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Cholesterol, the cornerstone of Dr. Keys's theory, is a mysterious yellowish, waxy substance, chemically a crystalline alcohol.

Scientists assume that cholesterol ( from the Greek chole, meaning bile, and sterios, meaning solid ) is somehow necessary for the formation of brain cells, since it accounts for about 2% of the brain's total solid weight.

They know it is the chief ingredient in gallstones.

They suspect it plays a role in the production of adrenal hormones, and they believe it is essential to the transport of fats throughout the circulatory system.

But they cannot fully explain the process of its manufacture by the human liver.

Although the fatty protein molecules, carried in the blood and partly composed of cholesterol, are water soluble, cholesterol itself is insoluble, and cannot be destroyed by the body.

`` A remarkable substance '', says Dr. Keys, `` quite apart from its tendency to be deposited in the walls of arteries ''.