Nancy A. Melville

July 13, 2011

July 13, 2011 (San Diego, California) — Vitamin D deficiency might be unusually high among American football players, with black players and players who have suffered muscle injuries showing significantly lower vitamin D levels, according to a study presented here at the American Orthopaedic Society for Sports Medicine 2011 Annual Meeting.

Researchers testing the vitamin D levels of 89 professional football players from the New York Giants National Football League (NFL) team in the spring of 2010 found that 27 players (30.3%) had deficient total 25-hydroxy vitamin D levels (below 20 ng/mL), and as many as 45 players (50.6%) had levels consistent with vitamin D insufficiency (between 20 and 31.9 ng/mL).

Only 17 players (19.1%) had values within normal limits (above 32 ng/mL).

"We found that an alarming percentage of players — 80.9% — had abnormal vitamin D levels of less than 32 ng/mL," said lead author Michael K. Shindle, MD, an orthopaedic surgeon from Summit Medical Group, in Berkeley Heights, New Jersey.

In addition, all players sustaining injuries that caused them to miss at least 1 practice or game had vitamin D levels that were significantly lower than players without muscle injury, he said.

"Among the 18% of players who sustained a muscle injury in the previous season, all had statistically significant lower vitamin D levels, compared with those without muscle injury. There were no other statistically significant differences between those who did and did not sustain the injuries."

The mean vitamin D level among players who did not sustain a muscle injury was 24.7 ng/mL (range, 9.0 to 46.0 ng/mL), and the mean level among the 16 players who did suffer a muscle injury was 19.9 ng/mL (range, 8.0 to 33.0 ng/mL) (< .04).

The team consisted of 31 white players and 58 black players. Blacks tend to have lower vitamin D levels than whites in the general population. The mean vitamin D level in white players was 30.3 ng/mL (range, 17.0 to 46.0 ng/mL), whereas among black players, the mean level was 20.4 ng/mL (range, 8.0 to 40.0 ng/mL) (P < 0.001).

"Up to 93% of African American players had abnormal vitamin D levels, compared with 31% of white players," Dr. Shindle said.

Vitamin D is believed to play an important role in skeletal muscle health. A primary source of the vitamin is sunlight exposure; dietary intake is considered only a minor source, because vitamin D is rare in foods other than fatty fish or fortified food products (such as milk and breakfast cereals), Dr. Shindle explained.

Since exposure to ultraviolet light has been shown to increase athletic performance, there is heightened interest in the status of vitamin D levels in athletes, he said.

"Athletic performance has been shown to be seasonal in nature, and corresponds with when vitamin D levels are at their highest and lowest points. It's also been shown that vitamin D can increase the number of type II, or fast twitch, muscle fibers," he said.

"Randomized trials looking at the association, however, have involved older individuals, so there is controversy about the association."

Limitations of this study include the fact that the muscle injuries were evaluated retrospectively, from the previous season, and that the study's figures reflect a single testing point in May, when levels can be low, Dr. Shindle explained.

He added that levels were checked again after 3 weeks of training camp, and although the percentage of deficient players had dropped to about 68%, the rate was still high.

Christian Lattermann, MD, associate professor of orthopaedic surgery and sports medicine, University of Kentucky in Lexington, agreed that the single-point assessment is a limitation and that further research is necessary to determine players' vitamin D levels throughout the year.

"I think this is a very interesting study, and it highlights a potential problem," said Dr. Lattermann, who moderated the session and is director of the university's Center for Cartilage Repair and Restoration.

"One thing I would be cautious about, however, is linking the deficiencies directly to muscle injuries based on these data. The study looks at the injuries retrospectively over the season, yet compares those to essentially a 1-time measurement," he said.

Dr. Lattermann added that overall vitamin D deficiency and insufficiency in the general population is relatively high, and previous studies have shown a comparatively high prevalence of vitamin D deficiency in athletes.

"Some reports say that up to 50% of people may be vitamin D deficient," he explained. "It only recently came to our attention that athletes commonly seem to have vitamin D deficiencies, but I would be very cautious to say that the data show a link to an increased rate of muscle injury."

"If there is a connection, it would be clinically very significant, but I would recommend that the researchers follow-up on this with a prospective study to verify the result."

Dr. Shindle noted that even when deficiencies are confirmed, caution is needed in regard to vitamin D supplementation in light of recent warnings from the Institute of Medicine and others about the association between excessive levels of vitamin D and fractures, pancreatic cancer, and overall death rates.

"As far as the NFL is concerned, any over-the-counter supplements are discouraged out of concern about...banned substances, so we don't recommend empiric administration of vitamin D. However, we do recommend that vitamin D levels be tested as part of routine care and athletes be treated with prescription, when appropriate."

Dr. Shindle has disclosed no relevant financial relationships. Dr. Lattermann reports being a consultant for Genzyme, Zimmer, and

American Orthopaedic Society for Sports Medicine (AOSSM) 2011 Annual Meeting: Abstract 46. Presented July 10, 2011.


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