Is vitamin D a wonder drug that can cure almost anything that ails us? Or is it a specialist in the toolbox of supplements that does one job really well—promoting strong bones? An analysis of more than 1,000 studies and interviews with experts leads to the latter conclusion.
The body needs vitamin D to absorb calcium and build healthy bones. A panel of experts produced a report in November, 2010 noting that while observational studies point to some positive impacts beyond bone health in preventing cancer, heart disease, autoimmune diseases and diabetes—ailments for which it has been touted in the past decade—no clinical trials have looked at these outcomes until now. The report proposed an increase in vitamin D from previous recommendations, but noted that the mega-doses some have preached for these illnesses are unwarranted and could actually be harmful.
"There is abundant science to confidently state how much vitamin D and calcium people need," said committee chair Catharine Ross, who chairs the Department of Nutritional Sciences at Penn State University, in a press release announcing results of the study, sponsored by the U.S. Institute of Medicine and the Canadian government. "We scrutinized the evidence, looking for indications of beneficial effects at all levels of intake. Amounts higher than those specified in this report are not necessary to maintain bone health."
Increased dosages can come from fortified food and supplements. Excessive calcium can lead to kidney stones, while excessive vitamin D can damage the kidneys and heart. "Evidence about other possible risks associated with routine vitamin D supplementation is still tentative," the press release continued, "and most studies have focused on very high doses taken short-term rather than on routine, long-term consumption of large amounts. However, some signals suggest there are greater risks of death and chronic disease associated with long-term high vitamin D intake, which informed the committee's conclusions about levels that consumers should not exceed."
Sun exposure is an important source of Vitamin D. The study determined that because many Americans and Canadians are covering up and/or using sunscreen while outside in an effort to combat skin cancer, the new, higher vitamin D recommendations assume little sun exposure.
The new Recommended Dietary Allowances are 600 international units (IUs) of vitamin D per day for individuals up to age 70, and as much as 800 IUs daily for those 71 and older. The amount of calcium needed, based on age, ranges from 700 to 1,300 milligrams per day, with teenagers and women over 50 and men over 70 needing the higher amounts.
The study committee determined, through a survey of blood levels, that a majority of Americans and Canadians are getting enough vitamin D and calcium, though some elderly may fall short and should increase their intake of Vitamin D-fortified foods and possibly take a supplement.
Susan Mayne is a professor of epidemiology at the Yale School of Public Health. She sat on the IOM Committee and says, "Vitamin D status is usually assessed by a blood measure and that's affected by almost every other risk factor for chronic disease status that we have—obesity, physical activity, race, supplement use, diet—perhaps more so than any other vitamin. Most nutrients are not affected by physical activity and obesity. When people are outside they're exposed to the sun and the sun produces vitamin D in the skin.
"Everybody assumes, 'It can't hurt me. I'll just take a supplement," she continues. "In fact, there are some data that suggest it could be harmful at higher doses, especially if you have reasonably good status to begin with. So one of the things the committee had to do was set upper limits of intake. Hypercalcemia, or harmful levels of calcium, is a well-known adverse outcome of excess vitamin D intake because vitamin D enhances the absorption of calcium. However, emerging evidence links excess vitamin D with other adverse chronic disease outcomes as well."
JoAnn Manson, a professor of medicine at Harvard University, was a member of the IOMIOM committee and is principal investigator in the first large-scale, randomized clinical trial evaluating the effect of moderate- to high-dose vitamin D (without calcium) in the prevention of cancer and cardiovascular disease. Called the VITAL study, it will enroll 20,000 men and women between now and 2016 to take 2,000 IUs of vitamin D plus Omega 3s.
She says previous observational studies of other supplements, like vitamin E and beta-carotene, looked promising for reducing cancer, but clinical trials showed otherwise, and even pinpointed increased risk of cancer or stroke, and possible increased overall mortality.
She notes that the evidence of any positive impact of vitamin D is "promising but unproven. With vitamin D in particular there are many confounding factors, like obesity and skin color," she adds. Vitamin D is stored in fat, making it less available in the blood serum, where it's measured. African Americans are often deficient because vitamin D is produced in the skin with exposure to sunlight, and darker skin pigmentation does not absorb as much sunlight.
Manson is still recruiting men over 50 and women over 55 with no prior history of heart attack, stroke or cancer, to be part of the study. For more information, visit vitalstudy.org.