Although it is unlikely that you or your family will ever suffer from an illness caused by pronounced dietary deficiency, the foods you eat can exert more subtle and, in the long run, no less harmful effects on your health. During the past few decades, scientists have identified several dietary factors that play important roles in the development of specific diseases. Diets high in certain types of fat, for example, appear to increase the risk of developing coronary heart disease and certain cancers, and, among susceptible people, too much salt in food is believed to increase the chances of developing hypertension (high blood pressure). Other scientific evidence suggests that the current average American diet—which is high in fatty foods and low in fruits and vegetables—can increase the risk of developing certain forms of cancer, especially cancers of the esophagus, colon (large bowel), prostate, and breast. Certain dietary patterns can increase the likelihood of dental caries (cavities). In addition, habitually eating more calories than the body uses for maintenance and physical activity produces obesity and increases the risk of several chronic diseases including noninsulin-dependent diabetes mellitus, a form of diabetes that does not usually require daily insulin injections but has many adverse complications and generally appears after age 40.

As the body of research on diet-disease connections has grown over the past half century, scientists, policymakers, officials of the food industry, consumer groups, and others have engaged in a debate about how much and what kind of evidence justifies giving dietary advice to the public. They have also argued about how best to control risk factors on which there is general agreement among scientists.

The central problem in this debate is one that characterizes all science: absolute proof is difficult to obtain. This is particularly true in a science such as nutrition, in which many factors—age, sex, genetics, social behavior, and cultural differences, for example—can play a role in what food we eat and how it affects our bodies.