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Vitamin D helps bones and muscles grow strong and healthy. Low levels of vitamin D can lead to problems such as rickets（佝偻）, a deformity （缺陷）mainly found in children. Osteoporosis（骨质疏松症）, 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（紫外线） 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（鲑鱼）, tuna（金枪鱼） and mackerel（鲭鱼）, 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 （风湿症）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： 维生素D的主要功能是调节体内钙、磷代谢，维持血钙和血磷的水平，从而维持牙齿和骨骼的正常生长就发育。儿童缺乏维生素D，易发生佝偻病，过多服用维生素D将引起急性中毒。
