diet and systemic diseases

The problems of undernutrition and overnutrition, as well as specific nutrient deficiencies, have been discussed; however, the composition of the diet, even in the absence of any of these problems, may make a significant contribution to the causation and progression of a number of diseases. A few examples suffice here.

Currently one of the most important and controversial issues is the contribution of diet to atherogenesis. The central question is, Can dietary modification prevent or retard the development of atherosclerosis (most importantly, coronary artery disease)? The average adult in the United States consumes an inordinate amount of fat and cholesterol daily, with a ratio of saturated fatty acids to polyunsaturated fatty acids of about 3:1.

Vegetable oils (e.g., corn and safflower oils) and fish oils contain polyunsaturated fatty acids and are good sources of cholesterol-lowering lipids. Fish oil fatty acids belonging to the omega-3, or n-3, family have more double bonds than do the omega-6, or n-6, fatty acids found in vegetable oils.

A recent meta-analysis of 11 studies with over 16,000 patients revealed that a diet enriched in omega-3 fatty acids (vs. placebo) significantly reduced the incidence of fatal myocardial infarction and sudden cardiac death.

There are other examples of the effect of diet on disease:
Hypertension is beneficially affected by restricting sodium intake.
Dietary fiber, or roughage, resulting in increased fecal bulk, has a preventive effect against diverticulosis of the colon.
People who consume diets that contain abundant fresh fruits and vegetables with limited intake of meats and processed foods have a lower risk of myocardial infarction.

One mechanism that may explain these epidemiologic observations is the association of hyperhomocysteinemia with increased intake of meats and decreased intake of vitamin B6, vitamin B12, and folate. Excess levels of homocysteine are hypothesized to contribute to atherosclerosis.

Calorie restriction has been convincingly demonstrated to increase life span in experimental animals. The basis of this striking observation is not entirely clearé

Even lowly garlic has been touted to protect against heart disease (and also, alas, kisses), although research has yet to prove the effect on heart disease unequivocally.