Vitamin D

Daily Value: 400 international units

Good Food Sources: Herring, sardines, salmon, fortified milk, eggs, fortified cereals

The D in vitamin D could very well stand for different. How else to describe the only nutrient that's both made by your body (vitamin D is synthesized through your skin by the action of ultraviolet light that's present in sunlight) and required in your diet?

Without it, another D word comes to mind: devastating. Children who don't get adequate vitamin D develop rickets, a condition characterized by flaring ankles and wrists that have noticeable knobby bumps and by weak, soft leg bones that bow under the child's own weight. Similarly, adults risk developing osteomalacia, a condition similar to rickets but occurring in developed bones. Some experts believe that not getting enough vitamin D can make osteoporosis worse. Osteoporosis is a bone-weakening disease that leads to fractures and tooth loss.

Vitamin D is responsible for getting the important bone builders calcium and phosphorus to the places in the body that they need to go to help bone grow in children and remineralize in adults. It does this first by making certain these minerals are absorbed in the intestines, second by bringing calcium from bones into the blood and third by helping the kidneys reabsorb the two minerals, says Binita R. Shah, M.D., professor of clinical pediatrics and director of pediatric emergency medicine at the State University of New York Health Science Center at Brooklyn. "When you see a case of rickets, the body is desperately trying to make bone, but adequate calcium and phosphorus aren't available. It's a poor effort. The result is a mass of unmineralized bone accumulation," says Dr. Shah.

Fortified milk is one ready source of vitamin D, although it takes a quart to provide the Daily Value. But you don't have to rely on diet alone to give you the vitamin D you need. Ten minutes of summer sun on your hands and face provides enough, says H. F. DeLuca, Ph.D., chairman of the Department of Biochemistry at the University of Wisconsin-Madison.

"It depends on where you are in relation to the equator, but in northern climates during the winter, the sun is at such an angle that the right rays don't penetrate the skin to make vitamin D," says Dr. DeLuca. "During the summer, you can store up quite a bit of vitamin D in your fat cells. If your diet is good, it will probably last you through the winter." Sitting next to a sun-filled picture window or driving in a car doesn't count; the glass filters out the rays you need.

Fortunately, kids are born with enough vitamin D to last them nine months. Adults aren't as lucky, as shown in a study at Columbia University in New York City. After evaluating the calcium and vitamin D status of elderly people who were entering nursing homes, researchers determined that a majority had low vitamin D levels. Nearly 85 percent had symptoms of osteoporosis.

"There is mounting evidence that vitamin D deficiency in elderly people is a silent epidemic that results in bone loss and fractures," reports Michael F. Holick, M.D., Ph.D., chief of the Section of Endocrinology, Diabetes and Metabolism and director of the General Clinical Research Center at Boston University Medical Center.

Vitamin D also plays a role in immune function, although it's still unclear just how that takes place.

Using Vitamin D Safely

When eight people in Massachusetts developed symptoms such as nausea, weakness, constipation and irritability just from drinking fresh vitamin D-fortified milk, doctors scratched their heads--that is, until they discovered that the milk had accidentally been fortified with more than 580 times the proper amount of vitamin D.

Such massive overfortifications are rare. But they're a good example of what can happen when you get too much vitamin D. Because the nutrient is stored in fat cells, long-term high doses can cause calcium to be deposited in the soft tissues of the body and can result in irreversible damage to the kidneys and cardiovascular system. Doses of 1,800 international units a day can cause stunted growth in infants and young children. High intake of vitamin D can even lead to coma.

Because vitamin D can be so toxic, you should never take more than 600 international units of vitamin D daily unless your doctor prescribes a higher dosage for you.