PTH and relation with vitamin D status.
Serum PTH concentrations ranged between 7.7 and 54.4 pg·mL−1 (23.8 ± 10.9 pg·mL−1) in the fall, between 9.4 and 50.8 pg·mL−1 (25.4 ± 9.3 pg·mL−1) in the winter, and between 12.8 and 48.5 pg·mL−1 (28.1 ± 9.9 pg·mL−1) in the spring (normal range = 12-54 pg·mL−1) and did not change significantly across time (P = 0.18). PTH concentrations were not different between indoor and outdoor athletes in the fall (24.8 ± 12.5 vs 23.3 ± 10.4 pg·mL−1), winter (25.2 ± 12.1 vs 25.5 ± 8.3 pg·mL−1), or spring (22.6 ± 11.1 vs 29.8 ± 9.1 pg·mL−1) (P > 0.10). As shown in Table 1, PTH concentration was not correlated with 25(OH)D concentration at any time point.
Intake of vitamin D-containing foods and supplements and relation with vitamin D status.
Frequency of consumption of selected vitamin D-containing foods and supplements along with their vitamin D content is shown in Table 2. Vitamin D status was correlated with MVI intake in the winter (r = 0.39, P = 0.025, n = 33) but not in the fall (r = 0.29, P = 0.062, n = 41) or spring (r = −0.14, P = 0.49, n = 25). In the fall, consumption of orange juice was negatively correlated with 25(OH)D status (−0.36, P = 0.02). No other correlations were found between vitamin D status and self-reported intake of vitamin D-containing foods.
Estimated vitamin D intake from food sources averaged 242 ± 161, 282 ± 206, and 204 ± 171 IU·d−1 from food sources in the fall, winter, and spring, respectively, and averaged 553 ± 471, 683 ± 610, and 489 ± 456 IU·d−1 when vitamin D supplements (including an MVI) were included. Neither total vitamin D intake nor intake from food alone differed across time or with sex or training location (indoor vs outdoor) (P > 0.05). Neither vitamin D intake from food nor food plus supplements was significantly correlated with vitamin D status in the fall (r = −0.05 and 0.03), winter (r = 0.02 and 0.00), or spring (r = −0.10 and 0.17) (P > 0.05), respectively. A very small percentage of athletes (4.9% in the fall, 6.1% in the winter, and 4% in the spring) consumed the current RDA of 600 IU from food alone. This increased to 26.8%, 18.2%, and 20% in the fall, winter, and spring when supplements were included. Interestingly, with the exception of two athletes during the winter, those who consumed >1000 IU·d−1 from food plus supplements had sufficient status (≥32 ng·mL−1).
UV exposure and relation with vitamin D status.
Reported leisure time spent outdoors changed significantly across time (P = 0.001) and averaged 4.5 ± 1.8, 3.6 ± 1.7, and 4.0 ± 1.8 h·wk−1 in the fall, winter, and spring, respectively. Reported tanning bed use averaged 0.24 ± 0.89, 0.03 ± 0.17, and 0.24 ± 0.66 h·wk−1 and did not vary significantly across time (P = 0.08). Time spent outside during practice or competition averaged 7.8 ± 6.4, 0 ± 0, and 4.9 ± 4.7 h·wk−1 in the fall, winter, and spring, respectively. Leisure time spent outdoors was not significantly (P > 0.10) correlated with vitamin D status at any time point. However, tanning bed use was correlated with 25(OH)D concentrations in the spring (r = 0.48, P = 0.016) but not winter (r = 0.20) or fall (r = 0.17) (P > 0.05), whereas time spent outside during practice or competition and total time spent outdoors were correlated with vitamin D status in the fall (r = 0.40 and 0.42, P < 0.01) but not the spring (r = 30 and 0.21, P > 0.05), respectively. Leisure time spent outdoors was significantly lower in indoor versus outdoor athletes in the winter (2.4 ± 1.5 vs 3.9 ± 1.6 h·wk−1, P = 0.02) and spring (2.8 ± 1.2 vs 4.4 ± 1.8 h·wk−1, P = 0.068) but not in the fall (4.1 ± 1.5 vs 4.8 ± 1.8 h·wk−1, P > 0.05). Reported tanning bed use, sunscreen use, or SPF typically applied were not different between indoor versus outdoor athletes at any time point.
Relation between vitamin D status and body composition.
Relations between vitamin D status and body mass, BMI, and percentage body fat are shown in Table 2. Pearson correlations between body mass or BMI were not found at any time point, although 25(OH)D concentrations tended to be correlated with body fat percentage in the fall and spring. When correlations were adjusted for sex, however, the relation between body fat and 25(OH)D concentration was significant in the fall (partial r = 0.44, P = 0.05) but not the winter (P = 0.19, P = 0.41) or spring (partial r = −0.33 P = 0.16).
Relation between vitamin D status and bone density.
Vitamin D status was not correlated (P ≥ 0.05) with total body bone density (r = 0.02) or bone density in the lumbar spine (r = 0.06) or dual femur (r = 0.02), which was assessed only in the spring. PTH, however, was significantly correlated with bone density assessed in the total body (r = −0.66, P = 0.001), lumbar spine (r = −0.51, P = 0.03), and dual femur (r = −0.55, P = 0.01).
Relation between vitamin D status and illness and injury.
Seven of 33 athletes who remained in the study at the winter collection point developed overuse injuries that were not due to contusion. This included one case of Achilles tendonitis, five stress reactions (two in the shin, two in the foot, and one in the femur), and a fracture of the foot, five of which occurred in athletes participating in cross-country or track and field. Frequency of injury was not related to vitamin D status but was significantly negatively correlated with total body bone mineral density (BMD; r = −0.50, P = 0.02).
Thirteen of 33 athletes also contracted at least one documented illness, which included the common cold, flu, or other URI, with 8 of the total contracting one illness, 3 contracting two different illnesses, and 2 contracting four. Vitamin D status in the fall, winter, or spring was not significantly correlated with incidence of injury but was significantly correlated with frequency of illness in the spring (r = −0.40, P = 0.048; Fig. 3) but not in the winter (r = −0.33, P = 0.065; Fig. 3) or fall (r = -0.15, P = 0.39).
The purposes of this study were to examine the vitamin D status of male and female college athletes during the university academic year to determine if vitamin D status (circulating concentrations of 25-hydroxy vitamin D) was related to dietary intake, training, and lifestyle habits and/or body composition and to evaluate whether insufficient status was linked with compromised bone density or increased risk for illness or inflammatory injuries. We found that vitamin D status varied across the year, with a higher percentage of athletes having insufficient or deficient status (25(OH)D < 32 ng·mL−1) in the winter (63.6%) compared with the fall (12.2%) and spring (20%). Interestingly, 75.6% and 36.0% of athletes maintained status within the more optimal range (>40 ng·mL−1) in the fall and spring compared with only 15.2% in the winter. We also found that low vitamin D status in the spring was correlated with frequency of illness-including URI, the common cold, influenza, and gastroenteritis. Although vitamin D status was correlated with outdoor practice or competition time in the fall and spring, reported MVI consumption in the winter, and tanning bed use in the spring, status was otherwise not significantly related to vitamin D intake, lifestyle habits, or body composition.
The main finding from this study was that a surprisingly low percentage of college athletes had insufficient status (defined as a circulating 25(OH)D concentration <32 ng·mL−1) in the fall and spring and more importantly that a high percentage of those with sufficient status maintained stores in the more optimal range (>40 ng·mL−1). In contrast to our findings, several groups of researchers studying runners (19; Willis, 2008, unpublished data), gymnasts (19,20), and other athletes (2,13) found that at least 37%-100% of athletes are vitamin D insufficient and that 1%-83% are deficient (25(OH)D concentration <20 ng·mL−1), depending on sport, geographic location, and season tested. A recent study from our laboratory, for example, found that 42% of distance runners training in Baton Rouge, Louisiana (30.5°N), were vitamin D insufficient and ∼1% had concentrations low enough to be deficient (Willis, 2008, unpublished data). Another study in 93 Middle Eastern sportsmen training in Qatar (25.4°N) found that 90% were vitamin D deficient (25(OH)D < 20 ng·mL−1). Although incidence of vitamin D insufficiency in the current study increased in the winter, as might be expected in athletes living at >35°-37° north or south latitude (6,15), the low prevalence of insufficiency and the high prevalence with optimal status in fall and spring may be explained by the sunny and mild climate of Wyoming during the spring, summer, and fall months. Such conditions allow athletes to train and/or perform leisure activities outdoors at close to solar noon when vitamin D synthesis is most effective (6). It is further possible that the lack of cloud cover and pollution as well as Laramie's elevation of >2195 m above sea level (31) allows increased endogenous synthesis because of an increased fractional strength of UVB radiation.
In support of our hypothesis, we found that athletes participating in indoor sports such as wrestling, basketball, and swimming had lower circulating vitamin D in the fall than athletes participating in outdoor sports, including football, soccer, cross-country or track and field, and cheerleading, and that vitamin D status correlated with estimated weekly outdoor practice time in fall and spring. This finding suggests that athletes who practice indoors are at increased risk for vitamin D insufficiency and deficiency, as are athletes who practice outdoors only in the early evening (13) or early morning (Willis, 2008, unpublished data) or who diligently apply sunscreen (which can reduce cutaneous synthesis by >95% (22). Although we also expected that 25(OH)D concentration would correlate with reported leisure-time sunlight exposure, we found that vitamin D status correlated with reported frequency of tanning bed use, as previously suggested (32), but only in the spring. Self-reported data via questionnaires, however, may not be the most accurate method for obtaining data on effective sun exposure (i.e., between the hours of 10:00 a.m. and 2:00 p.m., nondaylight savings time) or tanning bed use. Effective sun exposure is complicated by factors such as cloud cover and sunscreen use (6,15), whereas commercial tanning beds vary considerably with the type of UV light emitted (i.e., only beds that emit adequate UVB promote cutaneous vitamin D synthesis [15,32]). In future studies, the frequency and the length of time spent in leisure time and practice or competition between 10:00 a.m. and 2:00 p.m. nondaylight savings time and sunscreen application frequency and type should be collected through more detailed questionnaires, logs, or direct documentation. Direct measurement of UVB exposure with UVB detection devices placed directly on skin would also be of interest.
Our lack of a relation between vitamin D status and both frequency of intake of vitamin D-containing foods and estimated vitamin D intake from food is not surprising given that vitamin D is limited in our food supply. Only a few foods, including oily fish, naturally contain vitamin D, whereas milk, some fruit juices, margarine, and ready-to-eat cereals are among the few fortified food sources (5). In addition, food intake tables for vitamin D are inadequate, which makes intake evaluation difficult (5). In the current study, reported vitamin D intake from food alone averaged 242 ± 161, 282 ± 206, and 204 ± 171 IU·d−1 in the fall, winter, and spring and while higher than that previously reported in athletic populations, including ski jumpers, gymnasts, soccer players, ice skaters and runners (3,9,19,26,37), was lower than the current RDA of 600 IU (27). When vitamin D from supplements (including an MVI) was included, intake increased to more than approximately 500 IU·d−1 in the fall and spring and to almost 700 IU·d−1 in the winter, approaching the higher intake level (of 1000-4000 IU·d−1) recommended by researchers (4,6,16,17). The lack of a correlation with total intake from vitamin D (including supplements), however, is unexpected but may be explained by the food frequency methodology and/or by the fact that most circulating 25(OH)D is thought to originate from sunlight exposure rather than from dietary sources (6,15). We were intrigued, however, by our weak but significant relation between frequency of MVI intake and vitamin D status in the winter (and tendency for this same association in the fall). Although these findings suggest that a vitamin D-containing MVI may help athletes maintain vitamin D status, it is important to stress that previous studies indicate that-in the absence of sun exposure-a daily MVI (which typically contains 400 IU) is not enough to maintain 25(OH)D concentrations in the sufficient range (14,16,19).
Vitamin D status in the current study also did not appear to be significantly influenced by body weight or adiposity, although the expected negative association with body fat percentage tended to be present in the fall and spring. Previous studies in nonathletes have found that 25(OH)D concentrations are inversely correlated with body fat percentage (within the typical range of sedentary individuals) (29) and are commonly depressed with obesity (along with elevated PTH concentrations) (25,29). In a cohort of 302 healthy adults, for example, Parikh et al. (25), found lower serum concentrations of 25(OH)D in obese compared with nonobese adults. The decrease in vitamin D bioavailability is thought to be due to sequestration of vitamin D in adipose tissue after cutaneous synthesis or dietary intake, although the exact mechanism is not yet known. The reason for a lack of a significant correlation between vitamin D status and adiposity in the current study is most likely explained by the reduced range of body fat of our collegiate athlete population. Future studies in larger athletic populations, however, should evaluate whether athletes with excessively high body fat stores are at increased risk for vitamin D insufficiency, particularly during certain seasons of the year.
The final purpose of the current study was to evaluate whether vitamin D status is linked to bone density, overuse injury, or illness throughout the academic year. Studies in healthy individuals have found that serum 25(OH)D concentrations correlate positively with BMD (4,34), with the greatest BMD observed when serum concentrations are close to 40 mg·dL−1 (4). Among military recruits, risk for bone fracture is significantly associated with reduced serum 25(OH)D (11,28) and elevated PTH concentrations (35) and is reduced by supplementation with 800 IU·d−1 of vitamin D plus calcium (18). Although less is known about overuse or inflammatory injuries, preliminary findings from our laboratory found that the inflammatory marker tumor necrosis factor α rises exponentially in runners when serum 25(OH)D concentrations fall less than 32 ng·mL−1 (Willis, 2008, unpublished data). A previous German report also documented that athletes undergoing an extensive 6-wk program of UVB irradiation experienced a reduction in chronic pain because of sports injuries (30). Although the current study was unable to provide evidence for a link between low vitamin D status and compromised bone health and athletic injury, additional studies are warranted because detection of injury due to vitamin D deficiency was not ideal in our athletic population that mostly maintained 25(OH)D concentrations more than the 32-40 ng·mL−1 throughout the year. Our results, however, highlight the role of high-normal PTH (which is typically elevated when serum vitamin D concentrations fall less than 20-30 ng·mL−1) (15,17) in bone health and the probable link between bone density and bone fracture.
The finding that 25(OH)D concentrations in the spring were significantly associated with frequency of illness, on the other hand, is consistent with research, indicating that vitamin D up-regulates naturally occurring-and broad spectrum-AMP (6,12). AMPs exert a powerful immune response by compromising the integrity of the cell membrane of invading pathogens (10). These results are in agreement with Aloia and Li-Ng (1), who found that supplementation with vitamin D3 for 3 yr reduced self-reported incidence of influenza and the common cold and in support of other evidence that influenza epidemics and other wintertime infections may be brought on by seasonal deficiencies of AMP secondary to seasonal deficiencies in vitamin D (7). Although illness in the current study was documented as part of routine medical care by a staff member who was unaware of the athletes' study participation or vitamin D status, further evaluation in larger athletic populations is of interest. Future studies should document the time course of illness in relation to vitamin D status as well as the duration and the frequency of illness.
In conclusion, the current study demonstrates that healthy athletes can achieve adequate to optimal 25(OH)D concentrations in the nonwinter months through routine sun exposure, dietary sources, and supplements (which surpass the newly revised RDA of 600 IU) (27). Athletes living at distances away from the equator, however, need supplemental vitamin D during the winter to prevent the seasonal reduction in serum 25(OH)D concentrations. Maintaining sufficient vitamin D status may reduce the risk of common infectious illness, which can negatively impact athletic training and performance. Further research is needed to determine whether vitamin D status influences risk for overtraining and inflammatory injury.
Funding support: None.
The authors thank the athletes who volunteered to participate in the study. They also thank Inge Harper at the National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, Phoenix Epidemiology and Clinical Research Branch, for assistance in interpreting the studies of Bannert et al. (2) and Spellerberg (30).
This work was partially supported by research funds from the Department of Family and Consumer Sciences.
Results of the present study do not constitute endorsement by the American College of Sports and Medicine.
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BONE DENSITY; BODY COMPOSITION; PARATHYROID HORMONE; SUN EXPOSURE; 25(OH)D
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