Vitamin D: don’t worry about it

In the early 2000s, vitamin D became all the rage as a sort of cure-all. People began worrying that they weren’t getting enough of the vitamin, especially after Dr. Oz, on Good Morning America, declared that 100 million people were deficient in the vitamin. Sales of vitamin D supplements soared, as did rates of vitamin D testing. Forget testing and supplements: with some exceptions, you can rest assured that you’ve got plenty of vitamin D. 

A team from Harvard Medical School conducted the world’s largest randomized vitamin D trial that followed 26,000 healthy adults for 5.3 years. Half took supplements and half didn’t. The results showed that vitamin D did not make a dent in cancer or heart disease and did not prevent falls, improve cognitive function, reduce atrial fibrillation, change body composition, reduce migraine frequency, improve stroke outcomes, decrease age-related macular degeneration, reduce knee pain, or reduce the risk of bone fractures. Subsequent studies also showed that extra vitamin D didn’t reduce diabetes risk, respiratory infections, mortality rates, or the risk of invasive cancer.

The notion that most of us are suffering from vitamin D deficiency was based on the erroneous idea that 20 nanograms per milliliter (20 ng/ml) is the bare minimum for good bone health. That’s not true. Not only is 16 ng/ml a satisfactory level, a survey of the U.S. population revealed that most of us have levels of 20 ng/ml anyway. Nevertheless, more than 10 million vitamin D tests are still done annually in the U.S., even though these tests are not recommended by major medical organizations, such as the Endocrine Society and the National Academy of Medicine.

Vitamin D is important: it helps your body absorb and retain calcium and phosphorus, which is critical for building bone. We get our vitamin D mostly from the sun. When sunlight hits your skin, it starts a chain reaction, beginning with converting a compound in your skin into a vitamin D precursor—and proceeds from there. And you don’t need much. A 2010 study calculated that between April and October someone in Boston with 25 percent of their skin exposed would need between three and eight minutes of sunlight a day. Even if you’re not getting that much, your liver and fat cells store vitamin D for future use, which generally lasts for 10 to 12 weeks.

There are some exceptions: Because certain diseases, such as Crohn’s disease and cystic fibrosis, can cause vitamin deficiency, people with those diseases might need supplements, as might people who are hospitalized or have had gastric bypass surgery. My advice: unless you’re in a special category, don’t worry about it.

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