15.1: Fat-Soluble Vitamins

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In a world of instant news and a media system constantly on the look-out for eye-catching news, research findings have become a favorite source of copy. The more astonishing, the better.

This complicates life for the serious scientist. The accumulation of scientific knowledge is seldom a story of breakthroughs, and all findings must be subjected to a process of peer review and verification before they can legitimately join the body of acknowledged scientific fact. And even then, there must be care in defining exactly what has been learned.

Nutrition provides abundant examples. Animal studies are often the only practical form of much research, and the results may or may not apply to humans. There is always the tendency to believe (and the alert merchandiser will encourage the belief) that more of something will cure what inadequacy could have caused, even where there has been no inadequacy. If too little of vitamin X causes hair loss and diminished sexual desire, a surplus, even for the well nourished, will surely grow hair on the bald and enhance the libido of the most reluctant lover.

There are no better examples than the fat-soluble vitamins. They are wondrous indeed, but not in the way the supplement sellers often would like us to believe.

Like the water-soluble vitamins, the fat-soluble vitamins are chemical substances needed by the body in tiny amounts, which must be supplied from outside sources. But unlike the water-soluble vitamins, they can be stored in large amounts, especially in the liver. So dietary excesses—much smaller excesses than in the case of the water-soluble vitamins—can become toxic.

The vitamin overdoses stimulated by nutrition faddism become serious when we look at the fat-soluble vitamins. Some excesses can be dangerous at levels as low as five times the RDA, which—according to some supplement enthusiasts—is not excessive at all.

General characteristics of fat-soluble vitamins

Absorbed with dietary fats
Stored with body fat
Not readily excreted
More easily toxic (A and D)

The four fat-soluble vitamins—vitamins A, D, E, and K—play many diverse roles, roles which are quite different from the predominantly coenzyme roles of the water-soluble vitamins. Starting with vitamin A, we will see what these roles are.

As with the water-soluble vitamins, the more interesting and instructive aspects of the fat-soluble vitamins are selected for discussion. Also, to minimize tedious discussion, basic information about each vitamin is summarized in Table 15-1, and the RDAs are found in Appendix A.