Epidemiological studies of the relationship between diet and heart disease conducted in many different countries have shown that the countries with the lowest rates of heart disease can be lumped into two groups: those that eat a 'Mediterranean diet', such as Greece, Spain and Italy, and those that eat an 'Oriental diet', such as China and Japan. The factors common to both types of diet are:

1. they are low in saturated fat and animal products
2. they are low in processed foods
3. they are high in grains and vegetables.

The big difference between them is in the total fat content: the Mediterranean diet is not low fat, because of the olive oil used in cooking, whereas the Oriental diet is very low in all kinds of fat. The important point about olive oil, however, is that the fat is mono-unsaturated. There is overwhelming evidence that both of these diets are superior to the typical American or Northern European diets.

What is less firmly established is the benefit to be gained by simply restricting fat intake in the American diet. There is no dispute that switching to a low-fat diet can lower blood cholesterol levels, although the changes are often quite small; what is still hotly debated is whether moderate fat restriction can also prevent coronary heart disease. There have been at least 15 randomly controlled studies of the effects of a low-fat diet on heart attacks, with a net fall in blood cholesterol levels ranging from zero to 15%. Only five of them showed a significant reduction in the rate of heart attacks as a result of changing the diet, and only three studies showed a reduction in the overall death rate.

Studies using cholesterol-lowering drugs, which can produce bigger falls in blood cholesterol than dietary change, have been more encouraging and have shown that a cholesterol-lowering drug given to people who already have heart disease and a high blood cholesterol not only reduces the risk of further heart attacks, but also prolongs life.