What We Know About Vitamin D and Performance

Vitamin D is an essential nutrient that regulates some 1,000 processes in the body, and doctors have long known its importance to bone density and preventing the related illnesses like rickets and osteoporosis. Now researchers are beginning to study its role in athletic performance. While an extra dose of vitamin D might increase muscle strength and endurance, the science is far from settled. Still, many athletes are eyeing the vitamin for possible performance gains.

Given that the science is still young, is it worth hitting the drugstore? To find out, we reached out to some experts to understand what we know about the nutrient and what that means for athletes.

The Basics of Vitamin D

The main sources of vitamin D are sunshine and certain foods, including salmon, cod liver oil, and fortified cereal and dairy products. The USDA suggests an intake of 600 International Units per day, or 800 IU for adults over 70. You can easily meet that recommendation by spending 15 minutes outside on a sunny day.

In athletes, a vitamin D deficiency increases your risk of stress fractures, anemia, and a weakened immune system—all of which can hurt performance. In a study of 214 NFL players, scientists observed more muscle injuries in athletes with lower vitamin D levels. There’s no clear consensus about how widespread the deficiency is. In 2015 review, scientists found that about 56 percent of athletes had inadequate levels of the vitamin. Still, in a large-scale review, researchers at the National Academy of Medicine (then the U.S. Institute of Medicine) observed that, on average, Americans’ vitamin D levels appeared fine.

One cause of this discrepancy is that scientists don’t agree on the definition of “adequate” when it comes to vitamin D levels. The most common test for the nutrient measures a precursor version of its hormone form—the form of the vitamin that is actually used by the body. Sometimes this precursor doesn’t predict how much vitamin D exists in hormone form. Certain researchers, like those with the Endocrine Society, argue for higher concentration thresholds than than those of the National Academy of Medicine.

That said, some factors might increase your likelihood of a deficiency, such as living far away from the equator. During the Canadian winter, for example, it’s nearly impossible for your body to naturally produce vitamin D, says Dylan Dahlquist, a researcher and editor at Science Driven Nutrition. To prevent skin cancer, many people avoid unfiltered sun exposure entirely, says Graeme Close, a professor of sport and exercise sciences at Liverpool John Moores University. Though Close advises precautionary measures to avoid sunburns, ten to 15 minutes of UV exposure will help meet your daily vitamin D needs. Fair-skinned people can sustain sun-related skin damage in as little as five minutes, so vitamin D is no excuse to skimp on protection.

Skin tone is also a factor in how our bodies produce and use vitamin D. Higher-melanin skin blocks more UV rays, limiting vitamin D production. Some experts think this puts darker-skinned athletes at higher risk of a deficiency, but it’s still unclear what this means for bone health. “Some of the athletes with the strongest bones I have ever tested are darker skinned [and have] low vitamin D,” Close says. “It may be incorrect to have a blanket target concentration for all people.” So, while there’s a correlation between vitamin D and bone health and between dark skin and vitamin D absorption, the connection between dark skin and poor bone health is a leap that still isn’t backed by robust research.

Vitamin D and Performance

A number of studies in recent years have investigated a new possible function for vitamin D: boosting athletic performance. Preliminary research shows that supplementing with the vitamin could improve muscle strength, power, and recovery time. In a 2014 study, soccer players took a hefty dose of 5,000 IU per day for eight weeks. These supplements were linked to faster sprints and higher jumps. In another study of 14 rowers, a daily 6,000 IU regimen seemed to boost athletes’ maximum oxygen uptake, which is a measure of the body’s ability to use oxygen and thus generate energy.

The problem with these conclusions, aside from the small sample size in many of the studies, is that it’s still unclear whether the performance-enhancing benefits are related to remedying an existing vitamin D deficiency or increasing the nutrient to a greater, more optimal level. In other words, is it simply that the athletes in those studies were deficient? Or is there a performance-enhancing benefit of taking vitamin D even if you’re not deficient?

“In some cases, we are running before we have walked,” Close says. “In sports now, most athletes are taking vitamin D, and we have not seen a revolution in race times and injury rates.”

All the experts interviewed for this story agreed that performance-enhancing benefits of large doses, while intriguing, are still unclear.

Should You Supplement?

Getting enough vitamin D starts with a well-balanced, healthy diet, Dahlquist says. Still, it can take a lot of vitamin-rich foods to hit your daily needs, especially if you’re unable to get much sunlight.

According to Dahlquist, given the potentially high rate of deficiency, some supplementation doesn’t hurt. He recommends 1,000 to 2,000 IUs of the supplement (in the form of vitamin D3) per day, particularly in winter. This higher dose reflects the fact that athletes likely need more, since intense exercise burns through nutrients fast. Close advises against supplementing with more than 4,000 IU per day, however; beyond that amount, he found that the body stalls its use of the nutrient. In rare cases, regular supplementation of more than 4,000 IU a day can lead to vitamin D toxicity and a subsequent buildup of calcium in the bloodstream, called hypercalcemia. Symptoms include nausea, vomiting, and frequent urination.

Shane J. Nho, an orthopedic surgeon and sports medicine specialist, adds that he advises female athletes to take vitamin D and calcium to prevent osteoporosis later in life. “[The bones are] almost like a bank account,” he says. “Basically, we need to save enough so that in the future, as your bones become more brittle, you’ll have enough stored calcium and calcium uptake to prevent fractures.”

If you’re still concerned about your levels, get a blood test. The test isn’t perfect, but it’s cheap and easy, requiring just a small sample of blood from your fingertip, and can offer some valuable insights, Close says. If you choose this route, keep in mind that your results can vary with the seasons.

Overall, while the performance-enhancing benefits of vitamin D aren’t definitive, having at least adequate levels of the nutrient is important for overall health. Close and Nho say that, especially for competitive athletes, it’s important to watch for and correct deficiencies. And, Dahlquist argues, maintaining your health by supplementing can in turn help you maximize performance.

Outside Magazine: Fitness

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