The cholesterol problem, the egg and lipid metabolism in the laying hen.

There is little doubt that high blood serum lipid levels are related to a higher incidence of atherosclerotic disease in humans. Experimental evidence to date suggests that dietary intervention can reduce blood lipid levels in most cases and that some small reduction in occurrence of cardiovascular disease will probably result. On the other hand no reduction in total mortality has been demonstrated in the well constructed dietary studies. It appears that there is considerable variation in the human population with regard to their patterns of lipid metabolism. Some apparently regulate body production of cholesterol in response to dietary changes, others do not. Some seem to excrete excess sterols efficiently, while some do not. It seems likely, therefore, that dietary manipulation would be useful for those disposed by heredity and other conditions to accumulation of excessive sterols in the body. On the other hand drug control of cholesterol biosynthesis and/or sterol excretion may be more effective solutions to the problem of sterol accumulation. Irrespective of whether diet or drugs prove to be the best answer to control of sterol balance, these should be applied only to that segment of the population known to require such treatment. The egg is an important dietary source of cholesterol and as a result is used sparingly in low cholesterol diets. On the other hand normal egg consumption of two eggs per day does not appear to overload cholesterol balance in the healthy human adult since depression in cholesterol biosynthesis and increased sterol excretion will result. Investigation of the lipid metabolism of the laying hen has shown that most of the cholesterol found in the egg is synthesized in the liver where it is under both dietary and drug control. Most of the cholesterol deposited in egg yolk may be essential for embryonic development. Drugs that severely limit cholesterol biosynthesis probably also limit synthesis of adrenal and sex hormones and hence limit reproduction. Moderate depressions in lipogenesis achieved without feeding of large amounts of dietary fat may offer a means for moderating cholesterol deposition in eggs. On the other hand, it also seems clear that genetic selection could be used to moderate egg cholesterol concentration. In any event, a great deal more evidence from well constructed human diet studies will be needed before low cholesterol diets can be recommended to the general population as an aid to control of cholesterol balance and heart disease.