Vitamin D Status and Stress Fractures in Division III Male Cross Country Athletes
Estimates of stress fracture occurrence are as high as 21% among runners. Although one of the risk factors for stress fractures is poor vitamin D status, surprisingly few studies have examined serum vitamin D [25(OH) D] in athletes (Ruohola, J., Laaksi, I., et. al. (2006). Journal of Bone and Mineral Research, 21, 1483-1488.) The lack of measurement may be due to the expectation that humans will synthesize adequate amounts of 25(OH) D with UVB sunlight exposure and therefore, are not totally reliant on dietary sources of vitamin D. However, recent research challenges this assumption especially during periods of inadequate sunlight exposure [e.g. winter months] and questions the adequacy of the dietary recommendation or Adequate Intake [AI] for vitamin D (Willis, K., Peterson, N., and Larson-Meyer, E. (2008). International Journal of Sport Nutrition and Exercise Metabolism, 18, 204 - 224).
Purpose: To evaluate vitamin D status [dietary vitamin D intake and serum levels of 25 (OH) D] and bone density in male cross-country runners].
Methods: The research was approved by the Institutional Review Board of the College and all subjects signed an informed consent prior to data collection. Subjects aged 18 - 22 [n = 37] completed three day diet records during the initial week of practice. Diet records were analyzed using Diet Analysis Plus 7.0.1 software for energy availability and nutrient intake. Serum was frozen at -80oC and later analyzed for levels of 25(OH) D using an ALPCO ELISA assay. Heel bone density was determined using ultrasound with the Achilles InSight bone densitometer early in the season. Body composition measurements and blood samples were collected at the beginning and end of the season. Results: The initial mean serum 25(OH) D value in September was 116 +/- 42 nmol/L (optimal >75 nmol/L); only four runners (11%) were < 75 nmol/L. Mean dietary vitamin D intake was 7.2 µg/day ± 4.3 [AI for vitamin D = 5 µg (200 IU)]; however, nine subjects failed to reach the AI, and one individual obtained only 13%.
Results from the bone densitometer are presented as T scores, a standard deviation above or below the average value for a young adult. Mean T-score values for bone density of all subjects fell within the normal range of -1 to +1 or above. Mean calcium intake from the initial diet analysis was 1590mg/day ± 500 [AI = 1000 mg/day]. The mean December serum 25(OH) D was 79 +/- 28 nmol/L which was significantly lower (p=2.7*10-7) than the earlier measurement. Fourteen runners (38%) were <75 nmol/L by December and 17% (n=6) reported stress fractures.
Conclusions: Vitamin D status as measured by serum 25(OH) D declined significantly by December. Minnesota is above the 44th latitude; consequently the UVB rays are not sufficiently direct to maintain vitamin D levels during the winter months despite adequate dietary intake on average. These results support the need to consume dietary sources of vitamin D above the current AI or obtain controlled UVB exposure during the winter months and to monitor 25(OH) D status in runners to reduce stress fracture risk.
Jessica Walker presenting her research at the at the Northland Chapter of the American College of Sports Medicine, March, 2009, St. Cloud, MN
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