Vitamin D helps bones and muscles grow strong and healthy. Low levels of vitamin D can lead to problems such as rickets（佝僂）, a deformity （缺陷）mainly found in children. Osteoporosis（骨質疏松癥）, the thinning of bone, is a common problem as people, especially women, get older.

But more and more research is suggesting that vitamin D might also help prevent many diseases.

The easiest way to get vitamin D is from sunlight. The sun's ultraviolet（紫外線） rays react with skin cells to produce vitamin D. But many people worry about getting skin cancer and skin damage from the sun. As a result they cover their skin or wear sunblock or stay out of the sun.

Also, darker skinned people produce less vitamin D than lighter skinned people. Production also decreases in older people and those living in northern areas that get less sunlight.

Not many foods naturally contain vitamin D. Foods high in this vitamin include oily fish such as salmon（鮭魚）, tuna（金槍魚） and mackerel（鯖魚）, and fish liver oils.

Boston University researchers reported last year that farmed salmon had only about one-fourth as much vitamin D as wild salmon.

Small amounts of D are found in beef liver, cheese and egg yolks. And some people take dietary supplements containing the vitamin. But most of the vitamin D in the American diet comes from foods with D added, like milk.

In nineteen ninety-seven, the United States Institute of Medicine established levels for how much vitamin D healthy people need. It set the daily amount at two hundred international units from birth through age fifty. It set the level at four hundred I.U.s through age seventy, and six hundred for age seventy-one and over.

But some groups say these amounts are not high enough. They are hoping that the new research findings will lead to new recommendations.

Research in the last several years has shown that low levels of vitamin D may increase the risk of heart attacks in men and deaths from some cancers. Other studies have shown that people with rheumaticdiseases （風濕癥）often have low levels of vitamin D.

More doctors are now having their patients tested for their vitamin D levels. But as research continues, some experts worry that if people take too much vitamin D, it might act as a poison. Also, skin doctors warn people to be careful with sun exposure because of the risk of skin cancer.

 And that’s the VOA Special English Health Report, written by Caty Weaver. I’m Steve Ember.

P.S： 維生素D的主要功能是調節體內鈣、磷代謝，維持血鈣和血磷的水平，從而維持牙齒和骨骼的正常生長就發育。兒童缺乏維生素D，易發生佝僂病，過多服用維生素D將引起急性中毒。