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Put 10 doctors in a room, goes the old joke, and you’ll get 10 different opinions. Unfortunately, that has become the story with vitamin D — and it’s no laughing matter. All the conflicting advice about how much to take has left many of us unsure of what to do.

The stakes are high. Inadequate vitamin D levels can increase your risk of dozens of serious health problems, including cancer, heart disease, osteoporosis, asthma, Alzheimer’s disease, and even the common cold and influenza. And apparently, nearly all of us are at risk of vitamin D deficiency.

Should you take vitamin D? And if so, how much? Here’s the best advice culled from experts.

1) Take a vitamin D test.

Ask your doctor for a vitamin D blood test, which will eliminate the bulk of the guesswork — but not all of it. Because of individual differences in absorption and use, people may need to take differing quantities of vitamin D to achieve a healthy blood level. Make sure your doctor orders a “25-hydroxy vitamin D” test. Other tests might result in a false normal.

2) Look for optimal levels.

Although levels below 30 ng/ml indicate a deficiency, many physicians haven’t kept up with the research on vitamin D and believe that this level is just fine. The optimal level is at least 40 ng/ml and perhaps 50 ng/ml, says Robert P. Heaney, MD, of Creighton University in Omaha. But higher amounts, within reason, aren’t necessarily bad. Surfers, lifeguards, and people who spend a lot of time outdoors typically have levels of 70 to 90 ng/ml.

3) Spend time in the sun.

If you don’t currently have a significant deficiency, and if during the summer you spend a lot of time in the sun, with at least your arms and legs exposed, and you are not always slathered with sunscreen, you probably don’t need to take vitamin D supplements. Michael F. Holick, MD, PhD, of the Boston Medical Center, who wrote The Vitamin D Solution, suggests getting approximately 10 minutes of sun exposure (depending on time of day, season, latitudinal location, and skin pigmentation) before applying sunscreen. Vitamin D made from the sun actually lasts longer in the body, compared with vitamin D from supplements or foods (also note that with the exception of wild salmon and shiitake mushrooms, most foods aren’t great sources of vitamin D).

4) Supplement with vitamin D3.

If it’s fall, winter, or early spring, if you don’t get a lot of sun exposure, or if you know you are D-deficient, you should definitely take vitamin D supplements (most health pros recommend vitamin D3, also known as cholecalciferol). Your need will be greater if you are north of the latitude of Atlanta, since you will make little if any vitamin D from sun exposure during the months of November through March.

5) Take the right amount for you.

If you have not taken a vitamin D blood test and you’re looking for general guidelines, Holick suggests that children take 1,000 to 2,000 IU and adults take 2,000 to 3,000 IU daily. “The bottom line for me is that there is probably no evidence that these amounts pose any risk,” he says. John J. Cannell, MD, who heads the nonprofit Vitamin D Council, has this recommendation: Don’t drive yourself crazy with all the qualifications. “Just take 5,000 IU a day, unless you’re going outside to work or to the garden or beach.” The higher amount might be particularly helpful for people with a chronic illness, such as fibromyalgia, arthritis, or lupus, adds Ron Hunninghake, MD, chief medical officer of the nonprofit, nutrition-oriented Riordan Clinic in Wichita, Kan. “These high doses of vitamin D, while generally safe, should be monitored with follow-up blood level [tests],” he says. And what of the risks? For most people, vitamin D toxicity occurs after taking more than 40,000 IU daily for months, says Cannell. So as long as you’re being moderate in your intake, don’t sweat it.

This article was adapted from “The Vitamin D Debate.”

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