Sunshine a vital source of vitamin D


March 15, 1994|By Colleen Pierre, R.D. | Colleen Pierre, R.D.,Special to The Sun

A little bit of "sunshine vitamin D" may strengthen your bones. And a little too much winter may weaken them.

Vitamin D is unusual because we really don't get much of it from food. In fact, ultraviolet rays create vitamin D in the skin. Dynamic D helps our bodies absorb calcium from food and incorporate it into our bones. Without vitamin D, we cannot maintain strong bones, no matter how much calcium we consume. In fact, calcium supplementation is not helping women in early menopause. Since the mean vitamin D intake for U.S. women is only 1/2 the RDA, they may be suffering mild vitamin D deficiency.

At a recent conference on vitamins and women's health, Dr. Beth Dawson-Hughes reported that women from temperate latitudes, and north, have bone calcium losses from November through March, because the sun's ultraviolet rays don't reach the Earth.

In Baltimore, UV rays are adequate to maintain bone density, if we average about 15 minutes a day in sunlight with hands and face exposed.

The Recommended Dietary Allowance (RDA) for vitamin D is set by the National Research Council at 200 International Units (I.U.) per day from food.

Eight ounces of milk is fortified to contain 100 I.U. If you drink two glasses of milk a day, you've got it made. However, cheese and yogurt are not made from fortified milk, so they contain very little D. One ounce of Cheddar cheese, for example, offers only 3 I.U.

Oily fish offers considerably more. You get a hefty dose from three ounces of canned salmon (426 I.U.), sardines (255) or smoked eel (5442). A teaspoon of cod liver oil provides 400 I.U. But how often do you eat those foods?

A winter supplement may be a good idea.

Dr. Dawson-Hughes is chief of the Calcium and Bone Metabolism Laboratory at the U.S. Department of Agriculture's Human Nutrition Research Center at Tufts University School of Medicine in Boston. She was quick to point out that this research is the first of its kind and needs to be repeated and refined before clear supplementation answers are available.

In one study where women were treated with 400 I.U. of vitamin D, they experienced no winter decline in bone calcium, but stable bones might be achieved at 300 or even 200 I.U.

Elderly women are at even greater risk for bone calcium loss, because they have less sun exposure, aging skin does not create vitamin D so easily, and intestines don't absorb calcium from food as well. Dr. Dawson-Hughes says elderly women may need 800 I.U. per day. Caution is in order, however.

Vitamin D is fat-soluble and can be stored, so it can become toxic. The result? The same as with too little D, soft bones. Toxicity levels for children can begin with as little as five times the RDA. Exact toxicity levels for adults are harder to pin down.

Other factors do affect vitamin D needs. Black women have a 10 percent greater bone mass, even at birth, so are at lower risk to begin with. Dark skin (including skin that is deeply tanned) is less efficient at making vitamin D.

Colleen Pierre, a registered dietitian, is the nutrition consultant to the Union Memorial Sports Medicine Center and Vanderhorst & Associates in Baltimore.

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