Vitamin D is hot — and not just because it comes from the sun.

Once prescribed only to prevent or treat outright Vitamin D deficiency, rickets and osteoporosis, doctors have increasingly recommended Vitamin D supplements to treat or try to prevent a lengthy list of ills, including autism, certain cancers, diabetes, heart disease, multiple sclerosis, respiratory infections and cognitive decline in seniors. Sales of Vitamin D skyrocketed to $425 million in 2009 from just $40 million in 2001, according to the Nutrition Business Journal.

But is Vitamin D worth the hype? A recent report by the Institute of Medicine found that the news media and medical profession overreacted to the cascade of studies in the last decade associating low levels of Vitamin D with numerous ailments. The institute’s November 2010 report, which examined more than 1,000 studies and other reports, concluded that Vitamin D was essential for bone growth and maintenance but that the evidence of its benefits beyond that was inconclusive.

Most people aren’t deficient

Much of the evidence for Vitamin D comes from population studies, which have suggested a positive relationship between high blood levels of the vitamin and a reduced risk of disease. But they haven’t proved a cause and effect. Since Vitamin D can be created by exposure to sunlight and is usually higher in people who are thinner, it could be that people who have high levels of the nutrient tend to exercise more or have more healthful weights.

And there is confusion over what constitutes Vitamin D deficiency. The IOM report challenged the notion of a widespread deficiency of the vitamin or of the mineral calcium. So why do people think they’re deficient? One explanation is that most testing laboratories have changed their definition of a “normal” blood level. The lower limit used by many was 20 nanograms per milliliter; now, anything under 30 nanograms is considered an insufficient amount.

While some people truly are deficient in Vitamin D, the report concluded that the average American is not. Even assuming minimal sun exposure, the panel found that people are probably consuming adequate amounts of both Vitamin D and calcium because they take multivitamins and eat food that has been fortified with those nutrients.

The report’s recommendations — that children and most adults get 600 international units of Vitamin D a day, and that people older than 70 get 800 IU — were more than the targets set 14 years ago (200 IU a day for adults ages 19 to 50, 400 IU for those older than 50, and 600 IU for those older than 70). But they fall far short of the daily intake recommended by some experts — up to 3,000 IU a day for people living in areas such as the Northeast.

The risks

Some experts argue that the IOM’s report is overly cautious and that there is no downside to increasing Vitamin D levels. But Consumer Reports Health doesn’t agree with that advice for a number of reasons. The IOM report set the upper limit for Vitamin D at 4,000 IU for those 9 and older, after which the risk for harm begins to increase. At very high levels — above 10,000 IU a day — there’s a risk of kidney and tissue damage.

For African Americans, taking Vitamin D supplements could even be counterproductive. Last year a study by the Wake Forest University School of Medicine involving 340 African Americans with Type 2 diabetes found that those with the highest Vitamin D levels (few of whom were taking calcium or Vitamin D supplements) were more likely to have calcified plaque in their major arteries, a predictor of heart attack and stroke.

Bottom line

Until research confirms the benefit of higher doses, stick with the IOM’s recommendation: 600 IU for adults up to age 70 and 800 IU for those older than 70. Look for a Vitamin D product with the “USP Verified” mark, which means it meets standards of quality, purity and potency set by the nonprofit U.S. Pharmacopeia. For better absorption of the vitamin, take it with a meal containing some fat. Good food sources of Vitamin D include cod-liver oil, cooked button mushrooms, eggs, fortified milk and soy products, mackerel, sardines and wild Alaskan or sockeye salmon.

If you get some midday sun exposure during the warmer months and regularly consume foods rich in Vitamin D, you probably don’t need supplements. People who are middle-aged or otherwise at risk of Vitamin D deficiency, including those who are overweight or have darker skin, might need supplements. Even then, the amount in most multivitamins is probably enough.

Consumer Reports Health’s medical consultants say that the evidence suggests that screening as part of a routine regular exam is not yet justified. Instead, it should generally be limited to people who have some objective evidence or reason that they might be deficient in Vitamin D, such as having markedly weak bones, celiac disease or other ailments that impair the body’s ability to absorb the vitamin from food.

Copyright 2011. Consumers Union of United States Inc.