sugars fructose and glucose, but with fructose at a slightly higher concentration. In the production of HFCS, corn starch is broken down to glucose, and some of the glucose is converted to fructose. Fructose is sweeter than glucose, and HFCS is inexpensive, so many food manufacturers choose it for carbonated beverages, condiments, cereals, and other processed foods.

Some scientists, public health personnel, and healthcare providers believe that fructose has been important in the obesity epidemic. Since 1970 the number of overweight or obese Americans has dramatically increased and so has the consumption of foods containing HFCS. This correlation, however, is not conclusive as other factors have also changed in this time period: total calories ingested, total sugars ingested, sedentary lifestyle. Fructose is not used to produce energy in the body; it is mostly converted to fat in the liver—potentially contributing to insulin resistance and the development of Type 2 diabetes. Additionally, fructose does not stimulate the release of certain appetite-suppressing hormones, like insulin, as glucose does. Thus, a diet high in fructose could potentially stimulate fat deposition and weight gain.

In human studies, moderate fructose intake is not associated with weight gain. Other studies show that some fructose can improve glucose metabolism especially in people with Type 2 diabetes.^[5]

At this time conclusive evidence is not available on whether fructose is any worse than any other added sugar in increasing the risk for obesity, Type 2 diabetes, and cardiovascular disease.

Low-Carbohydrate Diets

Since the early 1990s, marketers of low-carbohydrate diets have told us that eating fewer carbohydrates promotes weight loss and benefits overall health. Low-carbohydrate diets include the Atkins diet, South Beach diet, Zone diet, and Earth diet. Despite the claims, there is little scientific evidence to support that low-carbohydrate diets are significantly better than other diets for long-term weight loss. A study in The Nutritional Journal concluded that all diets, (independent of carbohydrate, fat, and protein content) that incorporated an exercise regimen significantly decreased weight and waist circumference in obese women.^[6]

Some studies show that low-carbohydrate diets improve insulin levels and other risk factors for Type 2 diabetes and cardiovascular disease. The overall scientific consensus is that consuming fewer calories in a balanced diet will promote health and stimulate weight loss, with significantly better results achieved when combined with regular exercise.

Health Benefits of Whole Grains in the Diet

Consuming more complex carbohydrates is beneficial to health; replacing refined grains with whole grains decreases the risk for obesity, Type 2 diabetes, and cardiovascular disease. Whole grains are rich dietary sources of fiber, vitamins, minerals, healthy fats, and other beneficial plant chemicals (phytochemicals). A high-fiber meal compared to a low-fiber meal (see Figure 10.13) slows the absorption process, which affects the speed at which glucose enters the blood. Americans typically do not consume the recommended amount of whole grains, which is 50 percent or more of grains from whole grains.

Diets high in whole grains have been shown to decrease weight. Consuming more than two servings of whole grains per day reduces one’s chances of getting Type 2 diabetes by 21 percent.^[7] The Nurses’ Health Study found that women who consumed two to three servings of whole grain products daily were 30 percent less likely to have a heart attack.^[8]

The AHA makes the following statements on whole grains^[9]:

• Dietary fiber can help improve blood cholesterol levels and lower your risk of heart disease, stroke, obesity and even type 2 diabetes.
• If you’re trying to lose weight, fiber can help you feel full, which means you’ll be satisfied with fewer calories.

References

1. Welsh JA, Sharma AJ, et al. Consumption of Added Sugars Is Decreasing in the United States. Am J Clin Nutr. 2011; 94(3), 726–34. http://www.ncbi.nlm.nih.gov/pubmed/21753067.
2. Malik VS, Schulze MB, Hu FB. Intake of Sugar-Sweetened Beverages and Weight Gain: A Systematic Review. Am J Clin Nutr. 2006; 84(2), 274–88.https://ajcn.nutrition.org/article/S0002-9165(23)29007-3/fulltext
3. Hand L and Drexler M. Public Health Takes Aim at Sugar and Salt. Harvard Public Health Review. https://content.sph.harvard.edu/wwwhsph/sites/21/2012/12/HSPHFALL09review.pdfPublished 2009. Accessed February 21, 2025.
4. Van Horn L, Johnson RK, et al. Translation and Implementation of Added Sugars Consumption Recommendations. Circulation. 2010; 122, 2470–90. https://www.ahajournals.org/doi/10.1161/cir.0b013e3181ffdcb0
5. Elliott SS, Keim NL, et al. Fructose, Weight Gain, and the Insulin Resistance Syndrome. Am J Clin Nutr. 2002; 76(5),911–22. https://ajcn.nutrition.org/article/S0002-9165(23)05996-8/fulltext
6. Kerksick CM, Wismann-Bunn J, et al. Changes in Weight Loss, Body Composition, and Cardiovascular Disease Risk after Altering Macronutrient Distributions During a Regular Exercise Program in Obese Women. Nutr J. 2010; 9(59). https://pmc.ncbi.nlm.nih.gov/articles/PMC3000832/
7. de Munter JS, Hu FB, et al. Whole Grain, Bran, and Germ Intake and Risk of Type 2 Diabetes: A Prospective Cohort Study and Systematic Review. PLoS Medicine. 2007; 4(8), e261. https://pubmed.ncbi.nlm.nih.gov/17760498/
8. Liu S, Stampfer MJ, et al. Whole-Grain Consumption and Risk of Coronary Heart Disease: Results from the Nurses’ Health Study. Am J Clin Nutr. 1999; 70(3), 412–19. https://ajcn.nutrition.org/article/S0002-9165(22)04076-X/fulltext
9. “Get to Know Grains: Why You Need Them, and What to Look For.” American Heart Association. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/nutrition-basics/whole-grains-refined-grains-and-dietary-fiber. Updated June 25, 2024. Accessed February 21, 2025.

Carbohydrates and Diet Choices

Carbohydrates Recommendations

The Recommended Dietary Allowance (RDA) for carbohydrates for children and adults is 130 grams per day, which is the minimum amount the brain requires to function properly. Most people consume much more than this. The Acceptable Macronutrient Distribution Range (AMDR) for carbohydrates is 45 to 65 percent of total calorie intake. This means that a 2,000-kilocalorie diet would include 225 to 325 grams of carbohydrate. The World Health Organization and the AHA recommend that 10 percent or less of total calories comes from added sugars, which is 50 grams of added sugars in a 2000-kcal diet. RDAs for dietary fiber are 38 and 25 grams for adult males and females, respectively.

Dietary Sources of Carbohydrates

Carbohydrates made by plants are found in fruits, vegetables, legumes (beans and peas), and grains. Fungi and algae make carbohydrates as well, so mushrooms and seaweed will contain them. Mammalian milk contains lactose, a sugar, but meat does not contain carbohydrates. Fast-releasing carbohydrates are more prevalent in fruits, fruit juices, and dairy products, while slow-releasing carbohydrates are more plentiful in starchy vegetables, beans, and whole grains. Fast-releasing carbohydrates are common in processed foods, soft drinks, and sweets. On average, a serving of fruits, whole grains, or starches contains 15 grams of carbohydrates. A serving of dairy contains about 12 grams of carbohydrates, and a serving of vegetables contains about 5 grams of carbohydrates. Table 10.1 gives the specific amounts of carbohydrates, fiber, and added sugar of various foods.

Table 10.1 Carbohydrates in Foods (grams/serving).
Source: National Nutrient Database for Standard Reference. US Department of Agriculture. Updated April 2018. Accessed February 21, 2025.
Source: USDA Database for the Added Sugars Content of Selected Foods. US Department of Agriculture. Public domain. Published February 2006. Accessed February 21, 2025.

Foods	Total Carbohydrates	Sugars	Fiber	Added Sugars
Banana	27 (1 medium)	14.40	3.1	0
Lentils	40 (1 c.)	3.50	16.0	0
Snap beans	8.7 (1 c.)	1.60	4.0	0
Green pepper	5.5 (1 medium)	2.90	2.0	0
Corn tortilla	10.7 (1)	0.20	1.5	0
Bread, wheat bran	17.2 (1 slice)	3.50	1.4	3.4
Bread, rye	15.5 (1 slice)	1.20	1.9	1.0
Bagel (plain)	53 (1 medium)	5.30	2.3	4.8
Brownie	36 (1 square)	20.50	1.2	20.0
Oatmeal cookie	22.3 (1 oz.)	12.00	2.0	7.7
Cornflakes	23 (1 c.)	1.50	0.3	1.5
Pretzels	47 (10 twists)	1.30	1.7	0
Popcorn (homemade)	58 (100 g)	0.50	10.0	0
Skim milk	12 (1 c.)	12.00	0	0
Cream (half and half)	0.65 (1 Tbs.)	0.02	0	0
Cream substitute	1.0 (1 tsp.)	1.00	0	1.0
Cheddar cheese	1.3 (1 slice)	0.50	0	0
Yogurt (with fruit)	32.3 (6 oz.)	32.30	0	19.4
Caesar dressing	2.8 (1 Tbs.)	2.80	0	2.4

Carbohydrates Information on Labels

To determine whether the grains in a food are refined or whole, look at the ingredients list. Ingredients are listed by weight, the highest first. Whole wheat flour is listed as “whole wheat;” the term “wheat flour” refers to flour that has been processed to remove the bran. Enriched wheat flour refers to white flour with added vitamins and iron. Sweeteners include a variety of ingredients including HFCS, sugar, sucrose, honey, dextrose, and cane sugar. If you want to eat less processed foods, avoid products with long ingredient lists. On the front of food and beverages the manufacturers may include claims such as “sugar-free,” “reduced sugar,” “high fiber,” etc. The Nutrition and Labeling Act of 1990 has defined for the food industry and consumers what these labels mean (Table 10.2)

Table 10.2 Food Labels Pertaining to Carbohydrates. Source: Appendix A: Definitions of Nutrient Content Claims; Appendix B: Additional Requirements for Nutrient Content Claims. A Food Labeling Guide: Guidance for Industry. US Food and Drug Administration. Public domain. Updated January 2013. Accessed February 21, 2025.

Label	Meaning
Sugar-free	Contains less than 0.5 grams of sugar per serving
Reduced sugar	Contains 25 percent less sugar than similar product
Less sugar	Contains 25 percent less sugar than similar product
No sugars added	No sugars added during processing
High fiber	Contains at least 20 percent of daily value of fiber in each serving
A good source of fiber	Contains between 10 and 19 percent of the daily value of fiber per serving
More fiber	Contains 10 percent or more of the daily value of fiber per serving

The FDA permits foods that contain whole oats (which contain soluble fiber) to make the health claim on the package that the food reduces the risk of coronary heart disease. The FDA no longer permits Cheerios to make the claim that by eating their cereal “you can lower your cholesterol four percent in six weeks.”

Suggestions for Healthy Carbohydrate Consumption

• Boost whole-grain intake by eating a whole-grain cereal for breakfast, using whole-grain bread to make a sandwich for lunch, and eating a serving of beans and/or nuts with dinner.
• Make at least half of all the grains you eat whole grains. A serving of whole grains is one slice of whole-wheat bread, one ounce of whole-grain cereal, or one-half cup of cooked cereal, brown rice, or whole-wheat pasta.
• Cornmeal is a whole grain so choose tortillas, corn cereals, and corn breads with cornmeal listed as the first ingredient.
• When baking, substitute whole-wheat flour or other whole-grain flour for some of the refined white flour.
• If you like bread at dinner, choose a whole-grain muffin over a Kaiser roll or baguette..
• Choose whole-grain pastas and brown rice, cook al dente, and add some beans and vegetables in equal portions.
• Try new grains such as barley, quinoa, and bulgur.
• Eat snacks high in fiber, such as almonds, pistachios, raisins, and air-popped popcorn.
  Add an artichoke and green peas to your dinner plate more often.
• Calm your “sweet tooth” by eating fruits, such as berries or an apple.
• Replace sugary soft drinks with seltzer water, tea, or a small amount of 100 percent fruit juice added to water or soda water.

The Food Industry: Functional Attributes of Carbohydrates and the Use of Sugar Substitutes

In the food industry, sweeteners and fiber are used to give foods characteristics including sweetness, viscosity, bulk, coating ability, solubility, consistency, texture, body, and browning capacity. Differences in chemical structure of different carbohydrates confer their varied functional uses in foods. Starches, gums, and pectins are thickening agents in jam, cakes, cookies, noodles, canned products, imitation cheeses, and other foods. Molecular gastronomists use slow-releasing carbohydrates, such as alginate, to give shape and texture to their fascinating food creations. Adding fiber to foods increases bulk. Simple sugars are used not only for adding sweetness, but also to add texture, consistency, and browning. In ice cream, the combination of sucrose and corn syrup imparts sweetness as well as a glossy appearance and smooth texture.

Due to the consumers’ wish for lower-sugar products, sugar substitutes are found in many foods and beverages. Sugar substitutes may be from natural sources or artificially made. Artificial sweeteners must be approved by the FDA for use in foods and beverages; examples include saccharin, aspartame, acesulfame potassium, neotame, advantame, and sucralose. Stevia is a naturally-derived sugar substitute from the Stevia plant native to South America. Sugar alcohols, such as xylitol, sorbitol, erythritol, and mannitol, occur naturally in some fruits and vegetables. However, they are industrially synthesized for use as food additives. The FDA requires that foods disclose the fact that they contain sugar alcohols but does not require testing. Compared to sucrose, artificial sweeteners are significantly sweeter (in fact, by several hundred times); many sugar alcohols are less sweet than sucrose (see Table 10.3). Artificial sweeteners are so sweet that they are used in very small amounts, adding very few calories to foods. Stevia is not digested or absorbed in the human small intestine. Sugar alcohols may be digested by intestinal microbes, with some products absorbed by humans. They contribute about 2 kcals/gram, which is half the calories of sucrose. The metabolism of sugar alcohols by microbes may lead to intestinal gas – an unpleasant side-effect for some consumers!

Table 10.3 Relative Sweetness of Sugar Substitutes

Sweetener	Trade Names	Sweeter than Sucrose (times)
Saccharine	“Sweet-N-Lo”	300.0
Aspartame	“NutraSweet,” “Equal”	80-200.0
Acesulfame-K	“Sunette”	200.0
Neotame	n/a	7,000.0–13,000.0
Advantame	n/a	20,000
Sucralose	“Splenda”	600.0
Stevia	n/a	250.0–300.0
Xylitol	n/a	0.8
Mannitol	n/a	0.5
Sorbitol	n/a	0.6
Erythritol	n/a	1.0

Sugar Substitutes and Health

Foods and beverages containing sugar substitutes may reduce the consumption of simple sugars, which could benefit weight management. Sugar substitutes are not consumed by mouth bacteria, and therefore are associated with less risk of tooth decay than sugar. The FDA allows packaging for chewing gum with artificial sweeteners to carry health claims that it promotes oral health.

Some studies suggest that artificially-sweetened foods increase appetite for sweet foods and may lead to increased weight gain. The most common side effect of sugar substitutes is gastrointestinal upset, a result of their incomplete digestion. Since the introduction of sugar substitutes to the food and beverage markets, the public has expressed concern about their safety.

In the early 1970s scientific studies were published that demonstrated that high doses of saccharin caused bladder tumors in rats. This information fueled the ongoing debate of health consequences of artificial sweeteners. Since this study, investigations in rats, monkeys, and humans have not found any relationship between saccharine consumption and bladder cancer. In 2000, saccharin was removed from the US National Toxicology Program’s list of potential carcinogens.^[1]

There have been health concerns over other artificial sweeteners, most notably aspartame (sold under the trade names of NutraSweet and Equal). The first misconception regarding aspartame was that it was linked with an increase in the incidence of brain tumors in the United States. It was subsequently discovered that the increase in brain tumors started eight years prior to the introduction of aspartame to the market. Aspartame has been highly tested. It is approved for use as an artificial sweetener in over ninety countries.

Studies suggest that use of artificial sweeteners changes the way the body cells react to glucose and insulin, leading to an increased risk to develop type 2 diabetes.^[2]

Aspartame is made of aspartic acid and phenylalanine; when digested, it is broken down to aspartic acid, phenylalanine, and methanol. People who have the rare genetic disorder phenylketonuria (PKU) have to avoid products containing aspartame. Individuals who have PKU do not have a functional enzyme that converts phenylalanine to the amino acid tyrosine. This causes a buildup of phenylalanine and its metabolic products in the body. If PKU is not treated, the buildup of phenylalanine causes progressive brain damage and seizures. The FDA requires products that contain aspartame to state on the product label, “Phenylketonurics: Contains Phenylalanine.” More about sugar substitutes is in Table 10.3.

Table 10.6 Sweeteners

Sweeteners with Trade or Alternative Names	Calories	Source/Origin	Consumer Recommendations	Controversial Issues	Product Uses
Aspartame (NutraSweet, Equal)	4 kcal/g	Composed of two amino acids (phenylalanine + aspartic acid) + methanol.Two hundred times sweeter than sucrose.	FDA set maximum Acceptable Daily Intakes (ADI):50 mg/kg body weight = 16 12 oz. diet soft drinks for adults. *Cannot be used in products requiring cooking. People with PKU should not consume aspartame.	Children have potential to reach ADI if consuming many beverages, desserts, frozen desserts, and gums containing aspartame routinely.	Beverages, gelatin desserts, gums, fruit spreads.
Saccharin (Sweet ‘n’ Low)	0 kcal/g	Discovered in 1878. The basic substance is benzoic sulfinide.Three hundred times sweeter than sucrose.	ADI: 5 mg/kg body weight.*Can be used in cooking.	1970s, high doses of saccharin associated with bladder cancer in laboratory animals. In 1977, FDA proposed banning saccharin from use in food protest launched by consumer & interest groups warning label listed on products about saccharin and cancer risk in animals until 2001 when studies concluded that it did not cause cancer in humans	General purpose sweetener in all foods and beverages.Sold as Sweet ‘n’ Low in United States; also found in cosmetics and pharmaceutical products.
Acesulfame K (Sunnette, Sweet One)	0 kcal/g	Discovered in 1967. Composed of an organic salt, potassium (K). Structure is very similar to saccharin’s.It passes through the body unchanged which means it does not provide energy. Two hundred times sweeter than sucrose.	ADI: 15 mg/kg body weight.Body cannot digest it. *Can be used in cooking.		Chewing gum, powdered beverage mixes, nondairy creamers, gelatins, puddings, instant teas and coffees.
Cyclamates (Sugar Twin)	0 kcal/g	Thirty times sweeter than sucrose.Discovered in 1937.	No ADI available.	1949, cyclamate approved by FDA for use. Cyclamate was classified as GRAS (Generally Recognized As Safe) until 1970 when it was removed from GRAS status and banned from use in all food and beverage products within the United States on the basis of one study that indicated it caused bladder cancer in rats. Approval still pending for use in the United States since the ban. Canada and other countries use this sweetener.	Recommended as a substitute for table sugar for diabetics in 1950s, baked goods.
Sucralose (Splenda)	1 Splenda packet contains 3.31 calories = 1g	First discovered in 1976. Approved for use in 1998 in the United States and in 1991 in Canada.Derived from sucrose in which three of its hydroxyl (OH) groups are replaced by chlorine (Cl−). Six hundred times sweeter than sugar.	ADI: 5 mg/kg body weight.*Can be used in cooking.		General purpose sweetener, baked goods, beverages, gelatin desserts, frozen dairy desserts, canned fruits, salad dressings, dietary supplements; currently recommended as a replacement for table sugar and additive for diabetics.
Stevioside (Stevia, Sweet Leaf)	N/A	Derived from stevia plant found in South America. Stevia rebaudiana leaves.	Classified as GRAS.Considered to be a dietary supplement and approved not as an additive, but as a dietary supplement.	Used sparingly, stevia may do little harm, but FDA could not approve extensive use of this sweetener due to concerns regarding its effect on reproduction, cancer development, and energy metabolism.	Sold in health food stores as a dietary supplement.
Sucrose, Sugar	~4 kcal/g	Extracted from either sugar beets or sugar cane, which is then purified and crystallized.	It is illegal to sell true raw sugar in the United States because when raw it contains dirt and insect parts, as well as other byproducts. Raw sugar products sold in the United States have actually gone through more than half of the same steps in the refining process as table sugar.	Over-consumption has been linked to several health effects such as tooth decay or dental caries and contributes to increased risk for chronic diseases.	Biscuits, cookies, cakes, pies, candy canes, ice cream, sorbets, and as a food preservative.
Honey	3 kcal/g	Made from sucrose. Contains nectar of flowering plants. Made by bees.Sucrose is fructose + glucose; however, honey contains more calories than sucrose because honey is denser.	*Considered safe for baking and cooking.Infants under twelve months old should not be given honey because their digestive tracts cannot handle the bacteria found in honey. Older children and adults are immune to these effects. Honey contains some harmful bacteria that can cause fatal food poisoning in infants.		Sweeteners in various foods and beverages such as sodas, teas, alcoholic beverages, and baked goods.
HFCS, high fructose corn syrup	Dry form: 4 kcal/g; Liquid form: 3 kcal/g	Corn is milled to produce corn starch, then the cornstarch is further processed to yield corn syrup.		Controversial because it is found ubiquitously in processed food products, which could lead to overconsumption. Study results are varied regarding its role in chronic disease.	Soft drinks, desserts, candies, jellies.
Sugar Alcohols: Sorbitol, Xylitol, Mannitol	2–4 kcal/g.Not calorie free	Sugar alcohols.Sorbitol is derived from glucose.	Less likely to cause tooth decay than sucrose.Sugar alcohols have a laxative effect.	May cause diarrhea and gastrointestinal distress if consumed in large amounts.	Provide bulk and sweetness in the following sugar-free items: cookies, jams, jellies, chewing gum, candies, mints, pharmaceutical and oral health products.

Regulation

Prior to introducing any food additive, it is rigorously tested and must be legally approved by the FDA. The FDA regulates artificial sweeteners along with thousands of other food additives. The FDA is responsible for determining whether a food additive presents “a reasonable certainty of no harm” to consumers when used as proposed. The FDA uses the best scientific evidence available to make the statement of no harm, but it declares that science has limits and that the “FDA can never be absolutely certain of the absence of any risk from the use of any substance.”^[3]

The FDA additionally has established acceptable daily intakes (ADI) for artificial sweeteners. The ADIs are the maximum amount in milligrams per kilogram of body weight considered safe to consume daily (mg/kg bw/day) and include a large safety factor. Artificial sweeteners approved in the United States and their ADIs^[4] include:

• Acesulfame potassium (Sunett, Sweet One). ADI = 15 mg/kg bw/day
• Aspartame (Equal, NutraSweet). ADI = 50 mg/kg bw/day
• Neotame. ADI = 0.3 mg/kg bw/day
• Saccharin (SugarTwin, Sweet’N Low). ADI = 15 mg/kg bw/day
• Sucralose (Splenda). ADI = 5 mg/kg bw/day

References

1. Artificial Sweeteners and Cancer. National Cancer Institute. https://www.cancer.gov/about-cancer/causes-prevention/risk/diet/artificial-sweeteners-fact-sheet. Updated August 29, 2023. Accessed February 21, 2025.
2. Mathur K, Agrawal RK, Nagpure S, Deshpande D. Effect of artificial sweeteners on insulin resistance among type-2 diabetes mellitus patients. J Family Med Prim Care. 2020 Jan 28;9(1):69-71.
3. Overview of Food Ingredients, Additives and Colors. US Food and Drug Administration. https://public4.pagefreezer.com/browse/FDA/31-12-2022T07:59/https:/www.fda.gov/food/food-ingredients-packaging/overview-food-ingredients-additives-colors . Updated April 2010. Accessed September 22, 2017.
4. US. Food and Drug Administration. “Aspartame and Other Sweeteners in Food.” https://www.fda.gov/food/food-additives-petitions/aspartame-and-other-sweeteners-food Updated February 27, 2025. Accessed March 3, 2026.

Attributions

Adapted by Pattie S. Green, Ph.D., and Jonathan E. Pottle, Ph.D. at Tacoma Community College from Human Nutrition by University of Hawai‘i at Mānoa Food Science and Human which is licensed under a Creative Commons Attribution 4.0 International License. Specifically, the following parts were adapted here:

• Chapter 4. Carbohydrates Introduction
• Digestion and Absorption of Carbohydrates
• The Functions of Carbohydrates in the Body
• Health Consequences and Benefits of High-Carbohydrate Diets
• Carbohydrates and Personal Diet Choices