Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Summary of Background and Research Design
Background:Vitamin D is necessary for bone health and plays an important role in inflammation and immunity. Suboptimal vitamin D levels increase the risk of overuse and inflammatory injuries and increase susceptibility to illness. Vitamin D deficiency (25-hydroxy vitamin D, or 25(OH)D, < 20 ng/mL) and insufficiency (25(OH)D > 20 ng/mL to < 32 ng/mL) are endemic in the general population, and a surprisingly high percentage of athletes participating in outdoor or indoor sports have less than adequate levels of vitamin D.
Hypothesis:The authors of this study hypothesized that (1) athletes participating in indoor sports would have lower concentrations of 25(OH)D compared with those participating in outdoor sports, (2) that within the entire group of athletes studied sun exposure, sunscreen use, and vitamin D intake (from food and supplements) would be predictive of 25(OH)D concentrations, and (3) that lower 25(OH)D concentrations at any time point during the year would increase risk for low bone density, overuse and/or inflammatory injuries, and frequent illness.
Subjects:41 (18 men, 23 women) National Collegiate Athletic Association (NCAA) Division 1 athletes (12 indoor, 29 outdoor) from the University of Wyoming (2195 m, 41.3°N) participated in the study. Because of variability in the training schedules and conflicts, the number of subjects was N = 41 in the fall, N = 33 in the winter, and N = 25 in the spring. Mean ± standard deviation baseline characteristics for men and women, respectively, were age: 20.1 ±1.9 and 19.9 ±1.5 years, height: 183.9 ± 11.2 and 168.1 ± 9.6 cm, weight: 88.0 ± 19.6 and 59.6 ± 10.2 kg, and body mass index (BMI): 25.9 ± 4.4 and 20.9 ± 1.9 kg/m2.
Experimental design:Longitudinal, observational (no intervention)
Treatments and protocol:Blood samples were collected for analysis of 25(OH)D and parathyroid hormone (PTH) in the fall, winter, and spring. Dietary intake and lifestyle habits were assessed in these same time periods; subjects self-reported via questionnaire their frequency of intake of vitamin D-containing foods and supplements, frequency of leisure time spent outside, frequency of tanning bed use, frequency and type of sunscreen applied, and type of clothing typically worn outdoors. At study completion (spring only), body composition and bone density of the hip, lumbar spine, and total body were evaluated using dual energy x-ray absorptiometry (DEXA). Injuries and illnesses were documented throughout the academic year by athletic training staff and team physicians, and were reviewed by a certified athletic trainer blinded to the vitamin D status of the athletes.
Summary of research findings:
- Mean 25(OH)D status was significantly higher in outdoor vs indoor athletes in the fall (53.1 ± 17.4 vs 39.3 ± 8.9 ng/mL; P = .013) but not in the winter (31.9 ± 10.2 vs 26.3 ± 5.0 ng/mL; P = .15) or spring (44.6 ± 15.6 vs 33.1 ± 4.8 ng/mL; P = .09)
- Mean 25(OH)D concentration was 49.0 ± 16.6, 30.5 ± 9.4, and 41.9 ± 14.6 ng/mL in the fall, winter, and spring , respectively, and varied significantly across time (P = .001)
- The percentages and numbers of subjects by season that were 25(OH)D insufficient/deficient were: Fall 9.8% (n = 4)/2.4% (n = 1); Winter 60.6% (n = 20)/3.0% (n = 1); Spring 16.0% (n = 4)/4.0% (n = 1).
- Using a cut off of 40 ng/mL (lower limit of optimal for humans living in a sun-rich environment), 75.6% (n = 31), 15.2% (n = 5), and 36.0% (n = 9) of subjects had optimal vitamin D status in the fall, winter, and spring, respectively
- Vitamin D status did not differ by sex at any time point (P = .10)
- 25(OH)D concentrations correlated significantly with multivitamin intake in the winter (r = 0.39; P = .025) and tanning bed use in the spring (r = 0.48; P = .016), but were otherwise not related to dietary intake, lifestyle factors, or body composition
- Frequency of injury was not related to vitamin D status; however, 25(OH)D concentrations in the spring (r = –0.40; P = .048) negatively correlated with frequency of illness (eg, upper respiratory infection, the common cold, influenza, and gastroenteritis)
Interpretation of findings/Key practice applications:
In this study, a lower than anticipated percentage of college athletes had insufficient vitamin D status in the fall or spring, and a high percentage of athletes with sufficient vitamin D status maintained stores in a more optimal range (> 40 ng/mL). Compared with previous studies, the sunny and mild climate of Wyoming during the spring, summer, and fall months as well as a lack of cloud cover, pollution, and higher elevation may have increased the fractional strength of ultraviolet B (UVB) radiation in this study, resulting in an increased synthesis of endogenous vitamin D. The study demonstrated that indoor athletes had lower 25(OH)D concentrations compared with outdoor athletes, suggesting that the former may be at greater risk for injury or illness. Consistent with other research, a negative correlation was noted, at least in the spring, between frequency of illness and 25(OH)D concentrations. Thus, maintaining sufficient vitamin D status may reduce the risk of common infections and illnesses. Study limitations include use of self-reported data via questionnaires. Further research is needed to determine whether vitamin D status influences risk for overtraining and inflammatory injury.