发布时间:2010-05-24 15:46 浏览：5238 次 已上传31个录音
Vitamin D helps bones and muscles grow strong and healthy.
Low levels of vitamin D can lead to problems like rickets and osteoporosis.
Rickets is a deformity mainly found in children. Osteoporosis is the thinning of bone,
a common problem as people, especially women, get older.
Studies in recent years have suggested that vitamin D may also have other uses.
Studies have shown that low levels of D may increase the risk of heart attacks in men and deaths from some cancers.
Other studies have shown that people with rheumatic diseases often have low levels of D.
The easiest way to get vitamin D is from sunlight. The ultraviolet rays react with skin cells to produce the vitamin.
But many people worry about skin cancer and skin damage from 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 with high levels include oily fish such as salmon, tuna and mackerel, and fish liver oils.
Boston University researchers reported in two thousand seven that farmed salmon had a lot less vitamin D than 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 D in the American diet comes from foods like milk with the vitamin added.
These days, more doctors are testing for vitamin D levels in their patients.
But as research continues, some experts worry that if people take too much D, it might act as a poison.
Also, skin doctors warn people to be careful with sun exposure.
How much vitamin D does a healthy person need? Nutrition experts who advise the American government set the current recommendations in nineteen ninety-seven.
The daily amount is two hundred international units from birth through age fifty.
Then it rises to four hundred I.U.s through age seventy, and six hundred for those seventy-one and older.
But some groups say these amounts are not high enough.
The nutrition experts are taking another look at how much vitamin D and calcium people should get.
The Institute of Medicine of the National Academies expects to release a report by this coming May.
And that's the VOA Special English Health Report, written by Caty Weaver. I'm Bob Doughty.
天然含有维生素D 的食物并不是很多。维生素D 含量比较高的食物包括鲑鱼，金枪鱼和鲭等脂肪含量比较高的鱼以及鱼肝油。
那么，健康人需要多少维生素D 呢？美国政府营养专家在1997 年推出了目前的建议。
从刚出生到15 岁的孩子每天的摄入量为200 个国际单位，
从15 岁到70 岁则上升到400 个国际单位，71 岁及以上的人则需要600 个单位。