Of the four fat -soluble vitamins, vitamin D is the one concerned with efficient calcium and phosphorus absorption. Working with various hormones-- in particular, calcitonin and the parathyroid hormone--vitamin D's primary role is to ensure that calcium and phosphorous are absorbed from the intestinal tract into the bloodstream. There, the vitamin helps maintain concentrations of the two minerals in the right proportions (the amounts of calcium and phosphate in the blood--as well as the ratio between them--help determine normal bone growth). In addition, vitamin D also appears to act in the bones themselves, stimulating them to accept calcium into their bony matrices.

Strictly speaking, vitamin D is not one vitamin but is, instead, a group of vitamins--all derived from a parent compound, structurally similar to cholesterol--and it comes in a variety of forms. From a nutritional standpoint, two of these forms are especially important.

The first...