Vitamin D plays a central role in maintaining bone health, and recent studies suggest that it may protect against the development and progression of obesity and related chronic diseases and increase physical fitness (18). A significant portion of the literature has shown that a large percent of individuals with obesity, insulin resistance, and cardiovascular risk factors suffer from insufficient or deficient 25-hydroxyvitamin D (25OHD), the vitamin D metabolite most commonly used to assess vitamin D levels, including an estimated 59% of Americans and 60% of the worldwide population (21,22,27). Although most attention has been focused on the prevalence of vitamin D insufficiency in the general population, information about this hormone in the physically active population is lacking (11). In fact, 1 recent study conducted in the United Kingdom during the winter months revealed that up to 60% of rugby and soccer players and individuals who regularly trained in a gym had 25OHD levels <20 ng·mL−1, the serum concentration defined as adequate by the Institute of Medicine (11,18).
This research suggests that there is a negative relationship between serum 25OHD levels and adiposity in younger populations (14,15). Recent reports also suggest a positive relationship between 25OHD levels and cardiorespiratory fitness, defined by an individuals' duration during a graded exercise test on a treadmill (14,15). Although these studies provide some insight into the connection between vitamin D and improvements in body fat and measures of fitness, it is important to note that several limitations including failure to report maximal oxygen uptake, record sun exposure, monitor sunscreen use, and measure dietary intake of vitamin D may have confounded these observations.
Vitamin D has also been linked to improvements in muscle mass, strength, (5) and power (35). For instance, 25OHD deficiency in elderly individuals has been associated with myopathy; and D3 supplementation, the animal-based isoform of the vitamin, may improve muscle fiber synthesis and fiber type composition by increasing type II fibers (5). Furthermore, another study incorporating both active and inactive ambulatory individuals between the ages of 60 and 90 years revealed a positive association between 25OHD levels and lower extremity muscle function (8). Although these studies suggest a role of vitamin D in determining muscular strength and function in elderly individuals, few studies have examined this relationship in younger populations.
Though the relationships between vitamin D, body composition, and fitness measures (cardiorespiratory fitness, power, and strength) have been established independently in populations of various ages (16,24), a comprehensive study examining all of these factors together is yet to be completed. Accordingly, the purpose of this study was to investigate the relationship between serum 25OHD levels, body size, body composition, resting metabolic rate, aerobic fitness, and muscular power and strength in a young physically active population. We hypothesized that there would be a positive relationship between serum 25OHD and resting metabolic rate, measures of aerobic fitness and strength, and an inverse relationship between serum 25OHD and body size and composition.
Experimental Approach to the Problem
This cross-sectional study was designed to examine the relationships between serum 25OHD, body composition, and athletic performance in healthy college-aged students. Subjects reported to the laboratory 6 times. On the first day, all subjects signed an informed consent document, completed physical activity frequency and health assessment questionnaires, and height and weight were recorded. Subjects then completed tests to determine resting metabolic rate, serum vitamin D status, body composition, maximal cardiorespiratory fitness (V[Combining Dot Above]O2max), and anaerobic power and strength over the course of 5 additional visits that took place over a 14-day period. All tests were conducted during late summer in an effort to control for seasonal variation. At least 72 hours of rest were included between the anaerobic power, V[Combining Dot Above]O2max, and strength tests to allow for adequate recovery. Resting metabolic rate and blood collection took place on days 5–7 of the menstrual cycle for women.
Forty subjects (n = 20 men, n = 20 women) were recruited on a Southern U.S. university campus (30.46° N, 91.14° W) to participate in the study. To be eligible for the study, participants were required to be physically active in moderate to vigorous intensity activities for at least 3 days per week and must have maintained a consistent body weight for 3 months before the study. Subjects had no history of supplementing with vitamin D outside of what is included in a daily multivitamin (400 IU). This level of supplement is still fairly low and not likely to increase levels to a healthy range. In fact, only 1 subject was taking a multivitamin on a regular basis, and plasma 25OHD levels were considered low despite this supplementation. It is important to note that subjects were recruited based on gender to enroll equal numbers of men and women; however, subjects were not screened for 25OHD status before inclusion of the study. This project was approved by the Louisiana State University Institutional Review Board.
Descriptive data are presented in Table 1. Three subjects were Hispanic and all others were whites. All subjects were recruited and completed the study between July and September of the same year in attempt to minimize seasonal variation in sun exposure and resulting serum 25OHD. All men were significantly taller and heavier than all women and had a significantly lower body mass index (BMI) and percent body fat. There were no statistically significant differences in age between men and women. Furthermore, when participants were grouped based on 25OHD status alone, with those <35 ng·mL−1 being “low” and those >35 ng·mL−1 being “high,” there were no significant differences in measured parameters between groups. Only 39 subjects were included in data analysis, as one woman failed to complete all testing sessions.
Blood Collection and Analyses
During the assessment period, subjects reported to the laboratory after a 12-hour fast and having restrained from strenuous exercise for the previous 24 hours. A registered nurse collected 20 ml of blood in vacutainers containing no additive in the exercise biochemistry laboratory at Louisiana State University. Samples were centrifuged at 10° C (10 minutes, 1,000g) and plasma aspirated, aliquoted, and stored at −80° C until analysis. Vitamin D (25OHD) status was determined using enzyme-linked immunosorbent assay (ELISA) (Alpco Diagnostics, Salem, NH, USA) with a BioTek microplate reader (Model MQX200l; BioTek Instruments, Winooski, VT, USA). This ELISA kit correlates well with high-performance liquid chromatography (r = 0.943) and LC-MS/MS (r = 0.920) measures. Interassay assay variations were 0.55 and 0.98%, and intra-assay variation was 5.5%.
Sun Exposure and Dietary Intake
One week before blood collection, subjects were issued 2 surveys that captured their overall endogenous vitamin D production and exogenous dietary intake. The first questionnaire was designed to quantify the amount of time spent outdoors and in sunlight because endogenous vitamin D is produced by exposure to ultraviolet light. This method, set forth by Hanwell et al. (19), combines the time spent outdoors or in exposure to ultraviolet light into a numerical scale and was not modified from the original survey for this project. The second questionnaire was a 7-day dietary log that was analyzed for dietary vitamin D content using the United States Department of Agriculture database (27). This database reports both plant-based vitamin D2 and animal-based vitamin D3 content, allowing for maximal content values to be calculated.
Resting Metabolic Rate
Resting metabolic rate (RMR, kcal per day) was assessed through the analysis of oxygen consumption using a metabolic cart (Moxus Metabolic Systems, Pittsburgh, PA, USA). Briefly, subjects arrived at the laboratory after a 12-hour fast and having refrained from strenuous exercise for 24 hours. Subjects were asked to lie in a supine position in a thermoneutral environment while data were collected. The procedure continued until 10 minutes of steady-state data was collected, defined as ±5% of the respiratory exchange ratio (12). Resting metabolic rate was calculated through the modified Weir equation (38).
Body Composition—Dual-Energy X-ray Absorptiometry
Whole-body dual X-ray absorptiometry (DXA) scans were performed by a trained technician using a General Electric Lunar iDXA (General Electric, Milwaukee, WI, USA) and analyzed using enCORE software version 13.40 (GE Medical Systems, Milwaukee, WI, USA).
Cardiorespiratory Fitness Testing
V[Combining Dot Above]O2max was obtained using a modified Bruce Protocol (3). Participants were asked to refrain from alcohol and vigorous exercise for 24 hours before testing. This protocol required participants to walk or run on a treadmill, whereas the speed and incline was progressively increased until the subject reached fatigue. Exhaled gases were analyzed during the testing period using a metabolic cart (Moxus Metabolic Systems).
Anaerobic Power Measurements
Anaerobic power was measured on a cycle ergometer (Monark Ergomedic, Vansbro, Sweden) using the Wingate testing protocol (6). The protocol included a self-selected warm-up period during which the participants pedaled at a relaxed frequency against little or no resistance. The testing period required the subjects to pedal against a given resistance, calculated as 7.5% of the body weight of the subject in kilograms, for 30 seconds. This period was followed by a cooldown period that lasted as long as it was deemed necessary by the subject. Revolutions per 5 second intervals were counted and applied to the Wingate equations (6).
Strength and Power Testing
Strength measurements were assessed by 8 repetition maximums (8RM) for several lifts, including upright bench press, bicep curl, tricep pushdown, leg curl, leg extension, and upright row. For assessment of power, subjects were asked to perform vertical and horizontal jumps, with allowed countermovement before each jump. The best of 3 jumps was recorded and used for data analyses. Vertical jump height was determined as the difference between the height of outstretched hand at rest and the highest point the subject reached during the jump; horizontal jump distance was determined by initial landing spot of the subjects' heel on a marked tape measure (33).
Data were analyzed using SPSS (Version 19; IBM, Armonk, NY, USA). Descriptive statistics, including mean and SE were calculated for all outcome variables. Data was divided into groups based on gender and vitamin D status. Statistical power (n = 40) was observed to be 0.79, and was calculated based on the assumption that individuals obtaining the recommended amount of ultraviolet (UV) exposure would have average serum 25OHD concentrations of 40 ng·mL−1 (9). Subjects were further divided based on serum 25OHD concentrations above (high) or below (low) 35 ng·mL−1, which is considered adequate serum 25OHD for physically active individuals (10), resulting in 4 groups: “high” women (n = 10), “low” women (n = 9), “high” men (n = 9), and “low men” (n = 11). These groups were formed after data collection, as subjects were not screened based on 25OHD levels before inclusion of the study. A 1-way 4-level analysis of variance was conducted to determine significant differences among these groups. Before statistical testing, all data were examined for homogeneity of variance; the residuals were checked for normality with Shapiro-Wilkes test. Transformations were not necessary. Student's t-tests with a Bonferroni multiple comparison correction were used post hoc. Pearson's stepwise correlation analysis was used to determine relationships between all measurements. Multiple regression models using adiposity, gender, sunlight exposure, and dietary intake as cofounders were also tested. Significance was set at p ≤ 0.05. All values presented are mean ± SE.
Vitamin D Status: Measures of Intake and Serum Content
Twenty subjects (9 women, 11 men) presented with 25OHD serum levels below 35 ng·mL−1, which is considered to be below normal in young physically active individuals (10) (Table 2). The mean dietary vitamin D intake for all participants was just above 1,000 IU per week, and these values did not correlate with serum levels (Table 2). Intake was significantly higher in all men than in all women (p = 0.002); however, there was neither a significant difference in intake between the “high and “low” vitamin D groups (p = 0.93), nor was there a relationship between vitamin D intake and serum 25OHD content (r = −0.18). Although the recommended daily intake of vitamin D is 600 IU (31), or 4,200 IU per week, only 1 subject in the study met this criterion; however, serum 25OHD in this individual still fell below the normal 25OHD level.
Using the charting procedure for sun exposure, scores ranged from 11 to 52. For reference, in the study in which the survey measurement was proposed, the scores ranged from 0 to 41 (19). This survey was originally used with adults in Italy receiving variable amounts of sun exposure either in time or skin visible for UV contact, but meeting the proposed requirement for sufficient endogenous production. In our study, those subjects with higher sun exposure scores tended to spend considerable time outdoors, either training or walking on campus instead of driving. Those subjects with lower scores were those who had jobs that required them to spend most of the day indoors, with only minimal time spent outdoors on weekends or short workouts after work during the evening. Each subject was asked to report whether or not he or she used sunscreen on a daily basis.
Because most subjects were whites and no subjects were African-Americans, there were no adjustments made in the data for skin tone. In addition, there were no significant differences in sun exposure between the men and women or the “high” and “low” vitamin D groups (p = 0.66 and p = 0.81, respectively) (Table 2). We did not observe any significant relationships between sun exposure and any variables within our data set when divided by gender or “low” or “high” 25OHD status.
Body Composition and Resting Metabolic Rate
There was a significant negative trend between vitamin D status and BMI (r = −0.326, p = 0.033). Aside from the expected differences in percent body fat between genders (p ≤ 0.001), there were no other significant differences in percent body fat between groups (Table 1). Genders were analyzed separately because of the discrepancies in body fat between men and women (29). There were neither observed trends in body fat in the overall data set (r = 0.248, p = 0.109), in women (r = 0.216, p = 0.374) or in men (r = −0.031, p = 0.184), nor trends in the lean body mass (r = 0.088, p = 0.576).
As expected, there was a significant difference between RMR in men and women (p ≤ 0.001) (Table 2) but no significant differences in RMR between the “high” vitamin D group and the “low” vitamin D group (p = 0.94) (Table 2). There was no observed relationship between RMR and 25OHD overall (r = 0.248, p = 0.109), in females (r = 0.109, p = 0.656), or in males (r = 0.041, p = 0.863).
There was a statistically significant difference in V[Combining Dot Above]O2max between men in the “low” and “high” groups (p ≤ 0.01). When “low” and “high” groups were stratified by gender, high men had roughly a 20% higher V[Combining Dot Above]O2max compared with those who fell below the normal standard (Figure 1). This relationship was not observed in women. There was a significant relationship in the overall data set between 25OHD and V[Combining Dot Above]O2max (r = 0.360, p = 0.018), which remained significant in men (r = 0.477, p = 0.033) but not in women (r = −0.214, p = 0.379). Physical activity questionnaire data revealed that there was a mix of both endurance- and resistance-trained athletes in both the “high” (endurance n = 15; resistance n = 4) and the “low” groups (endurance n = 13; resistance n = 5).
Anaerobic power (p ≤ 0.001) was significantly higher in men than women; however, there were no significant differences in anaerobic power based on 25OHD status (p = 0.76 for women and p = 0.34 for men) (Table 2). The lack of significance was maintained when comparing absolute measurements or watts per body weight (kg) between the “low” and “high” 25OHD groups. There was no observed trend between 25OHD and anaerobic power (r = 0.112, p = 0.472) or 25OHD and relative anaerobic power (r = 0.236, p = 0.126). As expected, men had significantly higher 8RMs in all lifts compared with women (p ≤ 0.001)(Table 2). However, there were no significant differences between the “low” and “high” 25OHD groups in any of the 8RM measurements (Table 2), even when stratified by gender (Table 3). There were no observed relationships in the overall data set between serum 25OHD and any of the 8RM measurements (bench r = −0.057, p = 0.716; leg curl r = 0.055, p = 0.725; leg extension r = 0.77, p = 0.622; upright row r = 0.039, p = 0.802; bicep curl r = −0.057, p = 0.717; tricep pushdown r = 0.005, p = 0.974; vertical jump r = 0.206, p = 0.185; horizontal jump r = 0.090, p = 0.564; sit and reach r = 0.231, p = 0.136). There were no relationships between any of the 8RM measurements when stratified by gender (data not reported).
One of the most interesting findings in this study was the high percentage of subjects that presented with low levels of 25OHD. Even though hypovitaminosis D is prevalent in older and diseased populations, our results were especially surprising because all subjects in this study were young, fit, healthy, and living in an environment with a high potential for sun exposure (26). It was hypothesized that the individuals in the present study would consume more calories and spend more time in the sun, which would naturally elicit higher serum levels of 25OHD (14).
Our findings are somewhat similar to a recent study in the United Kingdom, which found that 62% of athletes (n = 71) presented with insufficient levels (<20 ng·mL−1) of 25OHD (11). Clinical studies have defined deficiency as serum levels below 10 ng·mL−1 and insufficiency as levels between 10 and 30 ng·mL−1 (30); however, depending on the research environment, these reference values may fall between 15 and 35 ng·mL−1 (10,13). Because vitamin D studies have been conducted in a wide variety of populations, it cannot be assumed that the same reference values apply to all individuals (27). The cutoff for deficiencies in active individuals is still widely debated (13,16,23,26). Some have suggested that physically active individuals should maintain a higher vitamin D status, perhaps as high as 50 ng·mL−1 (10), to achieve optimal health and performance benefits (39). However, some are hesitant to recommend an optimal serum level this high (34). Thus, we considered 35 ng·mL−1 as the normal reference value for the subjects in our study, a cutoff that has also been used as the reference value in a previous study with physically active subjects (10).
Dietary analyses revealed that all subjects had a suboptimal intake of dietary vitamin D. Because subjects who were supplemented with any pharmacological form of vitamin D were excluded from participation, it was difficult to examine a full range of serum 25OHD levels with other variables measured in this study. Studies have shown that it is difficult to achieve adequate vitamin D status using dietary measures alone, especially when considered independent of supplementation with multivitamins (22). Consequently, in the United States, many foods are required by the Food and Drug Administration to be fortified with vitamin D3, including but not limited to orange juice, dairy products, and cereals (27). Although the fortification of these foods helps to increase vitamin D intake, they cannot be considered a means for obtaining sufficient vitamin D (22). As a result, it is not surprising that the dietary intake of vitamin D is extremely low in the young adults enrolled in our study. It is also crucial to point out that our dietary data was analyzed using the USDA Nutrient Database for vitamin D content. Even though the amounts listed in the USDA nutrient database include amounts of both D2 and D3, D3 is regarded to be much more biologically active in humans (1). Therefore, the dietary analysis included the highest possible value because it considered both D2 and D3 forms. The Institute of Medicine's recommendation for vitamin D is 600 IU per day (31), and only 1 of the 39 subjects in the current study met this requirement with an average daily intake of 700 IU (4,644 IU per week). This is a common trend in several recently published studies that have reported similar results in young physically active individuals (17,40). It is also important to note that experts in the field of vitamin D metabolism feel that nutritional guidelines should be changed to further increase the recommended daily intake of vitamin D beyond the current level of 600 IU. Yet, the Dietary Reference Intake committee is hesitant to do so because of the lack of evidence concerning vitamin D toxicity and inability to determine a tolerable upper limit (22).
Vitamin D levels are also strongly related to UV exposure (28). Data in the present study was collected during the summer and early fall in the Southeast United States, where the number of days with sunlight is estimated at 218 per year, and up to 9 hours of strong sunlight during late summer months when data was collected. Current recommendations suggest that spending about 20 minutes in the sunlight during peak hours will produce about 20,000 IU of D3 (22), but this conversion is affected by a number of different factors including ethnicity, adiposity, age, and geographical location (36,37). Furthermore, conversion of vitamin D3 into serum 25OHD for metabolic action does not mirror the amount of production that occurs through UV activation (22). In our study, sun exposure was evaluated by a survey that allows for direct combination of time spent in the sun and overall exposure (19). We used this particular survey because many individuals in this study enjoy recreational activities outdoors, but do so under different conditions of time of day or wearing various different types of clothing, making analysis of time under UV exposure without accounting for these factors insufficient. The scores obtained in the present study varied significantly as both the function of time spent outdoors and the amount of skin exposed to sun. We found no correlation between sun exposure, dietary intake, and serum 25OHD levels. Surprisingly, we also found no significant relationship between amount of sun exposure and serum 25OHD, despite that fact that most subjects reported infrequent sunscreen use, which maximized the potential for endogenous vitamin D production. It is also important to note that none of the subjects reported wearing sunscreen on a regular basis. Collectively, the previous reports and the present results indicate that individuals with significant sun exposure may still be at risk for vitamin D deficiency (7,23).
Research has suggested that serum 25OHD concentrations are most directly related to dietary intake of vitamin D (17) and UV exposure (4). However, in this study, there were no relationships between serum 25OHD and dietary intake or serum 25OHD and sun exposure. Although this may seem questionable based on the historical knowledge of vitamin D metabolism, results of this study confirm that there may be no single determinant of serum 25OHD. Although it is known that other factors such as ethnicity are also determinants of serum 25OHD, there is also the possibility that individual differences in vitamin D metabolism may be involved as well (25).
The statistically significant positive relationship between serum 25OHD concentrations and V[Combining Dot Above]O2max revealed in this study is also noteworthy. Although 2 recent publications have suggested that this relationship may exist in men and women (14,15), the results of our study are novel as they distinctively show a positive association in a young physically active population. Some have speculated that those who are more physically fit will tend to spend more time outdoors in the sunlight and eat a diet with more vitamin D fortified foods, whereas those who are less fit will spend more time indoors and eat a less healthy diet (14). However, it is possible that 25OHD may play a role in increasing fitness over regular exercise alone (5). In fact, a recent study revealed significant improvements in 10-m sprint times and vertical jump distance when vitamin D deficient athletes were supplemented with 5,000 IU per day of vitamin D3 for 8 weeks (11). Furthermore, we show that men who had 25OHD levels above the recommended limit of 35 ng·mL−1 serum 25OHD concentrations had significantly higher (20%) V[Combining Dot Above]O2max values when compared with men below this cut point. Because of the fairly even distribution among preferred training modality between “high” and “low” groups (“high”—endurance n = 15, resistance n = 4; “low”—endurance n = 13, resistance n = 5), it is possible that training modality may not have been a factor.
Serum 25OHD and BMI were also negatively correlated in the present study. Previous studies have shown that low levels of 25OHD are related to high levels of body fat (2,16). Results in this study revealed that there were several nonsignificant relationships, including serum 25OHD and resting metabolic rate and body fat in the overall data set. In a recent study, increasing vitamin D status in 77 healthy but overweight women resulted in a reduction in body fat (32). Additionally, there was a nonsignificant negative relationship (p = 0.184) between 25OHD and body fat observed in men in this study, although this did not translate into a relationship when correlating 25OHD and RMR in men. There was no relationship overall between 25OHD and RMR, and a consensus on this relationship has not yet emerged in the literature. There were no observed relationships between 25OHD and lean mass, both in the complete data set and when stratified by gender.
In our study, there were no significant relationships among strength or power output and serum 25OHD. It is possible that this finding may be a function of regular exercise modality. For example, our results showed that men were more inclined to engage in a more structured routine with strict running distances and weight lifting protocols, whereas women were more likely to engage in a variety of running distances and events or other aerobic group physical activity classes. This gender-based variation in training regimen could conceivably influence the development of muscle strength and power differently in each group; hence leading to the differences observed between men and women in strength and power.
Our study includes several limitations worth mentioning. First, most subjects were whites with a few Hispanics and no African Americans. Interpretation of the results is also limited by the seasonal variation of serum 25OHD assessment, which may not represent a yearly average (10). Researchers have noted seasonal differences in performance, as early as the 1950s, with a specific study examining wrist flexor strength throughout the course of a year (10,20). It has been suggested that strength is increased during late summer months, whereas it reaches a minimum during winter months (20). More recent studies have shown similar phenomena, with increased maximal oxygen uptake, increased heart rate variability, an important marker of cardiovascular health, and peak muscular strength all increasing during times when sun exposure and 25OHD levels are highest (10). Even though a pattern has emerged in the literature, the seasonal variation of 25OHD and sun exposure and its effect on athletic performance cannot be determined as it pertains to this study. Although all subjects met the standard requirement for sun exposure to produce what is believed to be normal levels of vitamin D, the dietary intake of vitamin D was extremely low in these individuals. Finally, it is important to note that because of the cross-sectional nature of this study, it is impossible to draw broad conclusions based on the observed relationships or interpret why the phenomena occurred.
In conclusion, the present study shows a significant portion of young physically active individuals have low levels of serum 25OHD, which was not directly related to dietary intake or sun exposure. There was a significant positive correlation between aerobic fitness and serum 25OHD status and a significant negative correlation between serum 25OHD and BMI. These findings are also novel, as the outcome measures included in this project have not been investigated by a single study in a young healthy population.
Perhaps the most interesting result of this study is that about half of the subjects recruited for this study presented with suboptimal 25OHD concentrations, despite abundant sun exposure. It is assumed that most individuals in a sunny climate in the summer would produce significant amounts of vitamin D through activation by sunlight. However, emerging research from this study and others shows that this situation may not necessarily be the case. It is also interesting to note that although vitamin D has gained significant interest in the sports nutrition community, standards for optimal intake and serum concentration are lacking for young healthy individuals. Those who are experiencing health issues or a lack in athletic performance with unresolved causes may want to consider being evaluated for low levels of vitamin D.
The results of this study also revealed a significant positive relationship between serum 25OHD and cardiorespiratory fitness and a significant negative relationship between 25OHD and BMI. Although this study and others suggests that adequate vitamin D status may play a role in achieving enhanced cardiovascular fitness and muscular power and strength, more investigation in to this question is needed. In the meantime, it is important for physically active individuals to know their vitamin D status and realize that suboptimal levels of 25OHD may be associated with reduced athletic performance.
The authors thank Louisiana State University for funding this project.
1. Armas LA, Hollis BW, Heaney RP. Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab 89: 5387–5391, 2004.
2. Arunabh S, Pollack S, Yeh J, Aloia JF. Body fat content and 25-hydroxyvitamin D levels in healthy women. J Clin Endocrinol Metab 88: 157–161, 2003.
3. Balady G, Berra KA, Golding LA, Gordon NF, Mahler DA, Myers JN, Sheldahl LM. ACSM's Guidelines for Exercise Testing and Prescription. Philadelphia, PA: Lippincott Williams & Wilkins, 2000.
4. Barger-Lux MJ, Heaney RP. Effects of above average summer sun exposure on serum 25-hydroxyvitamin D and calcium absorption. J Clin Endocrinol Metab 87: 4952–4956, 2002.
5. Bartoszewska M, Kamboj M, Patel DR. Vitamin D, muscle function, and exercise performance. Pediatr Clin North Am 57: 849–861, 2010.
6. Beneke R, Pollmann C, Bleif I, Leithauser RM, Hutler M. How anaerobic is the Wingate Anaerobic Test for humans? Eur J Appl Physiol 87: 388–392, 2002.
7. Binkley N, Novotny R, Krueger D, Kawahara T, Daida YG, Lensmeyer G, Hollis BW, Drezner MK. Low vitamin D status despite abundant sun exposure. J Clin Endocrinol Metab 92: 2130–2135, 2007.
8. Bischoff-Ferrari HA, Dietrich T, Orav EJ, Hu FB, Zhang Y, Karlson EW, Dawson-Hughes B. Higher 25-hydroxyvitamin D concentrations are associated with better lower-extremity function in both active and inactive persons aged > or =60 y. Am J Clin Nutr 80: 752–758, 2004.
9. Bischoff-Ferrari HA, Giovannucci E, Willett WC, Dietrich T, Dawson-Hughes B. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr 84: 18–28, 2006.
10. Cannell JJ, Hollis BW, Sorenson MB, Taft TN, Anderson JJ. Athletic performance
and vitamin D. Med Sci Sports Exerc 41: 1102–1110, 2009.
11. Close GL, Russell J, Cobley JN, Owens DJ, Wilson G, Gregson W, Fraser WD, Morton JP. Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: Implications for skeletal muscle function. J Sports Sci 31: 344–353, 2013.
12. Compher C, Frankenfield D, Keim N, Roth-Yousey L. Best practice methods to apply to measurement of resting metabolic rate in adults: A systematic review. J Am Diet Assoc 106: 881–903, 2006.
13. Ducher G, Kukuljan S, Hill B, Garnham AP, Nowson CA, Kimlin MG, Cook J. Vitamin D status and musculoskeletal health in adolescent male ballet dancers a pilot study. J Dance Med Sci 15: 99–107, 2011.
14. Farrell SW, Cleaver JP, Willis BL. Cardiorespiratory fitness, adiposity, and serum 25-dihydroxyvitamin d levels in men. Med Sci Sports Exerc 43: 266–271, 2011.
15. Farrell SW, Willis BL. Cardiorespiratory fitness, adiposity, and serum 25-dihydroxyvitamin D levels in women. J Women Health (Larchmt) 21: 80–86, 2011
16. Gilsanz V, Kremer A, Mo AO, Wren TA, Kremer R. Vitamin D status and its relation to muscle mass and muscle fat in young women. J Clin Endocrinol Metab 95: 1595–1601, 2010.
17. Halliday TM, Peterson NJ, Thomas JJ, Kleppinger K, Hollis BW, Larson-Meyer DE. Vitamin D status relative to diet, lifestyle, injury, and illness in college athletes. Med Sci Sports Exerc 43: 335–343, 2011.
18. Hamilton B. Vitamin D and human skeletal muscle. Scand J Med Sci Sports 20: 182–190, 2010.
19. Hanwell HE, Vieth R, Cole DE, Scillitani A, Modoni S, Frusciante V, Ritrovato G, Chiodini I, Minisola S, Carnevale V. Sun exposure questionnaire predicts circulating 25-hydroxyvitamin D concentrations in Caucasian hospital workers in southern Italy. J Steroid Biochem Mol Biol 121: 334–337, 2010.
20. Hettinger T, Muller EA. Seasonal course of trainability of musculature [in German]. Int Z Angew Physiol 16: 90–94, 1956.
21. Holick MF. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am J Clin Nutr 80: 1678S–1688S, 2004.
22. Hollis BW. Circulating 25-hydroxyvitamin D levels indicative of vitamin D sufficiency: Implications for establishing a new effective dietary intake recommendation for vitamin D. J Nutr 135: 317–322, 2005.
23. Jacobs ET, Alberts DS, Foote JA, Green SB, Hollis BW, Yu Z, Martinez ME. Vitamin D insufficiency in southern Arizona. Am J Clin Nutr 87: 608–613, 2008.
24. Kremer R, Campbell PP, Reinhardt T, Gilsanz V. Vitamin D status and its relationship to body fat, final height, and peak bone mass in young women. J Clin Endocrinol Metab 94: 67–73, 2009.
25. Lips P. Vitamin D physiology. Prog Biophys Mol Biol 92: 4–8, 2006.
26. Mastaglia SR, Seijo M, Muzio D, Somoza J, Nunez M, Oliveri B. Effect of vitamin D nutritional status on muscle function and strength in healthy women aged over sixty-five years. J Nutr Health Aging 15: 349–354, 2011.
27. Moore C, Murphy MM, Keast DR, Holick MF. Vitamin D intake in the United States. J Am Diet Assoc 104: 980–983, 2004.
28. Norman AW, Bouillon R. Vitamin D nutritional policy needs a vision for the future. Exp Biol Med (Maywood) 235: 1034–1045, 2010.
29. Rattarasarn C, Leelawattana R, Soonthornpun S, Setasuban W, Thamprasit A. Gender differences of regional abdominal fat distribution and their relationships with insulin sensitivity in healthy and glucose-intolerant Thais. J Clin Endocrinol Metab 89: 6266–6270, 2004.
30. Rosen CJ. Clinical practice. Vitamin D insufficiency. N Engl J Med 364: 248–254, 2011.
31. Ross AC. The 2011 report on dietary reference intakes for calcium and vitamin D. Public Health Nutr 14: 938–939, 2011.
32. Salehpour A, Hosseinpanah F, Shidfar F, Vafa M, Razaghi M, Dehghani S, Hoshiarrad A, Gohari M. A 12-week double-blind randomized clinical trial of vitamin D3 supplementation on body fat mass in healthy overweight and obese women. Nutr J 11: 78, 2012.
33. Sayers SP, Harackiewicz DV, Harman EA, Frykman PN, Rosenstein MT. Cross-validation of three jump power equations. Med Sci Sports Exerc 31: 572–577, 1999.
34. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr 69: 842–856, 1999.
35. Ward KA, Das G, Berry JL, Roberts SA, Rawer R, Adams JE, Mughal Z. Vitamin D status and muscle function in post-menarchal adolescent girls. J Clin Endocrinol Metab 94: 559–563, 2009.
36. Webb AR. Who, what, where and when-influences on cutaneous vitamin D synthesis. Prog Biophys Mol Biol 92: 17–25, 2006.
37. Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: Exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol Metab 67: 373–378, 1988.
38. Weir JB. New methods for calculating metabolic rate with special reference to protein metabolism. J Physiol 109: 1–9, 1949.
39. Willis KS, Peterson NJ, Larson-Meyer DE. Should we be concerned about the vitamin D status of athletes? Int J Sport Nutr Exerc Metab 18: 204–224, 2008.
40. Willis KS, Smith DT, Broughton KS, Larson-Meyer DE. Vitamin D status and biomarkers of inflammation in runners. Open Access J Sports Med 3: 35–42, 2012.