The effect of diet on coronary heart disease (CHD) likely occurs through multiple biologic pathways, leaving single intermediate markers, such as low-density lipoprotein (LDL) cholesterol, with a limited role. Hu and Willett conducted a MEDLINE search of epidemiologic and clinical studies to examine the complex relationship between diet and CHD.
Few dietary trials with clinical end points exist. In their selection of 147 original investigations and reviews of metabolic, epidemiologic, and dietary intervention trials, the authors emphasized large prospective cohort studies with established intermediate end points. They examined fat, cholesterol, omega-3 fatty acids, trans-fatty acids, carbohydrates, glycemic index, fiber, folate, specific foods, and dietary patterns.
Overall, the search yielded the following findings. When monounsaturated or polyunsaturated fat replaces saturated fat, LDL cholesterol levels decrease, with little change in high-density lipoprotein (HDL) cholesterol levels. Trans-fatty acids raise LDL cholesterol levels and lower HDL cholesterol levels relative tocis-unsaturated fatty acids and may increase the risk of diabetes.
Consumption of fish and omega-3 fatty acids appears to lower the risk of CHD and sudden death, particularly as secondary prevention. Carbohydrates with a low glycemic index are likely to improve glycemic control and lipid profile. Glycemic load (the product of the glycemic index and carbohydrate content) is more strongly associated with higher fasting triglyceride levels and lower HDL cholesterol levels compared with glycemic index alone, with a strong positive correlation shown between glycemic load and CHD. Whole grains are associated with a lower risk of CHD than refined grains. Two randomized controlled trials suggest that folic acid supplementation lowers the risk of CHD.
Specific foods also have been shown to lower the risk of CHD, including the replacement of red meat with chicken, fish, and nuts. A significant inverse association has been established between fruit and vegetable consumption and risk of CHD. Potato consumption did not produce any benefits. In general, an overall diet with high fruit and vegetable intake, whole grains, poultry, and fish is superior to a diet of red meat, potatoes, french fries, and refined grains. The former diet, when ensuring fiber, folate, and omega-3 fatty acid consumption, with low trans-fatty acids and a low glycemic load, strongly predicts a decrease in CHD. According to the Nurses' Health Study, potentially 82 percent of CHD events could be prevented by moderate diet and lifestyle interventions.
The authors conclude that at least three dietary strategies are effective in preventing CHD: substitute unsaturated fats for saturated and trans-fats; increase consumption of omega-3 fatty acids from fish oil or plant sources; and eat a diet high in fruits, vegetables, nuts, and whole grains, and low in refined grains. Simply lowering the percentage of total fat in the diet is unlikely to lower cholesterol levels or reduce CHD risk. Combining the above dietary intervention with regular physical activity, maintaining a healthy weight, and not smoking may prevent the majority of CHD events in Western populations.
editor's note: Although there are few surprises in this review, the three dietary recommendations provide a useful summary for patient education purposes. Also important, in view of recent claims of efficacy for such high–saturated-fat diets as the Atkins diet, this study reiterates the impact of different kinds of fat on CHD risk. Notably, the authors do not recommend an overall lowering of the fat content of the diet.—c.w.