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For years, the media have trumpeted the vital role calcium plays in preventing osteoporosis - a bone-thinning disease that afflicts 10 million Americans. Almost 34 million more are estimated to have low bone mass, placing them at higher risk for osteoporosis. The calcium clamor is so loud it sometimes drowns out messages about another nutritional necessity - vitamin D. Without enough vitamin D, the body cannot absorb calcium and deposit it in the bones.
The human body makes its own vitamin D when ultraviolet-B (UV-B) radiation from the sun interacts with a chemical in the skin. About 15 minutes of unfiltered sun exposure a few times a week is usually enough. But the sunshine method isn't 100 percent reliable for everyone, particularly for older people who are at the highest risk for weakened bones and fractures. Reasons include:
The skin's ability to make vitamin D from UV-B becomes less efficient as we age. Darker skin blocks out up to 95 percent of UV-B rays. Sunscreen absorbs UV-B before the skin can form vitamin D. The earth's ozone layer screens out UV-B in northern latitudes during winter. It is not always possible to get outdoors on sunny days.
Americans ages 50 and older are more likely to have a vitamin D deficiency, especially during winter months. Researchers have found that not getting enough vitamin D leads to a greater occurrence of hip fractures. In such cases, the blood raids the skeleton for the calcium it needs, thus raising the risk of osteoporosis and fractures.
These and other findings indicate dietary sources of vitamin D become increasingly important as we age. In addition to strengthening bones, vitamin D can help you maintain a healthy immune system. According to the Institute of Medicine of the National Academy of Sciences, the recommended daily dietary allowance for vitamin D in adults ages 50 to 70 is 400 IU (international units); 600 IU for people older than 70 and 200 IU for younge adults and teens.
Sources of vitamin D
Vitamin D is available from three sources:
Fortified milk and products made from fortified milk (not yogurt) as well as some fish. Supplements, including multivitamins with vitamin D and calcium supplements with vitamin D. Sun-exposed skin.
By law, a quart of milk must contain 400 IU of vitamin D. Fortified breakfast cereals, egg yolks and fatty fish (including tuna, sardines and mackerel) are rich sources of vitamin D. Some breakfast and nutrition bars and drinks provide 100 IU to 140 IU of vitamin D per serving. Vitamin D is also in multivitamins and calcium supplements.
Before taking supplements, speak with your doctor to see if they are necessary and if so, how much you should take. Babies who are breastfed exclusively might also benefit from a supplement. Be aware that too much vitamin D can cause toxic side effects. Symptoms of vitamin D poisoning may include nausea, headache, excessive urination, high blood pressure, kidney stones, kidney damage and other problems.
This article was reviewed and updated June 2007.
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