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Vitamin D: How Much Do You Really Need?

A growing body of research spotlights vitamin D, the sunshine vitamin, as essential to overall health. But how much you need remains a topic of hot debate. Here, the lowdown on the newest studies.

Photography by: Anna Parini
With any given dose of sunlight, different people end up with different amounts of vitamin D in the blood, depending on their geography, genes, skin color, and age.

Every so often, something comes along that promises to relieve us of a serious ailment, whether cancer or heart disease. The latest hope under the sun is actually something that we get from the sun: vitamin D. Turns out, this modest nutrient may be instrumental in preventing (and perhaps even treating) not just one condition but an array of them—strong bones being only the beginning.


Versatile Vitamin D

Just by sitting outside in the sun, a fair-skinned person in a swimsuit without sunscreen will get thousands of international units, or IU, of vitamin D within 10 minutes at high noon on a bright summer day. But given concerns about skin cancer, doctors often advise getting the vitamin mostly from food and supplements (see 4 Everyday Ways to Get More Vitamin D). Regardless of its source, vitamin D is modified and activated in the liver and kidneys; it then travels in the blood to perform its many functions, says Michael F. Holick, a professor of medicine at Boston University School of Medicine. A 2010 paper published in Genome Research explained that vitamin D influences 229 genes and therefore can have wide-ranging effects throughout the body.


A Breast-Cancer Connection

In the 1970s, when researchers at the National Cancer Institute drew up a color-coded map to look at mortality rates in the United States, Cedric F. Garland and Edward Gorham (both now at the University of California, San Diego, School of Medicine) noticed a pattern: “The blue areas, which represented the lowest rates of cancer, were clustered in the lower, sunnier half of the U.S.,” says Garland. The death rates were highest in the Northeast. Follow-up studies found an association between higher vitamin D levels and a 50 percent reduction in breast-cancer risks.


Correlation is not causation, of course. But biology may, at least in part, offer an explanation. Under normal circumstances, cells stick together in a pattern to form organ tissue, explains Elizabeth Platz, an epidemiologist at Johns Hopkins Bloomberg School of Public Health and a senior editor of the American Association of Cancer Research journal Cancer Epidemiology, Biomarkers & Prevention. Research suggests that vitamin D helps to create a substance that aids in this adhesion. This has led some scientists to theorize that, with low vitamin D levels, cancer cells may be even less able to stick together, which in turn increases the chance of their metastasizing elsewhere in the body. Last year, a review study in the Journal of Clinical Endocrinology & Metabolism also noted a possible beneficial link between higher vitamin D levels in the blood and better outcomes for patients with colorectal cancer and lymphoma.


Fighting Off Flu and More

Why are flu viruses so much more contagious in winter than in summer? Doctors have long maintained that it’s because cold weather keeps us cooped up inside, making us easy targets. But some researchers suspect the seasonality is, in part, also due to variations in sun exposure. “Many immune cells have vitamin D receptors,” says Karin Amrein, an associate professor at the Medical University of Graz, in Austria. When the vitamin latches onto the cells, they destroy unfriendly microbes; it’s likely that they also prevent the immune system from ramming into overdrive and causing inflammation. This suggests that vitamin D deficiency may also promote risks for a range of inflammatory diseases, including rheumatoid arthritis, irritable-bowel syndrome, asthma, and cardiovascular conditions. In a Journal of the American Medical Association paper last fall, Amrein reported that high-dose supplementation for severely vitamin D–deficient patients in intensive care corresponded with lower mortality rates, compared with those of patients who received a placebo. “You can’t expect to never be sick again with vitamin D supplements,” says Amrein, “but if I had a loved one undergoing a high-risk surgery, I would make sure he had adequate levels.”


Hope for Heart Disease

When researchers at Washington University School of Medicine, in St. Louis, knocked out vitamin D receptors in the immune cells of mice, something curious happened. The rodents’ livers created too much glucose; their arteries were stopped up with plaque. The findings, published in March in Cell Reports, may help explain the correlation, often noticed by cardiologists, between low vitamin D levels and increased risks for high blood pressure, stroke, diabetes, and heart attack.

Most over-the-counter vitamin D supplements are D3, the same form that your skin makes in sunlight.

The Right Dose

For now, the Institute of Medicine (IOM), a division of the National Academy of Sciences, advises 600 IU per day (800 if you’re over age 70), which brings blood levels to about 20 nanograms per milliliter (ng/mL), believed to be needed for bone health. It’s waiting for results from larger randomized trials before making a recommendation for nonskeletal conditions. One such trial is the VITAL Study, which has enrolled 25,874 racially and ethnically diverse participants from all 50 states in hopes of finding the effect of 2,000 IU per day (in comparison with placebo and omega 3) on incidences of heart attack, stroke, cancer, cognitive decline, depression, and autoimmune disease. “We’re not just looking at markers in the blood associated with these conditions,” says JoAnn Manson, a professor of medicine at Harvard Medical School and the study’s lead researcher. “We’re looking at actual clinical events.” The first set of data is slated for release in late 2017.


In the meantime, experts disagree on what level of vitamin D is effective for disease prevention. The current research suggests it’s likely higher than the 20 ng/mL needed for bone health. If you want to hedge your bets before the larger studies conclude, some experts believe it may be helpful to take 2,000 IU per day, the dose chosen for the VITAL Study because it, according to Manson, “offers the best balance of efficacy and safety.” If you think you may be deficient, discuss the issue with your doctor. Based on your lifestyle habits and health condition, your doctor can discuss a protocol that incorporates an appropriate daily dose of sun, supplements, and vitamin D– rich foods—so you can feel your best, today and perhaps well into the future.

4 Everyday Ways to Get More Vitamin D

Vitamin D is unlike any other vitamin in that it’s found in relatively few foods, says Maria Mantione, an associate clinical professor of community pharmacy practice at St. John’s University College of Pharmacy, in New York City. To maximize the effect of each dose, follow these strategies.


Take your vitamin D supplement with fat

Since vitamin D is a fat-soluble substance, it’s more bioavailable when you take it with foods containing some fat, such as milk.


Eat vitamin D–rich foods

Focus on high–vitamin D items, including salmon (383 to 447 IU in three ounces) and canned tuna in oil (393 IU in one cup). And because a little adds up, supplement with eggs (37 IU in each yolk) and fortified milk (98 to 124 IU per cup).


Maintain a healthy weight

The fat-soluble nature of vitamin D means it can get trapped in excess body fat. If you’re obese, talk with your doctor about the possibility of increasing your daily vitamin D intake.


Get adequate calcium

Don’t forget, vitamin D needs calcium to maintain bone health (and vice versa). Take 1,000 milligrams a day of calcium if you’re under age 50, and 1,200 if you’re 50 or older.