What are carbohydrates?

Digestion and Absorption

Dietary carbohydrates include monosaccharides, such as glucose, fructose, and lactose; disaccharides (two monosaccharides linked together), such as sucrose and lactose; and polysaccharides (many monosaccharides linked together in polymers), such as starches and fiber.

Starches are first broken down in the mouth by salivary alpha-amylase and then in the small intestine by alpha-amylases of both salivary and pancreatic origin. The resulting simpler sugars
are further digested by enzymes linked to the inner lining of the small intestine: maltase, sucrase, and trehalase, which yield absorbable monosaccharides. These sugars cross the cells lining the small intestine via specialized molecular transport mechanisms and then diffuse into the intestinal capillaries and reach the bloodstream.

Metabolism

In the body, the main role of carbohydrates is energy production and storage. Carbohydrates can also be joined to proteins (glycoproteins, for cell-cell interactions) or fatty acids (glycolipids, which provide energy and can be markers for cellular recognition).

The body converts most digestible carbohydrates into glucose, which is a universal energy source for cells. Excess glucose is stored as glycogen (glycogenesis), which can then be broken down (glycogenolysis) when energy is needed. Glucose is maintained at a constant level in the blood by the interplay of insulin, gulcagon, and other hormones.

Carbohydrate-related diseases are often genetic in nature, linked to inborn errors in enzymes or cellular transporters. Examples are galactosemia, glycogen storage diseases, and lactose intolerance. Diabetes mellitus is a metabolic disorder characterized by excessive blood glucose. Type 1 diabetes is caused by insulin deficiency; type 2 can be the result of insulin resistance, impaired insulin secretion, and increased glucose production.

According to current recommendations, carbohydrates, preferably starches and natural sugars, should represent 40 to 60 percent of total calorie intake. Refined simple sugars provide calories but very little nutrition, and their intake should therefore be limited.

Perspective and Prospects

Food availability in developed countries has reached unprecedented levels, and the per capita consumption of carbohydrates, particularly in the form of refined sugars, increased dramatically in the late twentieth and early twenty-first centuries. Since the 1990s, the incidence of obesity has been climbing steadily, and so has the incidence of diabetes and related health problems. Current research in nutrition and carbohydrate metabolism is addressing the problem, which has reached epidemic proportions. Great progress is being made in dietary manipulations and drug development.

Bibliography

A.D.A.M. Medical Encyclopedia. "Carbohydrates." MedlinePlus, May 16, 2012.

Centers for Disease Control and Prevention. "Carbohydrates." Centers for Disease Control and Prevention, December 11, 2012.

Mayo Clinic. "Carbohydrates: How Carbs Fit into a Healthy Diet." Mayo Foundation for Medical Education and Research, February 8, 2011.

McGraw-Hill Encyclopedia of Science and Technology. 10th ed. 20 vols. New York: McGraw-Hill, 2007.

Sherwood, Lauralee. “The Digestive System.” In Human Physiology: From Cells to Systems. 7th ed. Pacific Grove, Calif.: Brooks, 2010.

Stanhope, K. L., and P. J. Havel. “Fructose Consumption: Considerations for Future Research on Its Effects on Adipose Distribution, Lipid Metabolism, and Insulin Sensitivity in Humans.” Journal of Nutrition 139, no. 6 (June, 2009): 1236S–41S.