Jan 29, 2015
The D Factor

Chances are some (if not most) of your athletes don’t get enough vitamin D to support optimal health and performance. But that’s easy to fix, and doing so can pay huge dividends.

By Randy Bird

Randy Bird, MS, RD, CSSD, CSCS is Sports Nutritionist for the University of Kansas Athletic Department. He can be reached at: [email protected].

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. Vitamin D deficiency, once thought to be conquered in the industrialized world, has made a comeback and is reaching epidemic proportions. Today, 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.

The good news is that vitamin D deficiency is quite easy to address. Once athletes understand why this readily accessible substance is crucial for their health and performance, they can make a few simple changes to their daily habits to ensure an adequate supply.


What we call vitamin D is actually a class of fat-soluble vitamins called secosteroids, and its primary function in humans is to promote calcium absorption. It also helps regulate inflammation and neuromuscular control.

There are two major forms of vitamin D: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The only good naturally occurring source of vitamin D2 is mushrooms that have been exposed to UV light. Whenever vegetarian or vegan products are fortified with vitamin D, the manufacturer uses D2.

Vitamin D3 is found in small amounts in a few animal based foods, and it can also be extracted from the lanolin of sheep’s wool and used to fortify foods or sold in supplement form. In addition, the human body produces its own D3 when skin is exposed to sunlight (specifically the sun’s UVB rays).

Research has shown that D3 is better than D2 at staying in the bloodstream in adequate levels, allowing the body to take advantage of its many benefits. Because vegetarians and vegans are less likely to consume D3 through food, they may benefit even more than other athletes from taking a vitamin D supplement.

Some people are at greater risk of vitamin D deficiency than others. Because the body’s own vitamin D factory depends on sunlight, the most vulnerable are those with limited exposure to the sun. Athletes who practice and play indoors, live in northern climates, and keep their skin covered when outside are unlikely to obtain much vitamin D from sunlight.

Sunscreen can also block the production of vitamin D. In fact, one study found that an SPF as low as eight decreased vitamin D production by 95 percent. (For more on sunscreen and vitamin D production, see “Under the Sun” below).

Skin tone affects vitamin D synthesis as well. People with darker skin (higher levels of melanin) have a reduced ability to produce vitamin D from exposure to sunlight. Dark-skinned people may need two to three times the exposure to sunlight to produce the same amount of vitamin D as fair-skinned people.

Lastly, individuals with difficulty absorbing dietary fat are at increased risk for deficiency, since vitamin D in food requires some dietary fat in the gut for absorption. Fat malabsorption can be caused by several medical conditions, including pancreatic enzyme deficiency, Crohn’s disease, cystic fibrosis, celiac disease, some forms of liver disease, and surgical removal of part of the stomach or intestines.

The list of health problems linked to vitamin D deficiency is staggering. It includes weakened bones, a compromised immune system, muscle weakness and pain, weight gain, depression, certain types of cancer, heart disease, and hypertension.

One of the most serious effects–bone weakening–occurs because vitamin D regulates calcium and phosphate concentration in the blood, which helps govern bone mineralization, growth, and modeling. For the general population, an inadequate supply can lead to diseases such as rickets, osteomalacia, and osteoporosis. For competitive athletes, additional concerns are chronic musculoskeletal pain, stress fractures, and heightened risk for trauma-related fractures.

The mental health effects of vitamin D are not as well understood, but some have been documented in the literature. One recent study published in the Journal of Internal Medicine found that people with vitamin D deficiency showed more depressive traits than those with normal levels. When some of the test subjects were treated with regular vitamin D supplementation for a year, their mental health improved significantly.


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. Small amounts of vitamin D3 are also found in beef liver, cheese, and egg yolks. The aforementioned UV-exposed mushrooms are also a dietary source, providing a moderate amount of D2.

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.

Some studies suggest that these antimicrobial peptides also help fight off the flu virus, and that seasonal fluctuations in vitamin D levels could explain the higher incidence of colds and flu in winter. One group of researchers found that 2,000 IU per day reduced occurrence of the flu by 70 percent. This finding needs to be repeated in future research, but for athletes who compete in winter when the flu is a constant threat, it suggests vitamin D supplementation may be as important as getting a flu shot.

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.


It is possible to get too much vitamin D. Most cases of vitamin D poisoning (also known as hypervitaminosis D) stem not from sunlight or food, but from overdoses of supplements. Toxicity can cause non-specific symptoms such as nausea, vomiting, poor appetite, constipation, weakness, and weight loss. More seriously, it can also raise blood calcium levels, causing heart rhythm abnormalities and mental status changes such as confusion. Over time, it may lead to deposits of calcium and phosphate in the kidneys, heart, and lungs, reducing their ability to function. In some cases, the kidney damage is irreversible.

A serum 25-(OH)D concentration consistently above 200 ng/mL is considered to be potentially toxic. Vitamin D toxicity is treated by discontinuing any supplementation and restricting calcium intake.

The maximum long-term safe dose of vitamin D is not known. Since 1997, the U.S. Dietary Reference Intake Tolerable Upper Intake Level (UL) of vitamin D for children and adults has been 2,000 IU per day, but many scientists believe that figure is outdated. A 2007 risk assessment suggested that 10,000 IU per day did not produce toxicity in healthy adults and therefore should be adopted as the tolerable upper limit. Meanwhile, research has shown that sustained intake of 100,000 IU per day is enough to produce toxicity in adults within a few months.

Because of vitamin D’s limited availability through food and the body’s natural limits on how much it produces through UV exposure, there is virtually no risk of reaching a toxic level by sitting in the sun all day eating salmon and drinking milk. But athletes who take vitamins and other supplements often follow the mistaken notion that if a little is good, more must be better. With this mindset, they could potentially achieve a toxic level of vitamin D. Check the labels and dosing of any supplements they may be using, and if you’re concerned about someone’s vitamin D level, recommend that they undergo a blood test.

Given all that we know about vitamin D’s positive impact on health, there is no excuse for the high rate of deficiency in today’s population. And since athletes demand more of their bodies than other people, it’s essential for them to achieve adequate vitamin D levels. Sound dietary choices and possibly a trusted multivitamin or vitamin D supplement are all that’s needed to ensure they are maximizing their health and athletic potential.


The human body produces some vitamin D on its own when skin is exposed to sunlight (specifically UVB rays), thus helping to prevent deficiency. But with the recent public health push for wearing sunscreen to reduce skin cancer risk, athletes may wonder which concern outweighs the other. Does sunscreen stunt the body’s ability to produce valuable vitamin D?

The short answer is “yes and no.” Some studies have found that sunscreen use significantly hampers endogenous vitamin D production, while others have followed subjects for extended periods and found that the effect of sunscreen is negligible.

The New York Times recently asked Henry Lim, MD, Chairman of Dermatology at the Henry Ford Hospital in Detroit and a spokesman for the American Academy of Dermatology, what to make of the conflicting information. He said the most reasonable interpretation is that sunscreen users typically don’t apply enough of the product to block all UV rays, so they still receive the vitamin D-producing benefits of sunlight. That isn’t surprising, since the National Institutes of Health reports that as little as five to 30 minutes of direct sun exposure twice a week may be enough to produce an adequate amount of endogenous vitamin D.

Lim concluded that the benefits of wearing sunscreen outweigh any potential harm from a vitamin D reduction. That’s especially true because vitamin D is easy to obtain otherwise through multivitamins, supplements, and diet.

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