For health’s sake, most people should increase their intakes of carbohydrate-rich foods such as whole grains, vegetables, legumes, and fruits—foods noted for their starch, fiber, and naturally occurring sugars. In addition, most people should also limit their intakes of foods high in added sugars and the types of fats associated with heart disease. A diet that emphasizes whole grains, vegetables, legumes, and fruits is almost invariably moderate in food energy, low in fats that can harm health, and high in dietary fiber, vitamins, and minerals. All these factors working together can help reduce the risks of obesity, cancer, cardiovascular disease, diabetes, dental caries, gastrointestinal disorders, and malnutrition.
Carbohydrates: disease prevention and recommendations
Fiber-rich carbohydrate foods benefit health in many ways. Foods such as whole grains, legumes, vegetables, and fruits supply valuable vitamins, minerals, and phytochemicals, along with abundant dietary fiber and little or no fat. The following paragraphs describe some of the health benefits of diets that emphasize a variety of these foods each day.
- Heart Disease
Diets rich in whole grains, legumes, and vegetables, especially those rich in whole grains, may protect against heart disease and stroke by lowering blood pressure, improving blood lipids, and reducing inflammation. Such diets are generally low in saturated fat, trans fat, and cholesterol and high in dietary fibers, vegetable proteins, and phytochemicals—all factors associated with a lower risk of heart disease. Foods rich in soluble fibers (such as oat bran, barley, and legumes) lower blood cholesterol by binding cholesterol compounds and carrying them out of the body with the feces. High-fiber foods may also lower blood cholesterol indirectly by displacing fatty, cholesterol-raising foods from the diet. Even when dietary fat intake is low, research shows that high intakes of soluble fiber exert separate and significant blood cholesterol–lowering effects.
- Diabetes
High-fiber foods—and especially whole grains—play a key role in reducing the risk of type 2 diabetes. The soluble fibers of foods such as oats and legumes can help regulate the blood glucose following a carbohydrate-rich meal. Soluble fibers trap nutrients and delay their transit through the digestive tract, slows absorption of glucose and prevents the glucose surge and rebound often associated with diabetes onset.
The term glycemic response refers to how quickly glucose is absorbed after a person eats, how high blood glucose rises, and how quickly it returns to normal. Slow absorption, the moderate rise in blood glucose, and a smooth return to normal are desirable (a low glycemic response). Fast absorption, a rise in blood glucose, and an overreaction that plunges glucose below normal are less desirable (a high glycemic response). Different foods have different effects on blood glucose. The glycemic index, a method of classifying foods according to their potential to raise blood glucose.
- GI Health
Soluble and insoluble fibers, along with ample fluid intake, may enhance the health of the large intestine. The healthier the intestinal walls, the better they can block absorption of unwanted constituents. Soluble fibers help to maintain normal colonic bacteria necessary for intestinal health. Insoluble fibers that both enlarge and soften stools such as cellulose (in cereal brans, fruits, and vegetables) ease elimination for the rectal muscles and thereby alleviate or prevent constipation and hemorrhoids.
Some fibers (again, such as cereal bran) help keep the contents of the intestinal tract moving easily. This action helps prevent compaction of the intestinal contents, which could obstruct the appendix and permit bacteria to invade and infect it. In addition, fibers stimulate the muscles of the gastrointestinal (GI) tract so that they retain their strength and resist bulging out in places, as occurs in diverticulosis. Insoluble fiber seems to be most beneficial in lowering the risk of diverticulosis.
- Cancer
Many studies show that, as people increase their dietary fiber intakes, their risk for colon cancer declines. A recent meta-analysis using data from several studies exposed a strong, linear inverse association between dietary fiber and colon cancer. People who ate the most fiber (24 grams per day) reduced their risk of colon and rectal cancer by almost 30 percent compared with those who ate the least (10 grams per day). Mid-range intakes (18 grams per day) reduced the risk by 20 percent. Importantly, fiber from food but not from supplements demonstrates this association, possibly because fiber supplements lack the nutrients and phytochemicals of whole foods that may also help to protect against cancers.
All plant foods—vegetables, fruits, and whole-grain products—have attributes that may reduce the risks of colon and rectal cancers. Their fiber dilutes, binds, and rapidly removes potential cancer-causing agents from the colon. In addition, the colon’s bacteria ferment soluble fibers, forming small fatlike molecules that lower the pH. These small fatlike molecules activate cancer-killing enzymes and inhibit inflammation in the colon.
Other processes may also be at work. As research progresses, cancer experts recommend that fiber in the diet come from 5 to 9 half-cup servings of vegetables and fruit daily, along with generous portions of whole grains and legumes.
- Weight Management
Fiber-rich foods tend to be low in fat and added sugars and therefore prevent weight gain and promote weight loss by delivering less energy per bite. Moreover, fibers absorb water from digestive juices as they swell, create feelings of fullness, interrupt hunger, and lower food intake. Soluble fibers is effective for appetite control. In a recent study, soluble fiber from barley shifted the body’s mix of appetite-regulating hormones toward reducing food intake. By whatever mechanism, as populations eat more refined low-fiber foods and concentrated sweets, body fat stores creep up. In contrast, people who eat three or more whole grain servings each day tend to have lower body and abdominal fatness over time.
Weight-loss products contains bulk-inducing fibers (such as methylcellulose), but pure fiber compounds are not advised. High-fiber foods not only add bulk to the diet but are economical, are nutritious, and supply health-promoting phytochemicals— benefits that no purified fiber preparation can match.
Harmful Effects of Excessive Fiber Intake
Despite fiber’s benefits to health, when too much fiber is consumed, some minerals may bind to it and be excreted with it without becoming available for the body to use. An adequate intake of minerals and high-fiber foods does not seem to compromise mineral balance.
People with marginal intakes who eat mostly high-fiber foods may not be able to take in enough food to meet energy or nutrient needs. The malnourished, the elderly, and young children adhering to all-plant (vegan) diets are especially vulnerable to this problem. Fibers also carry water out of the body and can cause dehydration. Advice clients to add an extra glass or two of water to go along with the fiber added to their diets. Athletes may want to avoid bulky, fiber-rich foods just prior to competition.
Recommended Intakes of Starches and Fibers
The DRI committee advises that carbohydrates should contribute about half (45 to 65 percent) of the energy requirement. A person consuming 2000 calories a day should therefore obtain 900 to 1300 calories’ worth of carbohydrate, or between 225 and 325 grams. This amount is more than adequate to meet the RDA for carbohydrate, which is set at 130 grams per day based on the average minimum amount of glucose used by the brain.
When it established the Daily Values that appear on food labels, the FDA used a guideline of 60 percent of calories in setting the Daily Value for carbohydrate at 300 grams per day. For most people, this refers intake of total carbohydrate. The Dietary Guidelines for Americans encourages people to choose fiber-rich whole grains, vegetables, fruits, and legumes daily.
Recommendations for fiber encourage the same foods just mentioned: whole grains, vegetables, fruits, and legumes, which also provide vitamins, minerals, and phytochemicals. The FDA set the Daily Value for fiber at 25 grams, rounding up from the recommended 11.5 grams per 1000 calories for a 2000-calorie intake. The DRI recommendation is slightly higher, at 14 grams per 1000-calorie intake—roughly 25 to 35 grams of dietary fiber daily. These recommendations are about two times higher than the usual intake in the United States.
As health care professionals, advise clients that an effective way to add dietary fiber while lowering fat is to substitute plant sources of proteins (legumes) for some of the animal sources of protein (meats and cheeses) in the diet. Another way to add fiber is to encourage clients to consume the recommended amounts of fruits and vegetables each day. People choosing high-fiber foods are wise to seek out a variety of fiber sources and to drink extra fluids to help the fiber do its job. Many foods provide fiber in varying amounts.
As mentioned earlier, too much fiber is no better than too little. The World Health Organization recommends an upper limit of 40 grams of dietary fiber a day.
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