The diet-heart or lipid hypothesis, based on the atherosclerosis theory of coronary heart disease, consists of a sequence of events involving dietary cholesterol and fats, blood cholesterol, atherosclerosis, and ultimately coronary heart disease. The most rigorous statistical studies have shown very weak or nonexistent relationships between diet or blood cholesterol and coronary heart disease.

Blood Cholesterol Levels and Coronary Heart Disease Rates

The underlying factor in the atherosclerosis theory of coronary heart disease is cholesterol in the human body. Cholesterol is found in all cell membranes and is especially prevalent in organs like the brain, liver, and kidneys; it plays a key role in the production of some hormones, steroids, and vitamin D; and it is converted to bile that is essential for digestion. Choles terol tends to accumulate in atheromas because it is a lipid, a fatty or greasy compound that does not dissolve in water or blood and so cannot be removed by circulating blood. Cholesterol was identified and labeled in the early nineteenth century and its presence in atheromas was noted a century later.

Cholesterol in the human body is obtained from both external and internal sources. About one-third of the cholesterol found in the intestine comes from the consumption of animal foods, primarily meat, eggs, and dairy products. Two-thirds comes from internal synthesis in the intestines and liver. The human body regulates the total amount of its blood cholesterol by balancing internal synthesis and dietary intake.

Cholesterol is transported from its sources in the intestine and liver to cells by flowing through the blood as lipoproteins, a soluble chemical combination of cholesterol and certain proteins. About 1950 John Gofman and his coworkers differentiated several types of lipoproteins according to their densities. The low density lipoproteins (LDL) transport 60%–70% of the cholesterol through the blood, the high density lipoproteins (HDL) transport 20%–30%, and the very low density lipoproteins (VLDL) transport 10%–15%.