A bit of sunshine is good for your bones.

You know calcium is essential for strong bones. But to enhance the amount of calcium that ultimately reaches our bones, you also need vitamin D.

Earlier this century vitamin D earned its reputation as an essential nutrient when doctors discovered a deficiency of the vitamin led to rickets. In this childhood disease bones fail to develop properly, leading to bowlegs and knock-knees.

The eventual development of vitamin D-fortified milk virtually eliminated vitamin D deficiencies in the United States. However, accumulating information raises new concern about vitamin D, suggesting some of you may not be getting enough.

Your body makes vitamin D from two sources--sunlight and food.

Most of the vitamin D your body makes starts with the sun. When you're exposed to ultraviolet (UV) rays, a chemical in your skin is changed into an inactive form of vitamin D.

Inactive vitamin D is carried by the blood to your liver, where it goes through another chemical change. Finally, your kidneys change vitamin D into the active form your body can use.

In its active form, vitamin D helps your body absorb calcium from your small intestine. Then it helps deposit calcium in your bones and teeth.

A prolonged deficiency of vitamin D and calcium can lead to osteoporosis, in which your bones become thin, brittle, and vulnerable to fracture. Less frequently, bones can develop osteomalacia (os-te-o-muh-la-shuh), an adult form of rickets characterized by soft and misshapen bones.

Despite the availability of the sun and vitamin D-fortified foods, several factors can interfere with getting enough of this essential nutrient:

* Too little sun. Because your body makes vitamin D from sunlight, deficiencies of the vitamin are uncommon in most people regularly exposed to the sun. Just 10 to 15 minutes of direct sun exposure to your face, arms, and hands three times a week stimulates adequate production of vitamin D.

However, people who are confined indoors because of health problems or who live in nursing homes can be deficient in vitamin D. Even living where weather limits the time you spend outdoors reduces year-round sun exposure and may jeopardize your vitamin D status.

During the winter you typically spend more time indoors. And when you are outside, warm clothing shields your skin against cold and sun. In addition, the sunlight you do receive is less intense.

A 1990 study found 80 percent of people ages 66 to 99 in the Boston area had reduced stores of vitamin D during the winter, largely because of less time in the sun and reduced intensity of the rays.

* Age. As you get older, your body turns UV rays into vitamin D less efficiently.

* Illness. Kidney or liver disease reduces your ability to change vitamin D into its usable form. Bowel diseases, such as sprue, that impair your ability to absorb fat can also limit absorption of vitamin D. Medications such as phenytoin (FEN-ih-to-in), sold as Dilantin and prescribed for epilepsy and abnormal or irregular heart rhythms, can also lead to vitamin D deficiency.

Aim for 10 to 15 minutes of summer sun three times a week. The vitamin D you make during the summer is stored in your liver for use in early winter.

Brief periodic exposures to the sun won't significantly increase your risk of skin cancer. When you spend more frequent, prolonged periods in the sun, wear a sunscreen.

In general, the darker your skin, the more time you need to spend in the sun to make the same amount of vitamin D. More pigment in your skin extends the time UV light takes to reach deep skin layers where your body makes vitamin D.

Most overdoses of vitamin D stem from taking too much of the vitamin in supplements. Because you store vitamin D, regularly consuming excessive amounts can be toxic. Vitamin D toxicity can lead to nausea, weight loss, irritability, and formation of calcium deposits in your lungs, kidneys, and soft tissues.

Vitamin D is like no other nutrient in that one of the best ways to get it has nothing to do with food. Although excessive sun exposure isn't healthy for your skin, a little bit of sun is good for your bones.

Fast Facts About Sun Stoppers

* Sunscreens block UV rays and inhibit vitamin D production. Still, use a sunscreen whenever you'll be in the sun longer than 10 to 15 minutes.

* Sunlight doesn't penetrate dense fabrics such as wool or denim. Loosely-woven materials such as silk and rayon allow UV penetration.

* Ordinary window glass blocks UV rays. Sitting near a sunny window won't boost vitamin D production.

[C] 1995, the Mayo Foundation for Education and Research. Reprinted with permission of the Mayo Clinic Health Letter. Distributed by the Los Angeles Times Syndicate.