Vitamin D deficiency has been linked to a panoply of health problems, ranging from osteoporosis and flu to auto-immune disorders and common cancers.
Now, a study in NFL football players suggests that lack of vitamin D may also raise the risk of muscle injuries in athletes.
The authors tested the vitamin D blood levels of 89 players from the NFL's New York Giants during the spring of 2010.
Their average age was 25, and the group included 31 white and 58 black (African American) players.
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According to lead researcher Michael Shindle, M.D., “Eighty percent of the football team we studied had vitamin D insufficiency. African American players and players who suffered muscle injuries had significantly lower levels.” (AAOSM 2011)
The findings come from a study presented in San Diego at the Annual Meeting of the American Orthopaedic Society for Sports Medicine (AOSSM), by researchers from New Jersey's Summit Medical Group and New York's Hospital for Special Surgery.
Many NY Giants players tested deficient in vitamin D, with injurious implications
Sixteen of the 89 tested players had spent time on the disabled list due to muscle injuries.
The average vitamin D level among these previously muscle-injured players was 19.9 nanograms per milliliter (ng/mL) … just below the official definition of vitamin D deficiency (20 ng/mL or less).
The 73 players with no history of time lost due to a muscle injury had an average vitamin D level of 24.7 ng/mL – which most experts would call insufficient or deficient – with their levels ranging from 9 ng/mL (very deficient) to 46 ng/mL (lower end of the optimal range).
These were the overall results of the blood tests on the 89 New York Giants players:
Twenty-seven (30.3%) players had levels considered deficient (less than 20 ng/mL).
Forty-five (50.6%) players had levels considered insufficient (20 to 31.9 ng/mL).
Seventeen players (19.1%) had levels considered normal (32 ng/mL or higher).
It is possible that the vitamin D levels might have been higher in a team from a sunnier city.
Ethnic divide seen in NFL players' vitamin D levels
The new study also detected a big ethnic divide in the vitamin D levels of the 89 players tested.
Vitamin D blood levels averaged 30.3 ng/mL among the white players, while the black players' blood levels averaged only 20.4 ng/mL.
Dark-skinned people of all ethnicities tend to have lower vitamin D levels.
Ultra-violet (UV) sunrays trigger vitamin D production in the skin and the melanin pigment in human skin blocks those rays.
Compared with fair-skinned people, dark-skinned people have higher levels of melanin pigment in their skin, so they make less vitamin D in response to the same amount of UV exposure.
Likewise, the team's average vitamin D levels might have been higher if the players had been tested later in the season, after a summer of sun exposure.
However, a summertime test would probably have presented a less accurate picture of the players' average annual vitamin D levels.
Based on the growing evidence that 20 ng/mL is too low to ensure adequate health, many vitamin D testing laboratories have raised their definition of a “normal” blood level to 30 ng/ml or more.
Yet, some of the players had less than 10 ng/mL, which falls very far short of the levels recommended by many vitamin D researchers.
For example, Boston University's Michael Holick, Ph.D., M.D. – a leading researcher and author of The Vitamin D Solution – says the optimal level is 40 to 60 ng/mL and that it takes 3,000 IU per day of dietary vitamin D to achieve and maintain that range.
Co-author Scott Rodeo, M.D., urged team physicians to heed the implications of their findings: “Screening and treatment of vitamin D insufficiency in professional athletes may be a simple way to help prevent injuries” (AAOSM 2011).
American Orthopaedic Society for Sports Medicine (AOSSM). Vitamin D Lower in NFL Football Players Who Suffered Muscle Injuries, Study Reports. June 10, 2011. Accessed at
Shindle MK et al. Vitamin D Status in a Professional American Football Team. ID 46 – 9849. AOSSM 2011 Annual Meeting, San Diego. Accessed at http://www.sportsmed.org/secure/reveal/admin/uploads/events/PODIUM%20ABSTRACTS%20Binder1.pdf