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Macronutrients in Health and Disease: Carbohydrates

Carbohydrate is the main energy source in the human diet, providing 50 percent or more of total calories in the form of starches and sugars. Carbohydrate–containing foods can be classified in several ways:

  • Simple vs. Complex Carbohydrate. The term simple carbohydrate refers to single (monosaccharide) and double (disaccharide) sugar molecules. Common monosaccharides include glucose and fructose, while common disaccharides include sucrose (table sugar). Examples of foods high in simple carbohydrate include table sugar, fruit, and milk.

Glucose C6H1206

Glucose
C6H12O6

Fructose C6H1206

Fructose
C6H12O6

 

Sucrose C12H22011

Sucrose
C12H22O11

A complex carbohydrate refers to multiple sugar molecules linked together by chemical bonds (polysaccharides, or “starch”). The health benefits of carbohydrate–containing foods are largely limited to those rich in complex (rather than simple) carbohydrate. Examples of foods high in complex carbohydrate include grains, legumes, starchy vegetables, pasta, and breads. Starches are complex carbohydrates that include cellulose, an important source of dietary fiber.

Cellulose

  • Refined vs. Unrefined Carbohydrates. Refining is a process by which the outer bran coating of grains is removed. In this process, brown rice is converted to white rice, for example, or whole wheat is converted to white flour. As a result, the food loses most of its fiber content. Note that a food can be rich in complex carbohydrate but also be refined. White rice and white bread, for example, are refined grain products, but retain their complex carbohydrate.
  • Glycemic Index. The glycemic index is a scale used to describe the effects carbohydrate–rich foods have on blood sugar levels. The glycemic index of a food is determined by feeding a portion of carbohydrate to healthy people after an overnight fast. Blood sugar is tested every 15 to 30 minutes over two hours, and the result is compared with that of feeding participants the same amount of pure glucose. Foods that are lower on the glycemic index tend to raise blood sugar more slowly than foods with a higher glycemic index.

These distinctions between various kinds of carbohydrate are clinically important. Diets high in sugars and refined carbohydrate may cause high blood sugar and increased triglycerides levels (the chemical form of fat in the blood). However, diets composed of low–glycemic–index foods and fiber tend to have the opposite effect. An analysis of 14 studies in individuals with diabetes showed that diets emphasizing low–glycemic–index foods improve control of blood sugar significantly. These studies showed a similar benefit for both type 1 and type 2 diabetes.

Diets that are high in carbohydrate and fiber and low in fat and cholesterol are particularly helpful for the prevention and treatment of several diseases, including obesity and weight–related diseases, such as diabetes and hypertension.

Studies indicate that people who consume approximately three servings per day of whole grain foods have a 20 to 30 percent lower risk of developing type 2 diabetes, compared with individuals consuming less than three servings per week. Low–fat, high–carbohydrate, high–fiber diets also significantly improve control of blood sugar. Such diets are also associated with significant improvements in blood cholesterol, blood pressure, and atherosclerosis and appear to be useful for preventing and treating some intestinal disorders, such as constipation, inflammatory bowel disease, peptic ulcer disease, and gastroesophageal reflux disease.

Although some weight–loss programs may imply that carbohydrates are responsible for weight or health problems, carbohydrates are essential to human health. Complex carbohydrates, in unprocessed or minimally processed forms, are abundant in the diets of countries where obesity and chronic diseases are rarely seen. Traditional Asian diets, for example, use rice and noodles as staple foods. Individuals following such diets have remarkably low risk of obesity, heart disease, diabetes, and most forms of cancer. In areas of changing dietary patterns, where carbohydrate–rich foods are being replaced by fat– and protein–rich foods, several chronic diseases have become much more common. In Japan, the Westernization of the diet occurring in the latter half of the 20th century resulted in a sharp decrease in rice consumption and an increase in meat and total fat intake, with corresponding increases in obesity, diabetes, cardiovascular disease, and other health problems. In surveys of Japanese adults over the age of 40, the rate of diabetes before 1980 was between 1 and 5 percent. By 1990, that number had gone up to 11 to 12 percent.

Carbohydrate Intolerance
Intolerance of certain kinds of carbohydrate is common.

Lactose intolerance is due to a deficiency of lactase, the enzyme in the gut that breaks down the milk sugar lactose. This enzyme normally disappears sometime after infancy. After lactase disappears, the consumption of milk or other lactose–contain products can cause bloating, cramping, diarrhea, and gas, which may be mistaken for a number of gastrointestinal diseases. Some authorities suggest limiting the consumption of lactose–containing products and using dairy substitutes, such as lactase–treated dairy products, soymilk, or rice milk. However, after the age of weaning, there is no nutritional need for either milk or milk substitutes (in fact, dairy consumption poses certain risks), and their use in the diet is based on preference, rather than nutritional requirement.

Other carbohydrate intolerances include sucrose intolerance, which is rare but may occur in individuals with kidney stones or HIV, and fructose intolerance, which may occur in individuals who eat large amounts of fructose (e.g., beverages with high–fructose corn syrup).

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