Dietary Supplements: Can Vitamin D Protect Bones?

Nutrition Action Healthletter spoke on the phone with Bess Dawson-Hughes, the director of the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University and a professor of medicine at Tufts. This is what she had to say about vitamin D and bone health.

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Q: Can taking calcium and vitamin D together prevent fractures?

A: Many trials have found that taking both lowers the risk of fractures. It’s hard to tell if vitamin D alone would lower the risk, because trials that used higher doses of vitamin D also used calcium. You can’t tell if it’s the combination or the higher dose of vitamin D that did it.

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Q: Why would vitamin D help prevent fractures?

A: Vitamin D is essential for absorbing calcium. People with low vitamin D blood levels have more rapid bone loss and more fractures.

And we know that vitamin D lowers the risk of falling. Falls are almost always the precipitating cause of hip fractures. So lowering the risk of falls is hugely important.

Q: How does vitamin D lower the risk of falls?

A: It helps build muscle. We did one study on healthy but frail older women with low starting vitamin D levels. We gave them vitamin D or a placebo for four months. We saw that the women who got vitamin D had larger cross-sectional muscle area than the placebo takers, suggesting that vitamin D increased their muscle mass.

We and others have also found vitamin D receptors in muscle biopsy studies. That also suggests that vitamin D matters for muscles.

Q: Does vitamin D improve strength?

A: We don’t know yet. We’re just about to start a new study in healthy older people with low-normal vitamin D levels. We’ll be giving them vitamin D and measuring power in the legs and arms every two months for a year to see if their performance improves.

Q: Can you get too much vitamin D?

A: Yes. Too much vitamin D can cause hypercalcemia—high blood levels of calcium—and increase your risk of kidney stones.

And there’s some evidence that if your blood levels of vitamin D are too high, you may have an increased risk of falls. Again, we don’t know why. So you can aggravate the very thing you’re trying to prevent.

Q: So it’s just like calcium?

A: Yes. Our bottom line is that it looks like 800 to 1,000 IU is the right amount for older people. That’s slightly more than the RDA—600 to 800 IU—but it’s the recommendation of the International Osteoporosis Foundation.

If you’re in a high-risk category for inadequate vitamin D—if you have very dark skin so your skin doesn’t make much vitamin D, or if you’re a shut-in, or if you’re obese—then you may need extra D. Vitamin D gets stored in fat tissue, so blood levels are lower if you’re obese.

But generally speaking, 800 to 1,000 IU is the right range.

Q: So most of us need a supplement?

A: Yes, unless you spend time in the sun and live in or south of Los Angeles or Atlanta or anywhere in between, or if you go to Florida in the winter.

Q: You can’t get much from food?

A: There’s not much vitamin D in the diet. It’s in foods that people don’t tend to eat daily, like salmon.

So supplements are pretty much the way to go. And I would take them with food, not on an empty stomach. Also, if you take vitamin D with a meal that has some fat in it, your absorption is greater than if the meal is fat-free.

Other relevant links:

• How much calcium should you be taking to protect your bones? See: Dietary Supplements: Calcium and Bone Health

• Shoot for the recommended levels of calcium and vitamin D. See: Calcium and Vitamin D Revisited

• Both aerobic and strength training are important for bone health. See: Exercise for Health: Does Exercise Help Bones?

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