Kaitlin Jackman and James Kohl, College of Nursing
Introduction
Vitamin D is a vital secosteroid that plays an important role in regulating more than 1000 vitamin D responsive genes (Owens, Fraser, & Close, 2014). Recognized for the important role vitamin D has in bone health, more recent studies have made strong arguments in support of optimizing vitamin D to improve cardiac and pulmonary function, reduce systemic inflammation, promote brain health and enhance the immune system. In a recent literature review, the presence of specific immunological cells were higher in athletes with adequate vitamin D levels compared with those who were deficient (Todd, Pourshahidi, McSorley, Madigan, & Magee, 2014). Athletes are at a higher risk for vitamin D deficiency, especially those at higher latitudes, in winter seasons, on indoor practice teams, and in athletes with larger body size. Vitamin D status must regularly be assessed in order to ensure athletes function at peak performance. When insufficient levels of vitamin D are present, the roles of immunologic cells are inhibited, resulting in a decreased immune response. Lower levels are also associated with increased severity and length of upper respiratory-tract infections, or URTIs (Constantini, Dubnov-Raz, Eval, Berry, Cohen & Hemila, 2014). As lower levels of vitamin D are associated with poor muscle, bone, cardiovascular and immunological health, athletes with deficiency in this vitamin are at risk for poorer overall health and at increased risk for injuries. In 2011, a study assessed the vitamin D levels of athletes in the National Football League and found that 51% of the players had serum levels of vitamin D that were considered insufficient. Players with lower levels were found to have increased musculoskeletal injuries when compared with those who had sufficient levels (Shindle, Voos, & Gulotta, 2011). The purpose of this pilot study was to compare vitamin D levels between indoor and outdoor intercollegiate athletes and determine if there is correlation between vitamin D levels and the self-reported number of upper respiratory infections and musculoskeletal injuries. For the purpose of this study a “normal” level of vitamin D is considered between 30-80 ng/mL. Levels less than 20 ng/mL are considered deficient while levels between 20-29 ng/mL are insufficient. However, several studies have determined that levels less than 50 ng/mL place athletes at risk for stress fractures and other musculoskeletal injuries (Shuler, Wingate, Moore & Giangarra, 2015).
Methodology
This is a descriptive pilot study utilizing mixed methods. A total of 133 healthy athletes were recruited from both indoor and outdoor, male and female collegiate teams between the ages of 19 and 28 years old. A total of 126 athletes consented to having blood levels drawn. Blood samples were drawn in April and tested in a local hospital laboratory and vitamin D, 25-hydroxy levels determined. Furthermore, a Qualtrics survey was developed and administered to these athletes regarding the self-reported prevalence of upper respiratory illnesses, visits to a physical therapist and musculo-skeletal injuries for the past athletic season. Results of the survey and blood samples were analyzed using the Tukey-Kramer multiple comparison procedure and analysis of the differences of least squares means using IBM SPSS statistical software version 23. Alpha was set at 0.05 for the accepted level of significance.
Results
The participant’s descriptive characteristics are shown in Table 1 and Table 2. All participants (n=133) completed the survey, however only 126 participants proceeded to having their blood sample drawn. Results for differences between indoor and outdoor athletes are reported in Table 3. Athletes reporting participation primarily in indoor sports had a mean vitamin D level of 36 ng/mL, whereas those reporting mainly outdoor sports had a mean level of 44 ng/mL. An adjusted P score for comparing athletes playing indoor and outdoor sports was 0.0130, meaning the pairwise test score was statistically significant. No statistical significance was found when comparing vitamin D levels to respiratory infections, muscle injuries, or bone injuries, however, this could be due to limitations in the study design.
Discussion
Approximately 30% of all athletes in this study were found to be insufficient in vitamin D. However, using the maximal sports benefit level of 50 ng/mL value as reported by Shuler et al (2105), 75-85% of athletes could potentially benefit from a dietary supplementation of vitamin D. Athletes with low vitamin D levels may be at risk for decreased athletic performance, such as delayed muscle response, decreased agility, and weakened bones, and a decreased immune response. Compared to outdoor team players, indoor team players were statistically more insufficient in vitamin D placing them at increased risk for significant health related issued.
There were several confounding variables and limitations in this study. First, Utah experienced an unusually warm winter in the school year of 2014-2015, and many athletes are likely to have received more sun exposure during that time than would be normal in typical years. Thus, a higher percentage of athletes may be vitamin D deficient during a typical year, due to less sun exposure in colder months. Secondly, the survey design may have possibly been confusing to athletes and led to skewed results due to inaccurate reporting of upper respiratory infections and number of visits to a physical therapist or sports injury specialist. Third, the duration of sun exposure as well as surface area exposed during practice and actual competition was not taken into account.
Conclusion
Vitamin D in an important nutrient that must be obtained either through dietary supplementation or exposure to the ultraviolet rays of the sun. Winter months at higher latitudes places athletes at greater risk for developing vitamin D insufficiency. Adequate vitamin D levels can aid in greater strength and agility, stronger bones, and increased endurance and immunity to common respiratory viruses. Approximately 30% of athletes participating in this study were deficient in this vitamin and could possibly contribute to increased injuries and poorer performance secondary to a weakened immune system. Thus, individual performance and consequently the performance of the entire teams overall could be potentially improved with the use of Vitamin D supplementation. More controlled studies are necessary to establish what the optimal level of vitamin D is in relationship to musculoskeletal injuries and a weakened immune system. Outdoor training could improve vitamin D sufficiency among all athletes. Given the relatively low risk and cost of testing, and the low morbidity of supplementation, obtaining baseline and routine levels of vitamin D could identify the subgroup of athletes who could benefit most from an oral supplementation schedule.
Table 1. Gender of athletes
Frequency | Frequency | Percent | Indoor (n=52) | Outdoor (n=74) |
Male | 58 | 43.61 | 17 | 36 |
Female | 75 | 56.39 | 35 | 38 |
Total (n) | 133 | 100 |
Table 2. Ethnicity of athletes
Ethnic Background | Frequency | Percent |
White | 104 | 78.20 |
African American | 9 | 6.77 |
Hispanic | 2 | 1.5 |
Pacific Islander | 14 | 10.53 |
Asian | 2 | 3.01 |
Table 3. Comparison of vitamin D levels between indoor and outdoor athletes
Vitamin D level ng/mL | Standard error | df | T value | |
Indoor athletes (n=52) | 36.0267 | 1.9595 | 122 | 18.39 |
Outdoor athletes (n=74) | 44.0625 | 2.0187 | 122 | 21.83 |
*Adjusted P 0.0130 pairwise tests
Table 4. Vitamin D comparison of indoor & outdoor, male & female athletes at optimal therapeutic range
Indoor Vitamin D <30ng/mL | Outdoor Vitamin D <30ng/mL | Indoor Vitamin D <50 ng/mL | Outdoor Vitamin D <50 ng/ml | |
Female | 11 (31%) | 7 (18%) | 31(89%) | 31(82%) |
Male | 6 (35%) | 5 (14%) | 14 (82%) | 24 (67%) |
Total | 17 (n=52) | 12 (n=74) | 45 | 55 |
% overall insufficient | 33% | 16% | 86% | 74% |