DEAR DR. DONOHUE Please discuss vitamin D and how older people can become deficient in winter. - W.L.

ANSWER - The vitamin D deck seems to be stacked against most older persons. That's important in light of the role of the vitamin in calcium absorption and utilization.The amount of D we get in food, while significant, is relatively small compared with what the body manufactures independently in a rather complex process. Sunlight causes skin cells to manufacture what is called the vitamin D pre-vitamin factor. The liver converts that substance into the actual vitamin.

Skin as it ages becomes less efficient in responding to sunlight and hence in production of the factor. In addition, older people tend to lessen their time out of doors or they may block the sun rays with sunscreen lotions. In one way or another this important vitamin source is lost. Limited intake of D-enriched foods (i.e., milk) contributes to the total deficiency picture. Natural D-rich foods include fish such as cod, mackerel and sardines.

As if all this were not enough to pose a threat of D deficiency, the lining of the digestive tract in older persons is a less efficient absorber of vitamin D from food sources.

All of these factors merit attention by older persons. If no ways can be found to correct a deficiency through food or sunlight exposure (be sure at least some sun exposure time is uninhibited by blockers) D supplementation may be needed.

A related question:

DEAR DR. DONOHUE - What is the difference between A and B ultraviolet light, and what is the kind the tanning salons deliver? - K.N.

ANSWER - A and B are two kinds of ultraviolet light (UVA and UVB). Only a small percentage of sunlight is ultraviolet, and of that percentage the A type predominates four to one over the B.

It is the UVB light that causes acute skin burning because it concentrates on the outer skin layers. The UVA rays penetrate more deeply and are more likely to cause tanning. In fact, the A type is what the tanning salons deliver.

However, by penetrating more deeply into tissue, the A light can have more dramatic long-range effects. Some authorities suggest that the A light can be the more insidious ager and damager of skin because it does not provide the overexposure warning signs of B light (burning and blistering).

The word from the experts is to avoid any overexposure to either form of ultraviolet light. One writer notes that our great grandmothers apparently realized these facts without having all the scientific information. But their parasols have long since gone out of vogue.