To Your Health

A. Vitamin D (also known as cholecalciferol) is different from other vitamins, and in fact it may not really be a vitamin. The body uses cholecalciferol to make an important steroid hormone, and this hormone does not activate cellular enzymes as most other vitamins do. Rather, it regulates the way the DNA in the cell nucleus operates, which in turn regulates the absorption and distribution of calcium and phosphorus. There is new evidence that receptors for vitamin D are present on many cells and that this hormone has repercussions that extend far beyond control of calcium and phosphorus.

Vitamin D's most dramatic effect is almost certainly the promotion of intestinal absorption of calcium, although it also assists in the absorption of phosphorus and magnesium. Calcium is absorbed very inefficiently in the absence of vitamin D, because the production of a number of the proteins that transport calcium from the intestine to the bloodstream depends on stimulation by vitamin D.

The best and most frequently used test for vitamin D is measuring the serum level of 25-hydroxyvitamin D (25-OH-D), because this test detects vitamin D excess as well as insufficiency. Before starting a vitamin D supplement, since it can be quite toxic, it would be wise to have your 25-OH-D level tested. Other test results, such as a rise in serum alkaline phosphatase, can suggest vitamin D deficiency but give no hint of excess. Also, other nutrient deficiencies, specifically of zinc, can affect alkaline phosphatase levels. The 25-OH-D test is fairly expensive, but a cheaper saliva test may soon be available.

If the test indicates a need for more vitamin D, the best way to get it is to expose your own skin to the ultraviolet rays of normal sunlight. This way of increasing vitamin D is self-regulating and individualized. In the southern tier of states, at noon, a light-skinned person can produce about as much vitamin D as they need for the day in as little as 10 minutes. Longer exposure is needed in northern states, or at other times of the day, or for people with darker skin. Exposure past 20 minutes may actually destroy some of the vitamin D, and it certainly increases the risk of sunburn and the risk of skin cancer that accompanies sunburn.

A lot of people living in America are working during the ideal times to go outside and generate some vitamin D through sunlight on our skins. If you decide supplements would be better for you, there are two forms of vitamin D: cholecalciferol, or vitamin D3, and ergocalciferol, or vitamin D2. Vitamin D2, a synthetic analogue of vitamin D3, is less biologically active and has a greater potential for harm. Unfortunately it is often used to fortify milk and other foods because it is less expensive.

Nursing mothers should be aware that the vitamin D content of their breast milk depends on their having enough of the vitamin to be able to donate some to their child. Rickets, a disorder most commonly caused by vitamin D deficiency, appears to be on the rise, especially in African-American children. Unless you suffer from one of the few diseases that are made worse by vitamin D supplements, you can likely take 1,000 IU/day of vitamin D3 with no ill effects.