If you're one of the millions of Americans who pop a calcium supplement each day to maintain strong bones, you're on the right track: Calcium helps ward off osteoporosis, a disorder that contributes to more than 1.5 million bone fractures annually. What you may not realize, however, is that if you're not getting enough vitamin D -- and chances are you're not -- that extra calcium you're taking may not be helping your bones as much as you think.
Scientists have known for years that bones need calcium to stay strong. They've been passing that message along to Americans, who have been diligently drinking milk and eating yogurt and swallowing calcium pills. They've also known that adequate levels of vitamin D are crucial for bone health, because the body uses vitamin D to help shuttle calcium across the intestinal wall into the blood.
For years, most medical specialists believed that if people consumed the recommended dietary allowances of vitamin D, then the calcium in their diets would be fully absorbed by their bodies. Recently, however, that thinking has been turned on its side. Several studies have found that even when people get the recommended daily amounts of vitamin D, their bodies are not absorbing all of the calcium they consume. Based on these studies, researchers are recommending that Americans boost their intake of vitamin D.
In a study published in the April 2003 Journal of the American College of Nutrition, researchers at Creighton University gave calcium supplements to 34 post-menopausal women who consumed recommended amounts of vitamin D regularly. Half were given 500 milligrams of calcium alone, and half were given the calcium after taking vitamin D supplements for three weeks.
After the women took the calcium supplements, researchers collected blood samples to determine how much calcium had been absorbed by their bodies. The women who took the vitamin D supplements absorbed 65% more calcium than those who didn't. "Higher vitamin D intake substantially improved calcium absorption," says Dr. Robert P. Heaney, an osteoporosis researcher at Creighton University and the study's senior author.
Heaney's results dovetail with the findings of a large study in England that was published in March 2003, in which researchers found that people between the ages of 65 and 85 who took vitamin D supplements suffered 22% fewer bone fractures than those who didn't.
These findings are important because osteoporosis is a major health problem. Ten million people in the U.S. have osteoporosis, and an additional 18 million have low bone mass that puts them at risk for developing full-blown osteoporosis in the future. One in two women and one in eight men will suffer an osteoporosis-related fracture in her or his life. And as baby boomers age, those numbers are bound to grow.
The fact is that many Americans don't even come close to meeting the recommended daily allowance of vitamin D -- about 200 international units, or IU, for adults up to age 50. (See chart.)
Dr. Bess Dawson-Hughes, an osteoporosis researcher at Tufts University in Boston, has done several large studies of vitamin D and calcium intake, and the results have suggested that bone density improves when patients increase calcium and vitamin D intake.
"The weight of the evidence is swaying toward higher levels of vitamin D," says Dawson-Hughes, director of Tufts' Bone Metabolism Laboratory. Like Heaney, she advises her patients to get 800 to 1,000 IU of vitamin D daily.
Supplements aren't the only way to increase vitamin D in your diet. The body can synthesize vitamin D from sunlight. In California, just 10 to 15 minutes a day of sunshine help your body manufacture the vitamin D it needs. People who always use sunscreen and cover every inch of their bodies with it may need longer exposure, because sunscreen prevents vitamin D synthesis.
Not everyone can count on the sun, though. In northern climes, sunshine helps only during the summer.
"Here in Boston, you could be in the sun all day October through March and you wouldn't make any vitamin D at all," Dawson-Hughes says.
And people with dark skin need more sun time to manufacture vitamin D because the pigment in skin blocks ultraviolet rays. For example, African Americans need six to 10 times more sun exposure for vitamin D synthesis than people with fair skin.
If concerns about skin cancer make you wary of sun exposure, you can increase your vitamin D levels by consuming such foods as milk, fatty fish, eggs and fortified cereals. Milk is an excellent source of both calcium and vitamin D (an 8-ounce serving provides 100 IU of vitamin D.) Calcium-fortified orange juice, which many non-milk drinkers choose to enhance their calcium intake, does not contain vitamin D. Taking vitamin D and calcium together is not necessary because vitamin D is a fat-soluble vitamin that is stored in fat and muscle and used when needed. For that reason, the National Academy of Sciences recommends that people consume no more than 2,000 IU of vitamin D a day.
Should people under the age of 50 increase their vitamin D intake? "We don't have enough data to have an answer for that. The onus is on the scientists to see whether it makes sense. It may help, or it may just be a waste of money," says Dawson-Hughes. She says more studies are needed to determine if extra vitamin D is beneficial to younger people.
Expect to hear more about vitamin D in the future. Researchers have noticed that older people who take vitamin D supplements fall less often, and they are interested in determining whether vitamin D helps improve muscle function.
What's more, epidemiological studies have hinted that people with higher vitamin D intake are less susceptible to cancer than those with low vitamin D intake.
Vitamin D also appears to act as an anti-inflammatory in some situations. "I think we'll be seeing benefits to parts of the body other than bone," says Dawson-Hughes. "The D world is opening up."
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How much vitamin D?
The recommended daily allowance for vitamin D varies with age, measured in international units, or IU:
* Ages 19 to 50: 200 IU
* Ages 51 to 70: 400 IU
* Ages 71 and older: 600 IU
A few selected sources of vitamin D:
* Milk (1 cup, vitamin D-fortified): 98 IU
* Egg (one whole): 25 IU
* Salmon (3.5 ounces cooked): 360 IU
* Cod liver oil (1 tablespoon): 1,360 IU
* Pudding (from mix with vitamin D-fortified milk): 50 IU
Source: National Institutes of Health