Vitamin D contributes to many important functions in the body, including the absorption of calcium and improvement of bone strength. According to a new review article published in the April 15, 2018, issue of the Journal of the American Academy of Orthopaedic Surgeons (JAAOS), vitamin D supplements help to increase muscle strength in athletes who are vitamin D deficient. Higher vitamin D levels have also been linked to reducing injury and improving athletic performance.

"Vitamin D deficiency commonly affects many people around the world," said lead study author and orthopaedic surgeon Geoffrey D. Abrams, MD. "With higher serum levels of vitamin D playing a role in muscle strength, injury prevention, and sports performance, it's essential for individuals to take necessary steps to ensure they're getting an adequate amount of vitamin D intake, whether through direct sunlight or other sources including fish, eggs, fortified dairy products, and dietary supplements. Studies also have shown that daily vitamin D supplements are proven to be more effective than weekly or monthly doses."

Vitamin D is measured using the 25-hydroxy vitamin D test using nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL). According to the Endocrine Society's clinical practice guideline for the evaluation, prevention, and treatment of vitamin D deficiency,

• less than 20 ng/mL (50 nmol/L) is deficient,
• 21 to 29 ng/mL (50 to 75 nmol/L) is insufficient, and
• 30 ng/mL (75 nmol/L) is considered a normal level.

People with very low vitamin D blood levels may be more likely to experience muscle cramps, bone pain, or joint pain. The review article authors highlight findings across several studies to determine the effect of vitamin D in three categories: 

• Muscle Strength. One study examined 310 participants with an average age of 24 years. Participants had a mean vitamin D level of 12.3 ng/mL and received either vitamin D3 or placebo supplements. Their upper and lower body strength were assessed with isokinetic dynamometers, a tool used to test the performance of different muscle groups. Results found that vitamin D supplementation in these athletes resulted in increased upper and lower body strength.
• Sports Performance. Another study investigated the effect of vitamin D on sports performance by looking at 80 American professional football players and their ability to obtain a professional contract. Seventy-seven percent of these athletes were vitamin D deficient or insufficient. The authors of that study found a statistically significant correlation between lower vitamin D levels and the athlete's release from a team due to poor performance or injury before the start of the regular season. An additional study assessed the effect of vitamin D on a group of athletes who received 5,000 International Units (IU) per day of vitamin D over an 8-week period. Seventy percent of these athletes had a vitamin D level of less than 20 ng/mL. Findings for this study showed that the group receiving vitamin D supplementation had increased vertical jump heights throughout the study, whereas the placebo group experienced no change.
• The risk of injury. Another study looked at 1,000 Royal Marine recruits in the United Kingdom to evaluate the association between vitamin D levels and risk of stress fracture. The study identified 92 stress fractures and found that recruits with vitamin D levels of less than 20 ng/mL had a 60 percent higher incidence of stress fractures than recruits with greater vitamin D concentrations.

"While vitamin D supplementation improves function and decreases fracture risk in people who are vitamin D deficient, it's important for individuals to be aware of the safe dosage amount, which varies with age and the status of an individual's current vitamin D level," explains Dr. Abrams. "We are not advocating for athletes to take additional vitamin D without first speaking with a doctor."

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