Carbohydrates

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.

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.

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.

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.

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 .