strong people

The Latest on D

I do a lot of public speaking around the country about our research on physical activity and nutrition. These talks give me a chance to catch up on people’s health concerns. One subject that seems to be on many people’s minds is vitamin D. What are the best sources? Can it help prevent osteoporosis? How much is enough?

Vitamin D is a fat-soluble vitamin that promotes the absorption of calcium and is therefore particularly important for healthy bones. Older adults need to be especially vigilant in protecting their bones to help prevent the onset of osteoporosis. One out of two older women and one out of five older men can expect to get osteoporosis if they live into advanced age.

New information is emerging all the time about this fascinating vitamin. Research by my colleagues at Tufts and elsewhere suggests that in addition to improving our bone health, vitamin D can preserve our muscle strength and increase our immune function, too. You can get vitamin D from three sources: the sun, the right foods, and supplements.

The Sun
The sun’s UV rays stimulate your skin cells to make vitamin D. Between April and September it may be possible for you to get your daily supply of vitamin D from the sun with just 10 to 15 minutes per day of full sun exposure on your arms and legs. This isn’t true for everyone, though. If you have dark skin pigmentation or if you wear sunscreen, you probably won’t be able to make as much vitamin D as your body needs. In addition, the older you get, the more difficult it is for your skin to convert the sun’s rays into vitamin D.

During the fall and winter months, the sun’s rays are neither strong enough nor at the correct angle to stimulate vitamin D synthesis in your skin. If you live in a northern state or province, you will not be able to rely on the sun as your source of vitamin D between October and March.

Food
Fortified milk and cereal are the best food sources of vitamin D. Unfortunately, many people don’t eat enough of these foods to maintain a healthy level of vitamin D. In fact, the Institute of Medicine reports that more than 50 percent of women in the United States are not meeting the current recommendations for adequate intake of vitamin D.

Supplements
I don’t usually recommend taking supplements, with the exceptions of calcium and vitamin D. There is ample evidence that a vitamin D supplement can make the difference between a steady loss of bone density and maintaining healthy bones for life. One of the best studies to illustrate the importance of vitamin D supplementation was done by my colleague Bess Dawson-Hughes and others at Tufts. They followed 389 men and women, age 65 and older, for three years. A daily supplement that contained 500 milligrams of calcium and 700 international units (IU) of vitamin D was given to about half the volunteers; the rest received a placebo.

The results were impressive. While the people in both groups had the same risk of falling, only 6 percent of those who took the supplements suffered fractures, compared with 13 percent who were taking a placebo.

How much should you take?
The current recommendations for daily vitamin D intake were established by the National Academy of Sciences in 1997: 200 IU for people up to age 50; 400 IU for people age 51 to 70; 600 IU for people over 70. These requirements will likely be increased during the next revisions of the guidelines.

I recommend choosing a vitamin D supplement that supplies 400 to 700 IU. Don’t go above 1,000 IU unless your doctor recommends it, because vitamin D can be toxic at high levels.

Vitamin D is an important part of bone health, but it’s just one piece of the puzzle: combining supplementation with good nutrition and adequate calcium intake, exercise, medication management (when necessary), and other lifestyle measures is critical to maintaining a strong and healthy body.

Miriam E. Nelson is director of the John Hancock Center for Physical Activity and Nutrition at Tufts and an associate professor at the Friedman School of Nutrition Science and Policy. She also holds an adjunct appointment at the Tisch College of Citizenship and Public Service. Strong Women, Strong Backs (GP Putnam’s Sons), her eighth book in the bestselling Strong Women series, was published in September.