Vitamin D's health benefits have gone from being highly touted to often refuted. Guided by a wave of new research, we're shedding light on what the nutrient actually does and how much you need.
Vitamin D Capsules
Credit: Peter Ardito

You might associate vitamin D with the label on your milk carton, or know it has something to do with the sun. And you may have read, or been told by a doctor, that low levels are linked to everything from fatigue to multiple sclerosis. (One prominent endocrinologist called vitamin D deficiency a global health issue in 2017, even offering it as a potential explanation for dinosaur extinction.) "In the test tube, vitamin D has some interesting effects," says Clifford Rosen, MD, a senior scientist at the Maine Medical Center Research Institute, in Scarborough. Laboratory, as well as animal and observational studies, do suggest a connection between lower blood levels and a higher risk of cancers (among them breast, colon, prostate, and lung), cardiovascular disease, diabetes, depression, and Alzheimer's. But findings from the large-ever randomized clinical trial-the gold standard for research-on vitamin D supplementation undercut some of the more miraculous claims. Federal health data also indicates that few of us have levels low enough to worry over.

"There's this supposed pandemic of deficiency, and an assumption that if some vitamin D is good, more is necessarily better," says JoAnn Manson, MD, preventive medicine division chief at Boston's Brigham and Women's Hospital and lead researcher of that five-year, 25,871-subject clinical trial, called the Vitamin D and Omega-3 Trial (VITAL), which was funded by the National Institutes of Health (NIH) and published last November in The New England Journal of Medicine. More than a quarter of older adults take a D supplement, and doctors ordered nine million tests for Medicare patients in 2016, at a costly increase of $13.3 million over the prior year. Then there are the megadoses of hundreds of times the recommended daily amount-and the over $1 billion we end annually on D supplements, compared with $248 million a decade ago, per one report. The bottom line: "We need to separate fact from fiction," says Manson.

What Does Vitamin D Do?

The vitamin is among 13 essential ones your body requires to function. Think of it as an invaluable assistant: It helps your muscles move, your nerves relay messages to your brain, your immune system fight infections, and-most important-your small intestine soak up calcium. Without it, says Rosen, you absorb only about 10 percent of the calcium you ingest; with it, up to 40. And calcium is vital to strong teeth and bones, as well as to healthy blood flow.

When it comes to disease prevention, we know vitamin D wards off osteomalacia, which causes bone pain and softness and muscle weakness. But beyond that, the jury's still out. The VITAL study concluded that taking supplements did not lower cancer or cardiovascular-disease rates, though it did reduce cancer deaths by about 20 percent. (The researchers plan to publish more findings concerning depression, cognition, and diabetes soon.) A separate review of 81 trials with 53,537 subjects found that supplements didn't prevent fractures, either.

How Much Vitamin D Do You Need?

A sufficient amount is between 20 or 30 and 50 ng/ml, according to the National Academy of Medicine and the Endocrine Society; over 50 ng/ml, you raise your risk of hypercalcemia, a surplus of calcium in the blood that can cause nausea and kidney stones. To support bone health and normal calcium metabolism, the NIH recommends that people under 70 get 600 IUs a day.

Who Is Truly Deficient?

Working long hours indoors or living up north isn't typically enough to put you in this group. But we do synthesize vitamin D less efficiently as we age, so the NIH suggests that those over 70 bump up to 800 IUs a day. And because D is fat-soluble, having Crohn's disease or another fat-malabsorption condition, or being obese, may mean you need more as well. People with darker skin might, too (more on that in a minute). But while deficiency is serious, it's not as widespread as some advocates claim: The most recent Centers for Disease Control and Prevention data found that just eight percent of Americans classify.

Where Do You Get Vitamin D?

You can get vitamin D in three different ways: the sun, your diet, and through supplements. Step outside on a sunny day and you'll reap the benefits-UVB exposure triggers our bodies to generate D. (Cholesterols under our epidermis take in the rays and get converted into a previtamin form; the liver and kidneys complete the job.) While you do need to be outside-windows block UVBs-you don't have to lie out in a swimsuit. "If you're Caucasian, 10 to 15 minutes of exposure around midday over 10 percent of your body-your hands, arms, and neck-is ample," says Bess Dawson-Hughes, MD, director of the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, in Boston. Very dark-skinned people may need two to three times longer. From October to March, you get less UVB exposure the farther you live above 33 degrees latitude, she explains (picture Dallas). That seasonal dip doesn't harm bones-studies show that even submariners fare perfectly well-but researchers are still investigating how it may affect other areas of your well-being. That's where alternative sources factor in.

Most people's main source of Vitamin D is through fortified foods. Cow's milk and nondairy alternatives, such as soy and almond milk, can contain up to 205 IUs per cup; that amount of fortified OJ has about 140. Vitamin D is also naturally present in a few foods. The highest levels are found in fatty fish like swordfish, salmon, and tuna (566, 447, and 154 IUs, respectively, per three-ounce serving) and some mushrooms: One-third cup of maitakes has 943 IUs; UV-enhanced creminis and portobellos-they're often labeled as such-have 1,072 and 953.

If you have a disease or condition that increases your risk of deficiency, a blood test will tell you how much D you need to take. If you don't, you might consider popping some if you know you get very little from the sun or your diet. However, until data indicates otherwise, experts suggest not exceeding 2,000 IUs. "Over 4,000, there's very little research on the long-term safety and health effects," Manson says. "But 2,000 a day appears to be safe, and may be beneficial."


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