Carbohydrates are compounds that consist of carbon, hydrogen, and oxygen, linked together by energy-containing bonds. There are two types of carbohydrates: complex and simple. The complex carbohydrates, such as starch and fiber, are classified as polysaccharides. Simple carbohydrates are known as sugars and they are classified as either monosaccharides (one sugar molecule) or disaccharides (two sugar molecules). All carbohydrates are made from these simple sugars. The term “complex” depends on the molecular structure—if the carbohydrate is composed of more than two simple sugars forming a straight or branched chain of monosaccharides, it is considered complex.


Refined and processed carbohydrates

Whole grain and high-fiber carbohydrates

White bread

100% whole wheat bread

White rice


White potatoes

Brown rice


Whole wheat pasta

Sugary cereals

Whole grain crackers

Cinnamon toast





Hulled barley


Whole wheat bulgur

Soft drinks

Bran cereals


Rye wafer crackers

Fruit drinks (fruitades and fruit punch)

English muffins

Dry beans and peas

Cakes, cookies, and pies

Navy beans

Dairy desserts

Kidney beans

Ice cream

Split peas

Sweetened yogurt


Sweetened milk

White beans

Pinto beans

Green peas


Whole fruits, fresh, frozen or canned


Low-fat milk


The primary role of carbohydrates in human nutrition is to supply the body with the energy needed to sustain life. Glucose is the body's primary source of energy, and glycogen is the stored form of glucose. In the human body, every cell depends on glucose for its fuel to some extent, with the cells of the brain and nervous system depending primarily on glucose as their energy source. For cells to function properly, the body must maintain blood glucose at high enough levels to support the cells. If glucose levels become too low, the body feels dizzy and weak; if levels are too high, a person may feel tired, confused, and have difficulty breathing. Blood glucose levels that are too low or too high may result in death if left untreated.


When food is eaten, the digestion of carbohydrates begins in the mouth, where an enzyme in saliva (amylase) breaks down the starch molecules into shorter polysaccharides and the disaccharide maltose. The food then moves into the stomach, where it mixes with the stomach's acid and other juices and digestion of carbohydrates stops. The majority of carbohydrate digestion occurs in the small intestine. Starch is further broken down into glucose chains and disaccharides by pancreatic amylase. Cells lining the small intestine secrete the enzymes maltase, sucrase, and lactase, which further split these disaccharides and polysaccharides into monosaccharides. The cells lining the small intestine can absorb these monosaccharides, which are then taken to the liver. The liver converts fructose and galactose to glucose. If there is an excess of fructose or galactose, it may also be converted to fat. Lastly, the glucose is transported to the body's cells by the circulatory system, where it can be used for energy.

When there is an excess of glucose, the muscle and liver cells often convert it to glycogen, which is the form in which glucose is stored in the body. The muscles store two-thirds of the body's glycogen solely for their own use, and the liver stores the other one-third, which can be used by the brain or other organs when needed. When blood glucose levels decline, the body breaks down some of its glycogen stores and uses the glucose for energy. If blood glucose (sugar) levels are too high, the excess glucose is taken to the liver where it is converted into glycogen or fat and stored for future use.


Fiber is the general term used to describe the non-starchy polysaccharides cellulose, hemicelluloses, pectins, gums, and mucilages in plant foods. It is also a term used for the nonpolysaccharides lignins, cutins, and tannins. They are usually categorized as either water insoluble, cellulose, hemicelluloses, or lignin. Gums, pectin, some hemicelluloses, and mucilages are called water soluble. Even though these compounds cannot be digested by human digestive enzymes, some are further broken down by the gut bacteria to short-chain fatty acids that yield a small portion of energy when metabolized and serve several important functions in human health. Insoluble fibers are found in high concentrations in wheat, cereals, and vegetables, and their main function in human physiology is to assist in the movement of waste through the intestines and to increase fecal weight. They also bind with bile acids, which reduces fat and cholesterol absorption. Soluble fiber helps decrease low-density lipoprotein (LDL) cholesterol levels, also called “bad” cholesterol, and delays the stomach emptying, promoting satiety. It can be found in barley, fruit, legumes, and oats.

Dietary fiber—
Also known as roughage or bulk. Insoluble fiber moves through the digestive system almost undigested and gives bulk to stools. Soluble fiber dissolves in water and helps keep stools soft.
A simple sugar that results from the breakdown of carbohydrates. Glucose circulates in the blood and is the main source of energy for the body.
The storage form of glucose found in the liver and muscles.
A naturally abundant nutrient carbohydrate found in seeds, fruits, tubers, and roots.
Any of a class of carbohydrates, such as starch, amylose, amylopectin, and cellulose, consisting of several monosaccharides.
A simple sugar, such as glucose or fructose, comprised of a single sugar unit.

Fiber is an extremely important part of the diet. It aids in weight control by displacing calorie-dense fats. Fiber also absorbs water and slows the movement of food through the digestive tract, promoting a feeling of fullness. It improves large intestine function and health and may help prevent colon cancer and lower blood cholesterol levels. It also helps to reduce the glycemic response (how fast foods are turned into glucose). Too much fiber, however, may result in abdominal discomfort and lower nutrient availability.

Recommended intake

The Institute of Medicine of the National Academy of Sciences has established ranges for the percentage of calories in the diet that should come from carbohydrates. The acceptable macronutrient distribution ranges (AMDR) take into account both chronic disease risk reduction and required intake of essential nutrients. For carbohydrates, the AMDR is 45%–65% of daily caloric need for people older than age one, though percentages can vary depending on age, general health, certain diseases and conditions, and activity level. Of this intake, it is further recommended that half or more of grains consumed come from whole grains. The Adequate Intake (AI) level for fiber is 14 g per 1,000 calories, or about 25 g per day for women and 38 g per day for men. The U.S. Department of Agriculture's (USDA) Dietary Guidelines for Americans suggests that half of a person's plate be filled with fruits and vegetables at each meal and recommends consuming legumes in place of red meat on occasion. According to the American Heart Association guidelines, the recommended sugar intake for adult women is 5 teaspoons (20 g) of sugar per day; for adult men, 9 teaspoons (36 g) per day; and for children, 3 teaspoons (12 g) a day.


A common concern among consumers is that high intake of carbohydrates will cause weight gain. Consuming too much of any particular food can cause an increase in weight, but eating a balanced diet with plenty of fruits, vegetables, and grains will help support weight management. Many of the carbohydrates consumed in the United States are made with high amounts of fat, sodium, and added sugars, which can promote weight gain. High-fructose corn syrup (HFCS) in particular is used in many foods in the United States. HFCS was first developed in the mid-1960s and, because of its unique physical and functional properties, was widely embraced by food formulators. The use of HFCS increased rapidly over the next 30 years, primarily used as a replacement for sucrose (table sugar). Since 1999, use of HFCS has declined somewhat due to noted adverse health concerns associated with its intake. Studies have shown that adults who consumed 25% of their daily calories as fructose or HFCS beverages (a percentage within current government guidelines) for two weeks experienced increases in serum levels of cholesterol and triglycerides (fatty acids in the blood). Other negative health claims associated with a high intake of fructose and/or HFCS include raised serum uric acid levels; increased risk of developing gout, dementia, cardiovascular disease, and high blood pressure; and accumulation of central fat storage. Smaller studies have also shown that it may play a role in the development of non-alcoholic fatty liver disease. Fructose is not only to blame—studies have also shown some of these same effects, such as central fat storage and increased blood pressure, with high intakes of other sugars. Regardless of form, when sugar displaces other nutrient dense foods and/or adds extra calories, usually through sweetened beverages, it has been shown to be a determinant of obesity and predicts the risk of developing cardiometabolic disease.

Body mass index (BMI)—
Also known as BMΙ, the index determines whether a person is at a healthy weight, underweight, overweight, or obese. The BMΙ can be calculated by converting the person's height into inches. That amount is multiplied by itself and then divided by the person's weight. That number is then multiplied by 703. The metric formula for the BMΙ is the weight in kilograms divided by the square of height in meters.
Cognitive behavioral therapy—
A type of psychotherapy in which people learn to recognize and change negative and self-defeating patterns of thinking and behavior.


Some people lack the enzyme lactase, which is required to break down the disaccharide lactose into glucose and galactose. This is termed lactose intolerance. People with lactose intolerance cannot consume dairy products, or else they will experience uncomfortable side effects such as bloating, abdominal discomfort, and diarrhea.

People on a high-fiber diet may lose minerals that become bound to phytic acid, which comes from husks of legumes, grains, and seeds and binds to zinc, iron, calcium, magnesium, and copper. These minerals then get excreted from the body unused, which could have health consequences, though this is not common in people consuming a well-balanced diet.


  • What are some signs that might child might have low or high blood sugar?
  • How can I reduce added sugars in my family's diet?
  • Can you recommend a nutritionist or dietitian to help our family eat better?
  • Would a low-carb or no-carb diet be a good choice for me?
  • I have lactose intolerance, how should I manage my carb intake?

Parental concerns

Parents can consult with a pediatrician, physician, registered dietitian (RD), or endocrinologist if they suspect that their child suffers from low or high blood sugar levels or if they have questions about children's diets. Parents should try to limit their child's consumption of added sugars, especially from sugared beverages, and refined grains. They should not attempt to place their child on a diet without consulting with a healthcare professional.

See also Adolescent nutrition ; Adult nutrition ; Carbohydrate addict's diet ; Sugar ; Whole grains .



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Eliasson, Ann-Charlotte. Carbohydrates in Food. 3rd ed. Boca Raton, FL: CRC Press, 2016.

Warshaw, Hope S., and Karen M. Bolderman. Practical Carbohydrate Counting. 2nd ed. Alexandria, VA: American Diabetes Association, 2008.

Wrolstad, Ronald E. Food Carbohydrate Chemistry. Hoboken, NJ: Wiley-Blackwell, 2012.


Lattimer, James M., and Mark D. Haub. “c.” Nutrients 2, no. 12 (2010): 1266–89. 212-1266 (accessed March 18, 2018).

Lin, W. T, et al. “Effects on Uric Acid, Body Mass Index and Blood Pressure in Adolescents of [sic] Consuming Beverages Sweetened with High-Fructose Corn Syrup.” International Journal of Obesity 37, no. 4 (April 2013): 532–39. (accessed March 18, 2018).

Moon, Mary Ann. “High-Carb, Low-Glycemic Index Diet Cuts Weight, Cardiac Risk.” Family Practice News 36, no. 17 (September 1, 2006): 15.


Harvard T.H. Chan School of Public Health. “Carbohydrates.” The Nutrition Source, Department of Nutrition, Harvard University. (accessed April 30, 2018).

MedlinePlus. “Carbohydrates.” U.S. National Library of Medicine, National Institutes of Health. (accessed March 19, 2018).


Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600,, .

Food and Nutrition Information Center, National Agricultural Library, 10301 Baltimore Ave., Rm. 105, Beltsville, MD, 20705, (301) 504-5414, Fax: (301) 504-6409,, .

U.S. Department of Agriculture, 1400 Independence Ave. SW, Washington, DC, 20250, (202) 720-2791, .

Ken R. Wells
Revised by David Newton

  This information is not a tool for self-diagnosis or a substitute for professional care.