MANIPULATION OF EGG-YOLK CHOLESTEROL - A PHYSIOLOGISTS VIEW

Concern about the cholesterol content of the human diet has led to many attempts to reduce the cholesterol content of eggs. Although this has involved a number of different approaches, including genetic selection and nutritional or pharmacological manipulation, the overwhelming evidence is that egg yolk cholesterol level is very resistant to change. The present review argues that this is because of the particular mechanisms involved in yolk formation. Yolk precursors are synthesized in the liver of the laying hen and transported in the plasma to the ovary where they are taken up into the developing follicles by receptor-mediated endocytosis. As a consequence, the cholesterol content of the yolk is primarily dependent on the cholesterol content of triglyceride-rich lipoproteins. Studies in mammals have shown that inhibition of cholesterol synthesis can reduce the rate of synthesis and secretion of lipoproteins by the liver, but has little effect on the composition of the lipoproteins that are secreted. This response, and the key role of cholesterol in the synthesis of steroid hormones, would seem to preclude the possibility of any substantial reductions in egg yolk cholesterol levels through manipulation of cholesterol metabolism. Possible alternative approaches are briefly discussed.