22. Egg yolk cholesterol and its functions in human health

Cholesterol is among the most studied nutritional components of egg yolk. Egg yolk is the major food source for cholesterol and has been considered as a food that should be avoided by many consumers. It has been known that serum cholesterol plays a major role on the development of cardiovascular diseases, but the effect from dietary cholesterol is not significant. Although cholesterol is blamed for various human diseases including cardiovascular diseases, it also has very important biological functions in the animal body. Cholesterols in egg as well as other food sources can be oxidized easily during prolonged storage. Cholesterol oxidation products are considered to have a stronger toxicity than natural cholesterol, and considered as the initiator of the cascade of coronary heart diseases. However, oxidation of cholesterol also reduces the bioavailability of cholesterol. There are many hypotheses on how cholesterol is involved in the development of atherosclerosis, which include cholesterol, response-to-injury, response-to-retention, and oxidation hypotheses. However, new emerging theories, which re-evaluate the roles of cholesterol on coronary heart diseases, are under way.