单项选择题

Get Right Amount of Vitamin D
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. Small amounts of D are found in beef liver, cheese and egg yolks. 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.
Which of the following is NOT mentioned in this article