A new national study finds that most adults, especially those over 50, fall short on recommended daily levels of vitamin D, an essential nutrient long known to preserve bones and now increasingly tied to protection against ailments from cancer to rheumatoid arthritis.
And no, just drinking more vitamin-D fortified milk or juice may not make up the deficit, many experts say, although it can help. Spending 10 to 15 minutes in the sun, done with proper care, might.
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The study is based on data drawn from a large, federally funded national health survey and analyzed by a team of scientists from Boston University and private industry. Presented yesterday at the Experimental Biology annual meeting in San Diego, the study found that vitamin D intakes peak during childhood and teenage years and then decline.
Women ages 19 to 50, as well as men and women 51 and older, ate the least food rich in vitamin D. Even when the team accounted for use of vitamin D dietary supplements, few older men and women reached recommended daily levels. The researchers concluded that the low intakes, especially for the aged, "warrant intervention."
At a time when researchers are discovering a widening role for vitamin D, "many lines of investigation indicate that most Americans do not have optimal levels of vitamin D, mainly because of low sunlight exposure," said Walter Willett, chairman of the Department of Nutrition at the Harvard School of Public Health in Boston. Willett convened a meeting in January with leading vitamin D researchers and vitamin manufacturers to review the latest findings. Since there are limited food sources of vitamin D, "the most practical way to increase our vitamin D levels is from supplements," Willett said.
Unlike other essential nutrients, vitamin D is made by the skin, which requires ultraviolet light to produce the vitamin from cholesterol. Those in the Washington area and others who live north of Newport News, Va., often don't get enough sun exposure year round to make sufficient vitamin D.
Concern over skin cancer means that more people are wearing sunblocks, which inhibit production of vitamin D. Dark-skinned people have to spend up to a couple of hours in the sun to make enough vitamin D. Light-skinned people can get what they need in about 10 to 15 minutes.
The skin's ability to make vitamin D declines significantly with age. For this reason, the National Academy of Sciences (NAS) set the latest vitamin D daily intake on an age-related scale: 200 International Units (IU) -- about the amount found in two eight-ounce glasses of milk -- for those 19 to 50 years of age; 400 IU for those aged 51 to 70 years; and 600 IU for people 70 and older. The NAS also set a tolerable upper intake of 2,000 IU for adults. Toxic levels have been reported at 10,000 IU or higher per day.
But a growing number of scientists believe that vitamin D intake should be at least 1,000 IU or higher.
"Fifty years ago, a bunch of guys got in a room and said, 'We know that a teaspoon of cod liver oil cures rickets in a child and it has 400 IU of vitamin D,' " said Bruce Hollis, a professor of pediatrics, biochemistry and molecular biology at the Medical University of South Carolina. "They transposed that amount onto adults. It was arbitrarily set with no evidence [in adults] at all."