This is a fat-soluble vitamin which is essential to the assimilation of calcium and phosphorus. It is of vital importance in the formation of good bones and teeth. By some magic of numbers the "authorities" have decided that there are vitamins D2 and D3 but no vitamin D1. Vitamin D2, or calciferol, is produced by irradiation with ultra-violet light of the sterol, ergosterol. "It is not the vitamin found in cod liver oil," although it serves the same purpose. Identical with vitamin D of cod liver oil is vitamin D3 produced by irradiation of the sterol, 7-dehydrocholesterol, stored abundantly in the skin. It may be stored in the liver. The production of this vitamin by irradiation of the provitamin stored in the skin accounts for part of the value of sun bathing.

The "authorities" emphasize cod liver oil and other fish liver oils, butter and cream as sources of this vitamin. Green vegetables, peas, peanuts, almonds and other nuts, wheat bran and many other vegetable foods are abundant in vitamin D.

Due to greater calcium and phosphorus metabolism in early life, vitamin D is most needed in the first year of life. Mothers need it during the last two months of pregnancy and during lactation. Sunbathing by mothers will also be or great value.