Jun 2, 2017
The D Factor
Randy Bird

If you follow nutrition and public health news, you may have noticed that vitamin D is getting more attention of late, and with good reason. An estimated 75 percent of Americans are vitamin D deficient.

Although there is little concrete data on the deficiency rate among athletes, it’s safe to assume they’re not far away from the general population. The hazards of a chronic vitamin D shortage–including heightened stress fracture risk, decreased immune function, and elevated blood pressure–are especially troublesome for those who play sports, yet vitamin D remains largely ignored in sports nutrition planning.

An individual’s vitamin D level can best be determined by a simple blood test that measures 25-hydroxyvitamin D (25-(OH)D). The optimal level of serum 25-(OH)D is thought to be around 50 nanograms per milliliter (ng/mL), and research has demonstrated that a level below 32 ng/mL is inadequate for good health. In the U.S., a federal panel is evaluating vitamin D research to update the daily recommendation, which now stands at 400 international units (IU) per day. Many scientists believe 400 IU is far too low, arguing that a daily intake of approximately 1,700 IU is needed to raise 25-(OH)D concentration in the blood to a healthy level.

It should be every athlete’s goal to have a vitamin D level greater than 32 ng/mL. Time in the sun is part of the equation, but dietary intake should be emphasized as well.

The challenge is that only a few foods naturally contain vitamin D. Fish and fish liver oils are among the best sources–wild salmon, for instance, contains roughly 1,000 IUs per 3.5-ounce serving, and farm-raised salmon typically has around 250 IUs.

Because of its scarcity in natural food, most vitamin D in the American diet comes from fortified foods. Almost all of the U.S. milk supply is fortified with 100 IU of vitamin D per cup, a practice that began in the 1930s to combat rickets. Other fortified foods include cereal, flour, some dairy products, and fruit juices (particularly those that are also calcium-fortified).

Unless someone frequently eats oily fish and consumes a lot of milk and other fortified foods and beverages, obtaining sufficient dietary vitamin D can be can difficult. Supplements are an easy and convenient solution in these cases: Both D2 and D3 are available in supplement form, and while either can be effective at raising serum 25-(OH)D levels, D3 appears to be more than three times as effective as D2.

Any time an athlete purchases a supplement, it’s important to check the label for banned or unsafe substances, and to choose a product that is certified for purity by an agency such as USP, NSF, or Informed-Choice. Dosing should be determined by an athlete’s serum 25-(OH)D test results. Research suggests that an intake of 1,700 to 2,000 IU per day is necessary to maintain a serum level over 32 ng/mL, though the figure varies based on body chemistry and other factors, such as time of year and climate (which determine exposure to sunlight). Follow-up blood testing after several months on a supplement can reveal whether an appropriate level has been achieved.

What can athletes expect once they resolve a vitamin D deficiency? One of the first observed benefits is often a stronger immune system, noticeable in the form of less frequent illness. This has two causes: First, the vitamin D receptor complex appears to control production of lymphocytes (a type of white blood cell). Second, researchers have found a link between vitamin D status and the release of antimicrobial peptides that attack the cell membranes of pathogens.

Recovery after workouts is another area where athletes often see improvement after augmenting their vitamin D supply. Intense exercise creates elevated levels of pro-inflammatory cytokines, which are linked to damage caused by overtraining. Vitamin D has been shown to reduce the production of these cytokines while increasing the production of anti-inflammatory cytokines. Thus, it’s possible that an adequate supply of vitamin D may speed recovery from intense training sessions.

There is limited research looking directly at vitamin D and athletic performance. Recent studies have found a link between vitamin D deficiency and loss of muscle strength in the elderly and in post-menarchal adolescent females, and one found that young women with the deficiency had increased fat infiltration into muscle tissue, but much more research is needed in this area. Still, the best available evidence suggests that athletes’ muscles may function better with a sufficient vitamin D supply in the body.

Randy Bird, MS, RD, CSSD, CSCS, is Director of Sports Nutrition at the University of Virginia.

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