Athletic Training and Sports Health Care

Original Research Supplemental Data

Assessment of Dietary Vitamin D Intake and Compliance With Recommended Vitamin D Supplementation in Division I Collegiate Athletes

M. Alison Brooks, MD, MPH; Rachel B. Parks, MS, RD; Jennifer L. Sanfilippo, MS, LAT; Neil C. Binkley, MD; Mikel R. Stiffler, MS

Abstract

The primary aim of this study was to assess dietary vitamin D intake and compliance with a recommended vitamin D supplementation program in a collegiate athlete population. Subsequently, associations between dietary intake, compliance with supplementation, and 25-hydroxyvitamin D [25(OH)D] levels were investigated. This study retrospectively reviewed vitamin D data for 256 athletes across 13 sports at one NCAA Division I University. Independent variables were gender, skin tone, sport, season of year, dietary intake of vitamin D, and supplementation compliance. The main outcome measure was serum 25(OH)D. Low vitamin D status was defined as 25(OH)D level less than 30 ng/mL. Supplementation was recommended for athletes with low status. In fall, 35.5% of athletes had levels less than 30 ng/mL. Mean 25(OH)D level declined (P < .001) between fall (40.7 ± 7.5 ng/mL) and winter (32.5 ± 7.3 ng/mL) in non-supplemented athletes. Supplementation increased 25(OH)D levels by 8.5 ± 9.5 ng/mL (95% confidence interval: 6.6 to 10.4) in 12 weeks. On average athletes reported moderate compliance, taking approximately half of their prescribed supplements. There was a weak correlation between percent supplement compliance and 25(OH) D levels (r = 0.257, P = .011). Athletes with better vitamin D status had higher intake of milk (among freshmen only, P = .042) and yogurt (among all athletes, P = .025). Increasing dietary intake of vitamin D-rich foods and moderate to good compliance with recommended supplementation may help collegiate athletes improve or maximize their vitamin D status. [Athletic Training & Sports Health Care. 2015;7(5):204–213.]

From the University of Wisconsin–Madison, Madison, Wisconsin.

The authors have no financial or proprietary interest in the materials presented herein.

The authors thank the Sports Medicine staff at the University of Wisconsin–Madison Department of Intercollegiate Athletics for their commitment to the welfare of the student-athletes and contributions to the Badger Athletic Performance program.

Correspondence: M. Alison Brooks, MD, MPH, University of Wisconsin–Madison, 621 Science Drive, Room Box, Madison, WI 53705. E-mail: brooks@ortho.wisc.edu

Received: January 12, 2015
Accepted: August 21, 2015

Abstract

The primary aim of this study was to assess dietary vitamin D intake and compliance with a recommended vitamin D supplementation program in a collegiate athlete population. Subsequently, associations between dietary intake, compliance with supplementation, and 25-hydroxyvitamin D [25(OH)D] levels were investigated. This study retrospectively reviewed vitamin D data for 256 athletes across 13 sports at one NCAA Division I University. Independent variables were gender, skin tone, sport, season of year, dietary intake of vitamin D, and supplementation compliance. The main outcome measure was serum 25(OH)D. Low vitamin D status was defined as 25(OH)D level less than 30 ng/mL. Supplementation was recommended for athletes with low status. In fall, 35.5% of athletes had levels less than 30 ng/mL. Mean 25(OH)D level declined (P < .001) between fall (40.7 ± 7.5 ng/mL) and winter (32.5 ± 7.3 ng/mL) in non-supplemented athletes. Supplementation increased 25(OH)D levels by 8.5 ± 9.5 ng/mL (95% confidence interval: 6.6 to 10.4) in 12 weeks. On average athletes reported moderate compliance, taking approximately half of their prescribed supplements. There was a weak correlation between percent supplement compliance and 25(OH) D levels (r = 0.257, P = .011). Athletes with better vitamin D status had higher intake of milk (among freshmen only, P = .042) and yogurt (among all athletes, P = .025). Increasing dietary intake of vitamin D-rich foods and moderate to good compliance with recommended supplementation may help collegiate athletes improve or maximize their vitamin D status. [Athletic Training & Sports Health Care. 2015;7(5):204–213.]

From the University of Wisconsin–Madison, Madison, Wisconsin.

The authors have no financial or proprietary interest in the materials presented herein.

The authors thank the Sports Medicine staff at the University of Wisconsin–Madison Department of Intercollegiate Athletics for their commitment to the welfare of the student-athletes and contributions to the Badger Athletic Performance program.

Correspondence: M. Alison Brooks, MD, MPH, University of Wisconsin–Madison, 621 Science Drive, Room Box, Madison, WI 53705. E-mail: brooks@ortho.wisc.edu

Received: January 12, 2015
Accepted: August 21, 2015

Inadequate vitamin D status has long been associated with impaired bone health,1 and more recently with other adverse health consequences including muscle weakness,2 reduced immune function,3 and increased injury risk.4 Vitamin D is obtained through both dietary intake and endogenous skin synthesis during sun exposure.5,6 Consequently, individuals who have a low dietary vitamin D intake, sun avoidance or little sun exposure, or higher skin melanin content resulting in reduced cutaneous vitamin D production are particularly at risk for deficiency or insufficiency.7

Measurement of circulating 25-hydroxyvitamin D [25(OH)D] is widely accepted as the best approach to define an individual’s vitamin D status.8 Although there is not consensus regarding the optimal serum 25(OH)D level, many experts recommend 25(OH)D cutoffs believed to enhance calcium uptake, increase bone mineral density, and optimize parathyroid hormone levels.1,7 Individuals with total serum 25(OH) D less than 20 ng/mL are commonly considered deficient, whereas levels between 20 and 30 ng/mL indicate vitamin D insufficiency and levels above 30 ng/mL indicate vitamin D sufficiency.1,7 Supplementation with oral vitamin D is recommended for individuals who are insufficient or deficient.8 The Centers for Disease Control and Prevention cites 32% of individuals in the United States having vitamin D deficiency,9 although higher rates of deficiency have been observed in those living in northern parts of the United States and in those with higher skin melanin content.10 For example, 48% of white preadolescent girls in Maine11 and 52% of Hispanic and African-American adolescents in Boston7 were found to be deficient during at least one season per year.

Most existing literature suggests diet has less effect on vitamin D status than does sun exposure, with estimates as low as 10% for food’s contribution to total vitamin D status.8,12 However, Heaney et al. challenged this belief and hypothesized that diet may provide upward of 75% of vitamin D input via undocumented food sources or preformed 25(OH)D.13 Few foods are currently recognized as natural vitamin D sources (eg, fatty fish, mushrooms, and eggs). Thus, dietary supplements and fortified foods such as milk, orange juice, and cereal are presumed to supply a majority of vitamin D in the diet.8 The lack of reference values in national food databases has limited researchers’ ability to precisely quantify vitamin D intake in international units (IU).14,15 Nevertheless, eating patterns are useful to document when tracking serum 25(OH)D levels, because certain dietary behaviors have been repeatedly associated with vitamin D status in previous studies.16–18 For example, regular milk consumption does not prevent deficiency, but has been correlated with higher vitamin D levels in children,16 college students,17 and African-American men.18 Collegiate athletes can have unique nutritional behaviors related to their high caloric demands, busy schedules, and/or frequent use of dietary supplements. Investigating the food intake of collegiate athletes may provide further insight into the prevalence of low vitamin D levels in this population.

Considering the prevalence of vitamin D deficiency in the general population, and the potential health implications of low vitamin D status, assessing 25(OH)D levels in athletes is of interest to researchers and clinicians alike. Inadequate vitamin D status in athletes may contribute to stress fractures,19,20 muscle weakness,21,22 reduced maximal oxygen consumption,23,24 and overall decreased athletic performance.25 Existing data suggest that athletes are no less at risk for vitamin D deficiency than the general population, although estimates range dramatically, with 3% to 62% of athletes having deficient levels, and there is limited and incongruent data, particularly regarding 25(OH)D levels of athletes in a northern climate.5,6,26–28 Given the athletic performance detriments potentially resulting from inadequate vitamin D status, it is now common clinical practice to recommend and/or prescribe vitamin D supplementation to athletes. However, it is unknown to what degree collegiate athletes are compliant with taking the recommended supplementation. Our primary aim was to assess dietary vitamin D intake and compliance with a recommended vitamin D supplementation program and subsequently investigate associations between dietary status, vitamin D supplementation compliance, and 25(OH)D levels in National Collegiate Athletic Association Division I athletes across 13 different sports at a single institution.

Methods

During the 2013–2014 academic year, the athletic department at a Division I university in the upper Midwest performed a pre-participation examination (PPE) that included serum 25(OH)D screening for all student-athletes in their first year of eligibility (freshmen) and also measured non-freshmen student-athletes on teams whose coaches elected to participate in the screening program (football, swimming, diving, and hockey). All data were collected by the university athletic department (through athlete self-report or athletic trainer report) and maintained in a performance database, which contains information aimed to improve clinical and athletic performance outcomes. All student-athletes signed a form consenting to have their information stored in the database prior to all testing. The database was accessed retrospectively for record review by study personnel following approval from the University’s Institutional Review Board.

Total serum 25(OH)D level was measured using a high performance liquid chromatography assay traceable to the National Institute of Standards and Technology standards.29 Individuals with below-target levels (< 30 ng/mL) were advised to begin a supplementation program consisting of ergocalciferol (vitamin D2) 50,000 IU once weekly for 8 weeks in addition to cholecalciferol (vitamin D3) 2,000 IU daily for 12 weeks. Supplemented individuals were re-tested in 12 weeks and self-reported compliance was documented at that time. The PPE screenings occurred between June and October 2013. Freshmen were also re-tested in winter (December–February) and spring (April–May) 2014 to determine seasonal variation. The testing schedule for freshman athletes is shown in Figure 1. Team screening of non-freshman athletes was performed during March and April 2014.

Testing schedule of freshmen athletes from fall 2013 through spring 2014.

Figure 1.

Testing schedule of freshmen athletes from fall 2013 through spring 2014.

A calcium and vitamin D questionnaire was administered to the student-athletes at or shortly after their initial and follow-up blood draws. This two-page survey queried intake of selected foods/food groups in the past week (Table A, available in the online version of this article). It was adapted from the validated Rapid Assessment Method (RAM) tool30 and several published vitamin D questionnaires.5,31 The survey was piloted prior to implementation; questionnaires were completed, reviewed, and discussed with approximately 50 college students and student-athletes to ensure that all food groups and serving sizes were clearly described. Because the vitamin D content of both natural and fortified food sources can vary considerably, dietary vitamin D intake was assessed as servings per week rather than calculated into international units per day.

Table A:

A. Milk – Yogurt – Cheese

A. Milk – Yogurt – Cheese

B. Fruits – Vegetables

B. Fruits – Vegetables

C. Breads – Cereals – Rice – Pasta

C. Breads – Cereals – Rice – Pasta

D. Meat – Fish – Poultry – Beans - Nuts

D. Meat – Fish – Poultry – Beans - Nuts

E. Other

E. Other

F. Supplements

F. Supplements

Outcome Measures

De-identified information including the student-athletes’ age, body mass index, body composition, gender, race, sport, year of eligibility, serum 25(OH)D levels, supplement compliance, and servings per week of vitamin D-rich foods was obtained from the database. Individuals were grouped by self-reported skin tone (light = Caucasian, Eastern Asian, Hispanic; dark = African American and Western Asian) for all analyses, because certain ethnicities had too few individuals to analyze independently. Vitamin D status was defined as follows: deficient = below 20 ng/mL; insufficient = 20 to 29 ng/mL; sufficient (above-target) = 30 ng/mL and above.

In athletes who received supplementation, self-reported compliance was reported as the total number of weekly 50,000 IU doses taken (8 possible) and the average number of days per week the daily 2,000 IU dose was taken. These values were used to estimate the total number of international units each student-athlete consumed over the 12-week supplementation period and their subsequent compliance rate. In accordance with previous literature, high compliance was defined as taking 80% or more of the prescribed supplementation and low compliance as taking less than 80%.32,33 To investigate seasonal variation in 25(OH)D from fall to winter and again from winter to spring, the change in serum 25(OH)D was calculated for individuals who were initially above-target and thus not supplemented and for individuals who reported low compliance with their prescribed supplements.

Nutritional categories extracted from the dietary questionnaire included weekly servings of yogurt, milk, soy milk, total dairy, orange juice, cereal, salmon, tuna, eggs, meats, energy bars, nutritional shakes, and vitamin/mineral supplements. Personal supplements were recorded in terms of “yes” or “no” because student-athletes were rarely able to recall the actual dosages of calcium/vitamin D in their supplements. To remain consistent with the 80% cutoff for high compliance, student-athletes were categorized as taking a multivitamin or vitamin D supplement if their nutrition questionnaire indicated consumption of their own personal supplement more than 5 days per week.

Statistical Analysis

A chi-square test was used to determine if the proportion of athletes with below-target vitamin D status (< 30 ng/mL) varied between genders, skin tones (light and dark), and those who did and did not regularly take a personal multivitamin or vitamin D supplement. A dependent t test was used to assess changes in 25(OH) D levels between seasons and after supplementation. An independent t test was used to assess differences in 25(OH)D levels after supplementation between the high compliance and low compliance groups. An independent t test and one-way analysis of variance were used to assess differences in nutritional intake by vitamin D status. A Pearson’s r correlation coefficient was calculated to investigate whether percent supplement compliance was correlated with 25(OH)D level or percent change in 25(OH)D level after supplementation. Statistical significance for all results was set a priori at a P value of less than .05.

Results

Serum 25(OH)D Levels

Demographic information for all student-athletes screened for vitamin D status is provided in Table 1. The average vitamin D level of incoming freshmen student-athletes was 34.8 ± 10.8 ng/mL. The overall prevalence of vitamin D insufficiency and deficiency at the PPE time point (June through October 2013) was 29% and 6.5%, respectively. Serum 25(OH)D levels of incoming freshmen by team are summarized in Figure 2. The men’s and women’s basketball, softball, men’s hockey, and football teams had the highest proportion of athletes who were vitamin D insufficient or deficient. Low vitamin D status was less common among golf, soccer, swimming/diving, and track athletes; none of these teams had incoming freshman who were vitamin D deficient. The proportion of freshman athletes with 25(OH)D levels below-target (< 30 ng/mL) at the time of their PPE differed significantly by gender (41.6% males vs 28.8% females, P = .045) and skin tone (90.9% dark skin vs 27.2% light skin, P < .001) (Figure 3).

Demographics for All Student-Athletes Screened During the 2013–2014 Academic Yeara

Table 1:

Demographics for All Student-Athletes Screened During the 2013–2014 Academic Year

Vitamin D status of incoming student-athletes in their first year of eligibility during the fall season, by team. Number of student-athletes is provided in parentheses. Omitted from the figure were individuals participating in men’s tennis (2), women’s tennis (1), women’s crew (1), and women’s volleyball (3) as all had vitamin D levels ≥ 30 ng/mL.

Figure 2.

Vitamin D status of incoming student-athletes in their first year of eligibility during the fall season, by team. Number of student-athletes is provided in parentheses. Omitted from the figure were individuals participating in men’s tennis (2), women’s tennis (1), women’s crew (1), and women’s volleyball (3) as all had vitamin D levels ≥ 30 ng/mL.

Vitamin D status of incoming student-athletes in their first year of eligibility during the fall season, by gender and skin tone. Vitamin D status varied significantly by gender (χ2 (2, n = 169) = 6.2, P = .045) and by skin tone (χ2 (2, n = 169) = 36.9, P < .001).

Figure 3.

Vitamin D status of incoming student-athletes in their first year of eligibility during the fall season, by gender and skin tone. Vitamin D status varied significantly by gender (χ2 (2, n = 169) = 6.2, P = .045) and by skin tone (χ2 (2, n = 169) = 36.9, P < .001).

Seasonal change in prevalence of low vitamin D status among all screened freshman athletes, regardless of supplementation, is shown in Figure 4. At least one-third of athletes had 25(OH)D levels below 30 ng/mL (insufficient or deficient) at all screening time points in fall, winter, and spring. However, the proportion of freshman athletes who were vitamin D deficient (< 20 ng/mL) doubled from winter to spring.

Seasonal variation in vitamin D status of incoming student-athletes in their first year of eligibility. aFall screening (169 athletes). bWinter screening (141 athletes). cSpring screening (112 athletes).

Figure 4.

Seasonal variation in vitamin D status of incoming student-athletes in their first year of eligibility. aFall screening (169 athletes). bWinter screening (141 athletes). cSpring screening (112 athletes).

Vitamin D status of the non-freshmen student-athletes who opted to participate in screening (Mar–Apr) is shown in Figure 5. At least 60% of football and hockey athletes had deficient or insufficient 25(OH) D levels; one in five male football or hockey athletes was vitamin D deficient. No swimming/diving athletes were deficient.

Vitamin D status of non-first year returners screened between March and April. Number of student-athletes is provided in parentheses.

Figure 5.

Vitamin D status of non-first year returners screened between March and April. Number of student-athletes is provided in parentheses.

A higher percentage of student-athletes with 25(OH)D levels above 30 ng/mL were already taking their own personal multivitamin or vitamin D supplement at the time of screening compared to those below-target (21.3% vs 13.6%, P = .186), although this did not reach statistical significance. Incoming freshman athletes with above-target levels at their fall PPE screening, and who were thus not supplemented, experienced a significant decrease (P < .001) in 25(OH)D levels from fall (40.7 ± 7.5 ng/mL, Jun–Oct) to winter (32.5 ± 7.3 ng/mL, Dec–Feb). Those with above-target levels at the winter screening did not experience significant changes in 25(OH)D levels between winter (36.2 ± 5.0 ng/mL, Dec–Feb) and spring (35.7 ± 10.4 ng/mL, Apr–May).

Assessment and Effects of Supplementation Compliance

There were 126 athletes over the course of the study who had levels less than 30 ng/mL and started the supplementation program. Three-quarters (n = 95) of these athletes completed a survey at their 12-week follow-up blood draw about their compliance with the recommended supplements, whereas one-quarter of the supplemented athletes did not complete the survey or were otherwise lost to follow-up. On average athletes took approximately half of their recommended supplements (5.7 ± 2.6 of 8 weekly 50,000 IU supplements; 3.6 ± 2.4 of 7 daily 2,000 IU supplements). Thirty-seven (38.9%) athletes were categorized as high (> 80%) compliance (7.8 ± 2.6 of 8 weekly supplements; 5.6 ± 1.4 of 7 daily supplements). Athletes in the high compliance group on average consumed 43,680 ± 3,145 IU per week compared to 23,115 ± 11,911 IU per week in the low (< 80%) compliance group. Overall, the mean 25(OH)D level of below-target individuals before supplementation was 23.4 ± 4.7 ng/mL and increased (P < .001) to 32.1 ± 10.8 ng/mL after supplementation, representing a mean change of 8.5 ± 9.5 ng/mL (95% confidence interval [CI] = 6.6 to 10.4). Both high and low compliance groups experienced an increase in serum 25(OH)D levels at their 12-week follow-up. No significant difference in 25(OH)D level or percent change after supplementation was observed between those with high versus low compliance. However, there was a weak but significant, positive correlation between percent supplement compliance and 25(OH)D level (r = 0.257, P = .011), as well as percent supplement compliance and percent change in 25(OH)D level (r = 0.249, P = .015) after supplementation.

Intake of Vitamin D-rich Foods

At the fall PPE, 143 of 169 incoming freshmen student-athletes completed the nutrition questionnaire (49% male, 87% light skin tone, 35 ± 11 ng/mL mean 25(OH)D, 34% below-target 30 ng/mL). Milk intake was the only food variable that differed significantly between below-target and above-target freshman athletes (6.3 ± 5.1 vs 8.3 ± 5.9 servings per week, respectively, P = .042).

Of the non-freshman student-athletes who participated in screening, 57 of 87 individuals completed the nutrition survey (91% male, 70% light skin tone, 26 ± 9 ng/mL mean 25(OH)D, 68% below-target). Non-freshman athletes with below-target 25(OH)D levels ate less yogurt (1.7 ± 2.1 vs 3.4 ± 3.8 servings/week, P = .02) and drank more orange juice (3.4 ± 4.1 vs 1.0 ± 1.6 servings/week, P = .02); they also drank fewer servings of milk but this difference was not significant (8.6 ± 7.3 vs 12.6 ± 12.9, P = .14). Of the non-freshman athletes who reported consuming orange juice in the previous week (n = 31), only 19% indicated that their juice was fortified with vitamin D.

Results from the freshman (fall PPE) and non-freshman returners (spring screening) are combined in Table 2. Yogurt, orange juice, and energy bar consumption differed significantly among vitamin D groups. Intake of fatty fish such as salmon and tuna was less than one serving per week in all groups. Intake of milk and eggs was approximately 7 and 5 servings per week, respectively; those with better vitamin D status tended to have higher intake of milk (P = .188) and eggs (P = .245), but this difference was not statistically significant. Total dairy was the nutritional category with the most servings (n = 20) per week.

Intake of Vitamin D-rich Foods Measured in Mean ± Standard Deviation (95% Confidence Intervals) Servings per Week, by Vitamin D Statusa

Table 2:

Intake of Vitamin D-rich Foods Measured in Mean ± Standard Deviation (95% Confidence Intervals) Servings per Week, by Vitamin D Status

Discussion

This study is the first to investigate the associations between dietary intake of vitamin D rich foods, compliance with vitamin D supplementation, and vitamin D status in collegiate athletes. In this cohort living in a northern climate, 25(OH)D level less than 30 ng/mL was common throughout the year with more than one-third of athletes having deficient or insufficient vitamin D levels. Indeed, 13.7% of those screened in spring were frankly vitamin D deficient. A vitamin D supplementation program effectively improved 25(OH)D levels. Athletes reported moderate compliance with the program, on average taking half of the recommended amount of weekly and daily supplementation. Non-supplemented athletes experienced a significant decrease in their 25(OH)D level from fall to winter. Better vitamin D status was associated with consumption of more servings per week of milk (freshman athletes only) or yogurt (both freshmen and non-freshman athletes).

The overall prevalence of vitamin D deficiency or insufficiency in this cohort is comparable to that reported in the general U.S. population.26 Low vitamin D status was more common in males and those with darker skin tones, which is in agreement with the current literature.26,34 However, our sample comprised more males than females with dark skin tone, so our findings for gender and skin tone may be inter-related and should be examined further in a larger collegiate population.

Low vitamin D status varied by sport; this may reflect the predominant ethnicity and gender of student-athletes on a given team. Within sports in which both genders were represented, low vitamin D status was more common among men than women. Although team differences may be partially due to training and competition environments, we did not investigate differences in outdoor versus indoor sports due to limited sample sizes for certain teams. Additionally, all sports in the upper Midwest are indoors for at least a portion of their training (eg, weightlifting) and become indoor sports entirely for several months in the winter due to the climate, dampening any potential differences in 25(OH)D level resulting from training environment and sun exposure.

Researchers have proposed that athletes may need 25(OH)D levels above what is recommended for the general population, upward of 50 ng/mL.35,36 If this is indeed true, most of the athletes included in this study are “low”; only 5.7% of this cohort overall, and 0.9% of the freshman athletes screened in winter, had 25(OH)D levels above 50 ng/mL. It must be emphasized that recommendations for such high 25(OH)D levels are controversial and that the impact of low vitamin D status on physical performance remains inconclusive, although several have suggested that elevated 25(OH)D levels improve muscle function and athletic performance,3,12,35 while preventing bone injury.3,19 Pairing vitamin D status with injury and/or performance data may elucidate what target 25(OH)D levels should be recommended for athletes.

In this cohort, compliance with recommended supplementation was moderate, with athletes taking at least half of the recommended supplements over a 12-week period. Compliance with weekly versus daily supplements was similar. Regardless of self-reported high or low compliance, serum 25(OH)D improved in those taking supplements, whereas levels fell in those not taking supplements, particularly from fall to winter. Because serum 25(OH)D levels in more than one-third of athletes screened was below 30 ng/mL, and was only rarely above 50 ng/mL, daily supplementation for all athletes is one approach that might be considered to optimize vitamin D status versus targeted screening of higher risk athletes or sports. At minimum, recommending supplementation for the highest risk months (late fall to spring) for those training and competing in northern climates may be indicated.

This study suggests that regular consumption of vitamin D fortified foods, particularly fluid milk and yogurt, is associated with better vitamin D status. Fortified dairy milk contains 100 IU vitamin D per 8 fluid ounces, and many non-dairy milks (eg, almond, soy, or coconut milk) contain that much or more vitamin D. The mean reported daily milk consumption of the student-athletes (1.0 servings) is comparable to previously reported intakes of college students (0.94 servings).37 Consistent with prior studies of milk intake and vitamin D status,16–18 student-athletes with 25(OH)D levels above 30 ng/mL tended to drink 2 to 4 more servings of milk per week, although the difference between above- and below-target freshman athletes reached statistical significance only at the fall PPE time point.

The connection between yogurt and vitamin D status is less clear because not all brands of yogurt are fortified, although required fortification of yogurt has been proposed as a national strategy for improving vitamin D status.38 The significant association observed between yogurt intake and optimal vitamin D status may have resulted from actual exogenous vitamin D contribution of fortified brands, or perhaps because yogurt intake is a surrogate measure for some other factor affecting 25(OH)D level (eg, gender, sport, or skin tone).

Limiting our analysis is the retrospective nature of these data, which prevented investigation of any causal relationships between diet, sun exposure, and vitamin D status. Additionally, certain sports such as women’s volleyball, men’s golf, and men’s and women’s tennis had only a few student-athletes represented. As a result, we were only able to analyze their results in aggregate with the larger group, which limits our ability to draw conclusions about the expected vitamin D status of individuals participating in these specific sports. It is also unknown if there are inherent differences in dietary and supplementation habits between freshmen and non-freshman athletes that may have influenced our findings. Future studies in the collegiate population should aim to evaluate these potential confounding differences by including a larger number of athletes from each year of eligibility.

We recognize the possibility that “bioavailable” 25(OH)D may be a better measure to evaluate vitamin D status, particularly in those with dark skin color.39 However, given the current controversy surrounding the veracity and utility of this measurement,40 we elected to measure only total 25(OH)D. The brief nutrition questionnaire was designed for convenience and maximal survey response. As a result, it provided only a crude estimate of self-reported vitamin D intake. A more thorough dietary assessment such as a weighed 3-day food log and reviewing supplement labels for vitamin D content may have better delineated differences in vitamin D intake between those with above- and below-target vitamin D levels.

Self-reported compliance may be considered a limitation due to poor recall. Nonetheless, those who were most compliant did demonstrate significant serum 25(OH)D level increases and the correlation between compliance rate and percent change in 25(OH)D levels was positive. Other variables including between-individual variation in response to oral supplementation, leisure time sun exposure, sunscreen use, and typical clothing worn while outside, in conjunction with supplementation compliance, may have affected the 25(OH)D changes seen. Although a research limitation, such confounders reflect real-world inputs to vitamin D status; as such, this study provides insight into the effect of vitamin D supplementation of student-athletes. Further work is indicated with a larger sample size with more rigorously documented compliance and dietary vitamin D intake to fully assess vitamin D supplementation in collegiate athletes. Future research investigating vitamin D status for all individuals on a team, not just those in their first year of eligibility, is certainly warranted.

Implications for Clinical Practice

Vitamin D deficiency or insufficiency is common in Division I collegiate student-athletes in a northern climate, particularly among males and those with darker skin tones. In athletes not receiving vitamin D supplementation, serum 25(OH)D levels declined from fall to winter and remained lower in spring. Twelve weeks of oral vitamin D supplementation raised serum 25(OH)D levels. Supplementation could be optimized in the winter and spring months to improve or maintain vitamin D status in competitive collegiate athletes. Student-athletes with 25(OH)D levels above 30 ng/mL tended to consume more weekly servings of milk and yogurt. Although sports nutrition education programming about dietary vitamin D likely will not inherently prevent or treat deficiency, it may help collegiate athletes maximize their vitamin D intake and status. Athletes could be encouraged to consume foods naturally rich in vitamin D when available (eg, salmon and eggs) and to choose fortified products such as milk, yogurt, and orange juice. A daily vitamin D supplement may be a good choice for all athletes, but certainly for those who avoid foods or food groups rich in vitamin D.

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Demographics for All Student-Athletes Screened During the 2013–2014 Academic Yeara

GROUPNBMILEAN MASS %bLIGHT SKIN TONEDARK SKIN TONE
All student-athletes
  Women10022.9 ± 2.970.4 ± 4.1937
  Men16827.2 ± 4.678.6 ± 6.112741
First year of eligibility
  Women8222.8 ± 3.169.8 ± 4.2757
  Men9925.7 ± 4.278.6 ± 6.58316
Basketball
  Women326.1 ± 5.465.2 ± 7.821
  Men623.8 ± 2.482.0 ± 1.542
Golf
  Women523.4 ± 3.868.3 ± 2.850
  Men219.6 ± 2.285.6 ± 7.420
Hockey
  Women524.2 ± 1.071.2 ± 3.650
  Men526.0 ± 1.580.7 ± 3.250
Soccer
  Women723.4 ± 1.870.1 ± 3.370
  Men623.8 ± 0.981.0 ± 3.260
Spirit
  Women820.9 ± 1.180
  Men326.5 ± 4.530
Swimming & diving
  Women1023.4 ± 3.791
  Men722.6 ± 2.670
Tennis
  Women118.8 ± 0.010
  Men225.3 ± 0.520
Track & cross country
  Women3122.1 ± 3.5274
  Men2223.4 ± 3.7202
Crew
  Women120.1 ± 0.010
Football
  Men2928.7 ± 4.077.7 ± 6.7209
Softball
  Women624.1 ± 0.472.7 ± 2.351
Volleyball
  Women322.2 ± 2.071.2 ± 3.030
Wrestling
  Men725.2 ± 3.577.8 ± 3.670
Second year of eligibility
  Women923.1 ± 1.772.0 ± 3.090
  Men2728.8 ± 4.477.7 ± 5.6198
Third year of eligibility
  Women623.9 ± 1.474.4 ± 2.460
  Men2627.9 ± 3.580.5 ± 4.4188
Fourth year of eligibility
  Women322.8 ± 1.072.1 ± 0.030
  Men1632.0 ± 4.777.0 ± 7.2106

Intake of Vitamin D-rich Foods Measured in Mean ± Standard Deviation (95% Confidence Intervals) Servings per Week, by Vitamin D Statusa

FOOD< 20 NG/ML (N = 19)20 TO 29 NG/ML (N = 68)≥ 30 NG/ML (N = 113)OVERALL (N = 200)P
Yogurtb1.9 ± 3.2 (0.3, 3.5)2.3 ± 2.5 (1.7, 2.9)3.3 ± 3.0 (2.8, 3.9)2.8 ± 2.9 (2.4, 3.3).025c
Milk4.7 ± 5.1 (2.3, 7.2)7.1 ± 6.2 (5.5, 8.6)7.8 ± 7.6 (6.4, 9.2)7.3 ± 7.0 (6.3, 8.2).188
Soy milk0.4 ± 1.0 (0.0, 0.8)0.3 ± 1.4 (0.0, 0.6)0.4 ± 1.5 (0.2, 0.7)0.4 ± 1.4 (0.2, 0.6).812
Total dairy18.0 ± 14.4 (11.1, 25.0)21.2 ± 9.8 (18.8, 23.6)21.0 ± 10.7 (19.1, 23.0)20.8 ± 10.8 (19.3, 22.3).496
Orange juiced2.4 ± 2.7 (1.1, 3.7)2.4 ± 3.4 (1.6, 3.2)1.4 ± 2.2 (1.0, 1.8)1.9 ± 2.7 (1.5, 2.2).048c
Cereal1.1 ± 2.0 (0.2, 2.1)3.4 ± 6.5 (1.8, 4.9)2.7 ± 3.0 (2.2, 3.3)2.8 ± 4.5 (2.2, 3.4).148
Salmon0.6 ± 1.0 (0.2, 1.1)0.3 ± 0.8 (0.1, 0.5)0.5 ± 1.6 (0.2, 0.8)0.4 ± 1.3 (0.3, 0.6).507
Tuna0.6 ± 1.5 (0.0, 1.4)0.6 ± 1.4 (0.3, 1.0)0.3 ± 0.8 (0.2, 0.5)0.5 ± 1.1 (0.3, 0.6).143
Egg3.9 ± 3.2 (2.3, 5.4)5.8 ± 5.6 (4.4, 7.2)5.1 ± 4.2 (4.3, 5.9)5.2 ± 4.7 (4.6, 5.9).246
Total meats8.7 ± 11.2 (3.3, 14.1)7.8 ± 6.0 (6.3, 9.3)6.4 ± 5.5 (5.4, 7.5)7.1 ± 6.4 (6.2, 8.1).226
Energy bare1.4 ± 1.7 (0.5, 2.2)3.1 ± 4.2 (2.1, 4.1)1.9 ± 2.0 (1.5, 2.3)2.3 ± 3.0 (1.8 ± 2.7).015c
Nutritional shake1.9 ± 3.1 (0.4, 3.4)3.4 ± 6.8 (1.8, 5.1)2.3 ± 3.9 (1.6, 3.1)2.7 ± 5.0 (1.9, 3.4).279

10.3928/19425864-20150831-06

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