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Institute of Medicine (US) Committee on Diet and Health; Woteki CE, Thomas PR, editors. Eat for Life: The Food and Nutrition Board's Guide to Reducing Your Risk of Chronic Disease. Washington (DC): National Academies Press (US); 1992.

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Eat for Life: The Food and Nutrition Board's Guide to Reducing Your Risk of Chronic Disease.

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Chapter 7Protein, Carbohydrates, And Chronic Diseases

As nutrients, protein is overemphasized and carbohydrates are underrated in terms of their importance in our diets. Most people in the United States now eat more protein than their bodies need, and, somehow, carbohydrate-rich foods such as potatoes and bread have gotten a bad name for being fattening. Yet protein from animal sources often comes in foods that contain significant amounts of saturated fatty acids. And the only carbohydrate-rich foods that are truly fattening are those laden with fats and added sugar, such as pastries, cookies, and baked potatoes heaped with sour cream.

In fact, there is some evidence that diets high in carbohydrates may help reduce the risk of heart disease. Thus the Eat for Life guidelines suggest not to eat any more protein than you already do and to increase the amount of carbohydrates you eat to compensate for the lower amount of fat that your diet will contain.

This chapter presents the evidence that increasing the proportion of carbohydrates in the usual American eating pattern at the expense of fat will tend to lower chronic disease risk. It also discusses some of the health claims made for dietary fiber, a group of substances that are made up mostly of complex carbohydrates.


Heart Disease

Health experts have suspected that protein from animal sources contributes to an increased risk of heart disease, since people who eat diets high in animal protein usually suffer more heart attacks than people who get their protein mainly from plant sources. But it may be that animal protein is not to blame at all. Any association of protein intake with high cholesterol levels and high cardiovascular disease risk in populations is largely explained by the levels of saturated fatty acids in the protein-containing food. Many foods of animal origin, such as meats and dairy products, contain saturated fatty acids. So in all likelihood it is the fat content of diets high in animal protein that causes an increase in heart attacks, not the protein.

There is evidence, though, that vegetable protein may exert some beneficial effects that animal protein does not. In one experiment, for example, volunteers with high serum cholesterol levels ate diets in which all their protein came from soybeans. Both serum cholesterol and LDL-cholesterol levels dropped substantially in this group.


As with heart disease, diets that are high in animal products appear to increase the risk for cancer. But, also as with heart disease, this seems to be more a factor of the animal fat associated with animal protein than the protein itself.


There seems to be little increased risk of osteoporosis from eating a high-protein diet. Consumption of large amounts of purified animal protein is associated with calcium loss in the urine. But few of us increase the amount of protein we eat by adding purified animal protein to our diets. Instead, we eat protein-containing foods like meat, which contains significant amounts of phosphorus, and phosphorus seems to minimize any effect protein has on calcium excretion. Therefore you do not need to worry that eating meat increases your chances of developing osteoporosis.


Dental Caries

There is little doubt that simple carbohydrates, otherwise known as sugars, are involved in causing dental caries. This is particularly true when sugar is eaten between meals and as part of sticky foods. It also appears, though, that most carbohydrates, as least to some extent, can cause cavities.

Carbohydrates play a role in tooth decay by acting as an energy source for the bacteria that live in the mouth. These bacteria break down sugar and produce acids that can remove minerals from teeth. The body has mechanisms for putting those minerals back into teeth, but once the bacteria get a good hold on a particular tooth, the microbe can break down a tooth's surface faster than the body can rebuild it. Eventually, if nothing is done to disturb acid production by the bacteria, a cavity develops.

Not all carbohydrates are as effective in feeding this process. Sucrose—common table sugar—appears to be the worst offender. Fructose, a sugar in honey and fruit, is not as bad, but it, too, serves as a good energy source for mouth bacteria. Foods rich in complex carbohydrates and fiber may actually help protect against cavities. Chewing them stimulates the flow of saliva and neutralizes the acids produced by the bacteria.

Developing tooth decay is not simply a matter of how much sugar you might eat, but when and with what foods. For example, eating cheese right after eating sugar seems to neutralize the acid that mouth bacteria produce.

Experiments show, too, that it is not the amount of sugar you consume, but how you eat it—sucking on hard candy all day would promote tooth decay more than eating one large piece of rock candy in 15 minutes. Soft drinks are less apt to promote cavities than hard candies—sugar in solid foods promotes cavities more than does sugar in liquid foods.

Noninsulin-Dependent Diabetes

Contrary to popular belief, diets high in carbohydrates are not associated with an increased risk of developing noninsulin-dependent diabetes. In fact, the opposite seems to be true: The risk of developing noninsulin-dependent diabetes decreases as the amount of calories from carbohydrates increases.

Compared to low-carbohydrate diets, diets high in carbohydrates improve the body's sensitivity to insulin. Therefore, many physicians recommend that their patients with noninsulin-dependent diabetes switch to a high-carbohydrate, low-fat diet. Such a switch seems to reduce the number of symptoms these people experience, although the best course of action for this disease is to lose weight. Another reason why a high-carbohydrate, low-fat diet may also benefit people with noninsulin-dependent diabetes is because it reduces their risk of developing heart disease, a major cause of death among people with diabetes.


The word "fiber" actually describes many different mixtures of carbohydrates and other large molecules present in almost all plant foods. An eating pattern that is high in fiber is high in vegetables, whole-grain products, fruits, and legumes (beans and peas) and low in animal products. A high-fiber eating pattern is high in complex carbohydrates and relatively low in fat.

Because the mix called ''fiber" may be made up of many different substances, depending on the source, it has been difficult to clarify the role of fiber in health and disease. What evidence we have appears to be positive.

There are two kinds of fiber: insoluble fiber, which exerts its effects primarily in the digestive system, and soluble fiber, which has effects on substances in the blood stream. Diets high in both kinds of fiber tend to be bulky, and since fiber itself does not contribute calories, foods high in fiber tend to contain fewer calories in the same volume of food. These characteristics of high-fiber diets may help assuage hunger and thus contribute to weight loss.

Fiber also stimulates the liver to produce more bile, thus aiding digestion. Insoluble fiber also causes digested food to pass through the intestines more quickly, thus contributing to "regularity" and reducing the length of time the gut lining is in contact with any potentially harmful substances contained in the digested food.

Heart Disease

People who eat diets that are high in fiber have significantly lower serum cholesterol and higher HDL-cholesterol levels than people who eat low-fiber diets. Comparisons of cholesterol levels among populations eating different levels of fiber—complete vegetarians, lacto-ovovegetarians, and nonvegetarians—showed that the high-fiber-consuming vegetarians had the lowest serum cholesterols, followed by the lacto-ovovegetarians, and finally the lowest-fiber-consuming nonvegetarians.

Studies show that it is soluble fibers (like those in fresh fruits, vegetables, and beans) rather than insoluble fibers (like those in the bran of wheat) that appear to have an effect on serum cholesterol. Indeed, studies have shown that guar gum, pectin, and oat bran—all soluble fibers—seem to lower LDL-cholesterol. The connection between high-fiber diets and fewer heart attacks is less clear. Some studies suggest that it is the low-fat content of high-fiber diets that reduces the risk of heart disease. Other studies, however, show that eating a diet high in fiber, regardless of its fat content, reduces the risk of developing heart disease. For now, the question of whether or not a high-fiber diet will protect you against heart disease remains unanswered. What is clear, though, is that a high-fiber eating pattern will help to lower your serum cholesterol and also help to lower your risk of heart disease.


Cancer of the large intestine is rare in Africa, where people eat diets high in fiber, which suggests that fiber may protect against colon cancer. But studies of groups of people who differ in the amount of fiber they eat have not proven this idea true. The conflicting results—some studies showed a protective effect and others showed no effect—may stem from the problems of comparing fibers from different sources.

Other Chronic Diseases

Fiber, according to what you might read in magazines and newspapers, seems to be the one dietary component that affords some protection against nearly every chronic disease known. While that may, indeed, prove to be true, the scientific evidence so far is sketchy. Various studies have shown, for instance, that diets high in fiber may benefit people with noninsulin-dependent diabetes and may even help prevent this disease. Other studies have suggested that high-fiber diets can lower high blood pressure and reduce the chances of developing gallstones. But all of these studies focus on fiber-rich diets, not fiber itself. It may be that other components of these diets are at work, perhaps in combination with fiber, in lowering the risk for these chronic diseases.

Can Too Much Fiber Be Bad?

Some health experts have raised the concern that high-fiber diets may make it difficult for the body to absorb important minerals from the digestive system. This does not appear to happen, however. For example, vegetarians eating high-fiber diets have normal levels of iron, zinc, copper, and selenium in their bodies. Similarly, the levels of iron, calcium, and magnesium are the same in people with diabetes who eat a high-fiber diet and in those who eat the average U.S. diet. The conclusion seems to be that there is little evidence that high-fiber diets alone will produce a mineral deficiency in people who otherwise consume a balanced diet.

You should be aware, however, that switching abruptly from a low-fiber diet to one high in fiber may make you feel bloated and nauseous and may cause flatulence. This is particularly true if the increased fiber comes from wheat bran and guar gum. These effects are temporary, though, and diminish after a few weeks. On the plus side, a high-fiber diet reduces constipation and contributes to more regular bowel movements. You are likely to be more comfortable if you gradually increase your intake of fiber-rich foods.

One final word on fiber—do not start taking fiber supplements based on what you have read here. There is an impressive amount of evidence showing the beneficial effects of eating a diet containing a large amount of fruits and vegetables (i.e., fiber-containing foods), and relatively low levels of meat and fatty products. We do not know, yet, whether the benefits of such a diet are the result of the large amounts of fiber and carbohydrates, low amounts of fat, other protective factors in plants, a combination of the three, or some other dietary factor. Therefore, although it is desirable to eat a diet containing fiber-rich foods, it is not necessary to take fiber supplements unless specifically advised to do so by your doctor.

Copyright © 1992 by the National Academy of Sciences.
Bookshelf ID: NBK235012


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