Vitamin D status in collegiate female athletes: Relationship to indoor vs. outdoor sports

Vitamin D is important for bone metabolism, muscle function and possibly athletic performance. Low serum 25(OH)D concentrations among adolescents and adults have been observed. Limited research exists on vitamin D status in female athletes living in the southern United States or on differences in serum 25(OH)D concentrations between athletes who play indoor or outdoor sports. The purpose of this study was to determine differences in fall 2009 serum 25(OH)D concentrations between indoor and outdoor sport athletes living at 33.2°N latitude and to examine changes in serum 25(OH)D concentrations of indoor sport female athletes from fall 2009 to spring 2010.

Athletes were recruited from a NCAA division II college by purposive convenience sampling. Vitamin D intakes were estimated using 3-day food records and analyzed using Nutritionist Pro software. Body composition and bone mineral density were measured by DXA scanning. Serum 25(OH)D concentrations were obtained using blood spot testing and measured by liquid chromatography-tandem mass spectrometry. Sun exposure and use of tanning beds were assessed with self-reported questionnaires. Independent-t-tests were used to compare serum 25(OH)D concentration differences between indoor and outdoor sport athletes. A paired-t-test was used to determine if there was a significant change from fall to spring in serum 25(OH)D concentrations of indoor sport athletes. A p value < .05 was significant.

Estimated vitamin D intakes for all athletes were below the recommended intake value (5 μg/d). In the fall, there was no significant difference (p = .52) found between indoor (n=17) and outdoor (n=19) sport athletes in serum 25(OH)D concentrations (mean ± SD) of 50 ± 20 and 46 ± 15 ng/ml, respectively. There was also no significant change (p = .31) in mean serum 25(OH)D concentrations in indoor athletes (n=11) from fall to spring.

The mean serum 25(OH)D concentrations of female athletes living at 33.2°N latitude were within optimal concentrations of 40–70 ng/ml. Although indoor athletes do not obtain similar sun exposure as outdoor athletes, no significant differences in serum 25(OH)D concentrations were observed. Summer UV exposure indicated on self-reported questionnaires may be a reason. No significant change in serum 25(OH)D concentrations was observed in indoor athletes from fall to spring, and may be due to UVB exposure from tanning bed use.