Johan Nilsson/AFP via Getty Images
A man sunbathes in Malmo, Sweden, in July. Humans (and other animals) make vitamin D when exposed to sunlight.
The Institute of Medicine is throwing cold water on the latest dietary supplement fad: big doses of vitamin D.
Humans make vitamin D when they are exposed to the sun. But many worry that clothing, indoor living and sunscreen are depriving most people from enough of the sunshine vitamin. It's also hard to get enough vitamin D from the diet, proponents say, despite fortification of milk and orange juice.
But the institute's Food and Nutrition Board, which makes official recommendations on dietary intake, says advocates of high-dose vitamin D are going overboard.
After two years of study and debate, the panel says children and most adults need 600 international units of vitamin D a day. People older than 70 need 800.
That's more than the previous targets, set 13 years ago, of 200 units a day for young adults, 400 for those older than 50.
But the new recommendations are much less than advocates of high-dose vitamin D claim is necessary. Many people believe daily doses of 1,000 to 4,000 units can prevent a long list of ailments — cancer, heart disease, diabetes, influenza and other infections, autism, immune disorders such as multiple sclerosis, and more.
The Institute of Medicine panel rejects those claims. The only benefit, it says, is to maintain healthy bones.
"The evidence was inconsistent and inconclusive as to a benefit of vitamin D in preventing cancer, cardiovascular disease, diabetes, autoimmune disorders and many other health outcomes beyond bone health," says Dr. JoAnn Manson, chief of preventive medicine at Brigham and Women's Hospital in Boston and one of the 14 IOM panel members.
"The Internet will say that vitamin D has all these benefits," adds Dr. Cliff Rosen of Maine Medical Center in Portland, another panelist. "But the evidence really isn't there."
The panel also found no national epidemic of vitamin D deficiency. Many proponents of high-dose supplements believe there is one, and many of the IOM experts thought it might be true.
"It was very surprising," Rosen says. "I think there were a lot of us who came in thinking that requirements should be much higher or that [average American] blood levels were not nearly as high. But it's very good news for the general population."
The new report says people's blood levels of vitamin D don't need to be higher than 20 nanograms per milliliter of blood. Leading proponents aim for a blood level of 30 or even 40.
If 30 were the right number, more than half of the U.S. population could be considered deficient in vitamin D.
"Let's face it, everybody wants a number," Rosen says. "Everybody wants to know their vitamin D level these days. And so when you look and you say, 'Gee, my level's 25 and it should be 30, and it doesn't seem harmful to take a supplement and more is better, then I should be doing that.' And that's exactly what happened."
The Institute of Medicine experts worry that taking vitamin D in large doses over a long period might harm some people. The evidence is inconclusive, but the panel points to studies hinting at higher levels of pancreatic and esophageal cancer. Panelists say there's reason to worry about excess deposits of calcium in arteries from too much vitamin D.
Dr. Michael Holick of Boston University, who discovered the active form of vitamin D 40 years ago and is a leading proponent of high doses, isn't backing away from his conviction that most people need at least 3,000 units a day. That's what he takes, and what he recommends to his patients. Sometimes he prescribes 50,000 units of vitamin D a week.
"My recommendation is very simple," Holick says. "I don't see any downside to increasing your vitamin D intake. When I've been recommending for the past decade that people take more than the [officially recommended] 200 units, there was a lot of skepticism. Now they're recommending three times what we recommended in 1997.
"I suspect a decade from now that they'll be recommending another three- or fourfold higher increase," Holick predicts.
Members of the Institute of Medicine panel say that's possible -– if rigorous studies back up the proponents' claims. Manson is launching a $22 million federally financed study that will involve 20,000 people across the country. They will be randomly assigned to take daily doses of vitamin D and omega-3 fatty acid (linked in many studies to lower levels of heart disease) –- together or alone. Others will get placebo pills.
But it will be five or six years before the results of that study are ripe, Manson says.
The Institute of Medicine panel also reconsidered calcium intake, because vitamin D and calcium together are necessary for healthy bone development and maintenance throughout life.
The panel says children ages 1 to 3 need 700 milligrams of calcium a day, while those ages 4 to 8 need 1,000 milligrams. Adolescents need no more than 1,300 milligrams, while adults up to age 50 need 1,000 milligrams. Starting at age 51, women need 1,200 milligrams of calcium a day, as do both men and women older than 70.