The active form of vitamin D (calcitriol) is a secosteroid hormone synthetized in humans, with autocrine regulation and a nuclear receptor (15,33). Apart from the maintenance of calcium and phosphorus balance, many pleiotropic functions have been discovered, such as regulatory function of the kidneys, heart, immune system, anti-inflammatory, anti-apoptotic, antifibrotic, cell differentiation, and proliferation (25). Besides the classical target tissues, the vitamin D receptor has been found in the immune, reproductive, and endocrine systems, all types of muscles, brain, skin, and liver (33).
There is growing body of evidence for a role of vitamin D in muscle function and its influence on the athletic performance as well as injury profile and recovery (7,10,15,16,23). Several mechanisms (both genomic and nongenomic) underlying the effect of vitamin D on skeletal muscle function have been proposed. In general, these mechanisms interfere with regulation of metabolic processes, transcription, and gene expression in the skeletal muscles, thereby inducing changes in calcium handling (regulation of membrane calcium channels), myocyte differentiation, expression of contractile proteins, hypertrophy, and improved mitochondrial function (15–17). In elderly women, vitamin D deficiency has been linked to type II muscle fiber atrophy, which could be reversed by vitamin D supplementation leading to increased percentage and size of type II fibers as well as muscle strength (29).
Despite the potential role for vitamin D in muscle performance and the high rates of vitamin D deficiency among athletes and nonathletic individuals (9), the results of studies investigating the impact of vitamin D supplementation on muscle strength and performance in both nonathletic and athletic populations are mixed. In older populations, several observational, cross-sectional or longitudinal studies reported the association between 25(OH)D status and various measures of physical performance (6,11,22,34,36). A higher 25(OH)D concentration was associated with better musculoskeletal function in lower extremities in men and women more than 65 years (6,36), or only in women (11). Houston et al. (22) have found that participant (70–79 years) with 25(OH)D < 50 nmol·L−1 had poorer physical performance, lower knee extensor, and grip strength than those with 25(OH)D levels >75 nmol·L−1. In a longitudinal study conducted by Visser et al. (34) persons aged 65 years and older with 25(OH)D levels below 25 nmol·L−1 were more likely to develop sarcopenia than those with 25(OH)D > 50 nmol·L−1 at 3-year follow-up. In line with these observations, the supplementation of vitamin D together with calcium over a 3-month period has been shown to improve musculoskeletal function in people more than 60 years of age (5). However, recent systematic reviews and meta-analyses showed that an increasing serum 25(OH)D concentration in nonathletes had inconsistent effects on grip strength, lower- and upper-limb strength (4,26,31,32). In younger adults or adolescents, the 25(OH)D has been associated with a faster muscle recovery after a bout of intense exercise (2), greater improvements in jump height and velocity (35), or a handgrip strength (14). However, in a randomized controlled trial undertaken in the school children, a vitamin D supplementation for 1 year did not lead to an improvement in grip strength (13).
In athletes, both cross-sectional and intervention studies produced equivocal results. No relationship between 25(OH)D concentration and muscle function was found in well-trained professional football players (18) and adolescent competitive swimmers (12). A few intervention studies have aimed to investigate the effect of vitamin D on muscle function and performance in the athletes providing inconclusive results (8,9,30,37,38). These studies differed markedly with respect to period of supplementation (from 8 days to 6 months), doses of vitamin D, baseline levels of 25(OH)D, and training regimens (both between- and within-study) during the experiment. Close et al.(8) found no significant effect of vitamin D supplementation on 1-RM bench press and leg press, vertical jump height, and 20 m sprint in club-level athletes. However, earlier, the same authors demonstrated significant effects of vitamin D on 10 m sprint times and vertical jumps in highly trained football players (9).
The aim of this double-blind, placebo-controlled study was to investigate the effect of vitamin D supplementation in response to High Intensity Interval Training (HIIT) in well-trained football players. Direct (Wingate test) and indirect (squat jump height, sprint times) measures of muscle power and explosive strength were used as primary outcomes in the study.
Experimental Approach to the Problem
The experiment was performed during 8-week training cycle of preparatory season in winter, from January to March, during the time when the solar exposure in Poland is very low. The subjects were divided into 2 groups: the experimental one that was supplemented with vitamin D (SG) and another that was not supplemented with vitamin D—the placebo group (PG). All the players were subjected to the same football training described as High Intensity Interval Training (HIIT) that involved endurance, speed, and strength drills (Table 1). Just before and after the experiment, blood samples for vitamin D level were taken from the players. Wingate test was performed to determine peak power and total work. Speed and explosive power were asessed by means of the sprint (5, 10, 20, and 30 m) and vertical jumps (squat jump, SJ, countermovement jump, CMJ), respectively. Small-sided games and interval run at anaerobic threshold (AnT) were performed on the field with synthetic surface. The intensity of the effort was determined by heart rate (HR) that was equal or higher than AnT value but did not exceed 90% HRmax. Small-sided games were performed on the field size 32 × 22 m (3 vs. 3 on Tuesday) and 44 × 33 m (6 vs. 6 on Thursday) with 120 m2 of the surface per player. The subjects of both groups played 4 games, 4 minutes each with 3 minutes active break that involved marching and muscle relaxing drills. On Fridays the players performed 4 series, 5 minutes each of interval run with 3 minutes active break (like during small-sided games). On the other days of the 1-week training cycle, the players were subjected to speed and explosive strength drills (Table 1).
Forty-two young football players entered the experiment, but only thirty-six (age: 17.5 ± 0.6, range 17–19 years, body mass 71.3 ± 6.9 kg, BMI 22.2 ± 1.8 kg·m−2) were analyzed due to the random incidents. They showed high sports level and participated in Central League of top junior teams in Poland. The subjects were divided into 2 groups: the placebo one, n = 16 (that was subjected to HIIT only), and the experimental, vitamin D3 supplemented one, n = 20 (that was subjected to HIIT and vitamin D3 supplementation). The selection to the groups was based on peak power results attained before the experiment (so that both groups were homogenous), and position on the field (equal number of the players of the same positions in both groups). The participation in training sessions of the analyzed players reached 95%. Before, during and after the experiment, the tested subjects lived in the school dormitory and were nourished in the same way. The diet of the players was standard but included increased amount of vegetables, fruits, and dairy products (sports diet). One month before and during the experiment, the players did not take any vitamin or other sports supplements. All the subjects had valid medical cards. The protocol was fully approved by Ethical Committee of the local Medical Association in Gdańsk (Nr KB—1/14). All subjects and their parents or guardians were provided with detailed information about the research procedures and gave their written consent form.
Vitamin D3 Supplementation
Each subject from the supplemented group was given the vitamin D3 bottle (Vigantol; Merck, Germany) and was asked to take 10 droplets per day (around 5000 IU/day) in the morning for the 8 weeks during the study. Placebo group received identical bottles containing sunflower oil. The dispensation procedure was double-blind. Supplementation and testing were carried out in late winter, when natural level of 25(OH)D was likely to be minimal. During the study, the subjects were asked several times if they had taken the vitamins as instructed. All the subjects reported that they had followed the instructions. It was confirmed by the measurements of 25(OH)D before and 8 weeks after the supplementation.
Peak Power and Total Work
A 30-second Wingate test on a cyclo-ergometer (Ergomedic 894E; Monark, Sweden) was used to assess the peak power and total work. A relative load corresponding to 7.5% of the subject's body mass was applied. Before performing the test, the participants completed a 10-minute warm-up, including pedaling at a frequency of 60 rotations per minute (RPM), with a relative load of 1.2 W·kg−1 and 3 rapid accelerations between seventh and 10th minutes. After the warm-up, the subjects performed 5 minutes of stretching and relaxing exercises and then started the test (3).
Before the test, the players performed a 20-min warm-up, involving two 5 m and 10 m, and one 30 m sprints. The sprint times were recorded by double photocells (Smart Speed electronic system; Fusion Sport, Cooper Plains, Australia) positioned at the starting (0 m) and finishing lines (5 m, 10 m, 20 m, and 30 m) at a height of 0.7 m and 0.9 m. The subjects performed 2 maximal attempts for the 5, 10, 20, and 30 m distances. Only the best (the shortest) times were used in the subsequent analysis. Each participant started from a standing position, with his front leg on the starting line (0 m). The resting periods were 90 seconds after the 5 m, 120 seconds after 10 m, 180 seconds after 20 m and 240 seconds after the 30 m sprints.
Before the test, the students performed a 20-minute warm-up involving 5 vertical jumps. The test comprised 2 maximal vertical jumps without (squat jump, SJ) arm swings and 2 with arm swings (countermovement jump, CMJ). In a squat jump, participants settled down in a full squatted posture with knees close together and maximally flexed. After that, the knees and hips were extended to jump vertically off the ground with arms resting on hips. A countermovement jump was performed from an upright standing position with arms up. The vertical jump was preceded by downward movement (countermovement phase) until a full squatted posture with arms swinging back. In a propulsive phase of the jump, the knees and hips were extended and arms swinging upward. The resting period between jumps was 2 minutes. Only the best (the highest) jump was used in the subsequent analysis.
Data were analyzed using a within-subject modeling (http://www.sportsci.org) and analysis of covariance (ANCOVA). First, pretraining values were compared using an independent sample t test for unequal variances, whereas the training response was analyzed using a dependent sample t test. Next, the posttraining and pretraining changes (change scores) were calculated for each individual and the mean change score was compared between SG and PG with the t test (unequal variances) and ANCOVA (the pretraining value was included in the model as a covariate). In addition, to estimate the magnitude of the supplementation effect, the mean change score in nonsupplemented individuals was subtracted from the mean change score in supplemented individuals. The difference in mean change score was then standardized with a pretraining standard deviation (SD) calculated for all supplemented and nonsupplemented individuals (19). The difference in mean change and standardized difference were reported with 95% confidence limits. Besides the mean effects, we estimated the magnitude of individual responses to training, as described by Hopkins (21). SDIR, a measure of individual response was calculated as the square root of the difference between squares of the standard deviations of the change scores in the supplemented and control groups. Intraclass correlation coefficients were used to assess reliability of measures between 2 trials for sprint runs and jump (20). All other analyses were performed using STATISTICA (version 12; StatSoft, Inc. (2014), www.statsoft.com). A p value ≤ 0.05 was considered significant.
The presupplementation plasma concentration of 25(OH)D did not differ between supplemented and placebo groups (48.5 ± 8.6 vs. 47.5 ± 16.2 nmol·L−1, p = 0.817). At baseline, 22 of all participants (61.1%), 12 in the supplemented group (60.0%) and 10 in the placebo group (62.5%) had 25(OH)D plasma below 50 nmol·L−1. In the supplemented group the mean change of the 25(OH)D was 57.8 ± 21.7, compared with −4.0 ± 12.7 nmol·L−1 in the placebo group (p < 0.0001, t test, Figure 1). Nine participants in the supplemented group (45.0%) achieved 25(OH)D level ≥ 100 nmol·L−1 at the end of the study. Pretraining intraclass correlation coefficients for test–retest reliability were 0.68, 0.80, 0.93, 0.91, 0.97, and 0.98 for 5 m sprint, 10 m sprint, 20 m sprint, 30 m sprint, SJ, and CMJ, respectively. Pretraining peak power and total work measured during Wingate test as well as power measurements from vertical jumps and sprint runs are listed in Table 2. There were no significant differences between supplemented and placebo groups for any power-related characteristics at baseline.
All power-related variables, except the 30 m sprint running time, improved significantly in response to HIIT (Table 3). However, the mean change scores (the difference between postraining and pretraining value) did not differ significantly between placebo group and vitamin D supplemented group (Table 4).
In addition, the effect of vitamin D supplementation was estimated by subtracting the mean change score in the placebo group from the mean change score in the supplemented group. It was then standardized using the standard deviation of the whole cohort. The magnitude of vitamin D supplementation effect was meaningless for all power-related variables (below 0.2 for all variables, except for 20 m and 30 m sprints, 0.26 ± 0.56 and 0.24 ± 0.38, respectively). In addition, as indicated by the SDIR and 95% confidence limits, the variation in response to training in the SG was comparable with variation seen in the PG for all power-related measures (Table 4).
The main finding of the present study is that a vitamin D supplementation did not modulate the adaptation of the power-related variables to the 8 weeks High Intensity Interval Training. Specifically, the training gains were similar in both supplemented and nonsupplemented players. The magnitude of the effect of supplementation (calculated as the difference in mean change in each group) was meaningless, merely exceeding the value of 0.2 (for 20 m and 30 m sprint), a threshold for the smallest standardized difference in mean. So far, only few intervention studies investigating the effect of vitamin D on muscle strength and power in athletes have been conducted (8,9,30,37,38). Three of these studies have reported positive effects of vitamin D supplementation on direct or indirect measures of muscle performance (9,37,38). Close et al. (9) found greater improvements in 10 m sprint time and vertical jump height in the supplemented group (n = 5) as compared with nonsupplemented group. Close et al.'s (9) study and our study are comparable with respect to participants examined (highly trained football players), time of duration (8 weeks during inadequate ultraviolet exposure), dose of vitamin D (5,000 IU/day), rate of vitamin D deficiency (60% in both studies), and restoration rate of vitamin D deficiency (60% of players reached the 25(OH)D level of 100 nmol·L−1 or higher, compared with 45% in our study). Based on currently existing research, it has been suggested that the benefit of vitamin D supplementation in athletes may be limited to individuals with significant vitamin D deficiency only (10). Similar observations come from studies on nonathletic populations. In the meta-analysis of 17 randomized controlled studies a significant effect of vitamin D supplementation on muscle strength could be found only in pooled data from the 2 studies of vitamin D deficient participants (<25 nmol·L−1) (31). Thus, a sufficient baseline vitamin D status may be an important limiting factor for the vitamin D- induced performance improvements (23). Indeed, in the 2 of the 3 studies that observed positive effects of vitamin D supplementation on muscle function in athletes, most participants had either 25(OH)D baseline level consistent with vitamin D deficiency (9) or 25(OH)D concentration were not available for analysis (37). In this regard, our results are in opposition to the “baseline 25(OH)D hypothesis” as the percentage of athletes with presupplementation vitamin D deficiency (<50 nmol·L−1) in contradicting studies was equal (60%, 3/5 vs. 12/20). Thus, the lack of positive effect of vitamin D supplementation in players in our study cannot be explained by their better vitamin D status. Therefore, different physical characteristics of the participant at the time of entry into the study (initial fitness), different range of conditioning methods, or other yet undefined factors may be among the possible reasons for the discrepancy between the 2 studies. It has been suggested that the vitamin D supplementation may be most effective and beneficial in older populations characterized by a low baseline physical fitness (24). Close et al. (9) suggested that older subjects may be more sensitive to vitamin supplementations than young people.
The third positive study was conducted in 22 national level judokas (38). The treatment group (n = 11) exhibited a significant (13% on average) increase in isokinetic concentric quadriceps and hamstring muscle strength as compared with placebo group. However, the results of the Wyon et al.'s (38) study cannot be directly compared with our and Close et al.'s study (9) due to significant methodological differences. The observation period was only 8 days, and the vitamin D was given in a single high dose (150,000 IU). Although, the participants continued their full-time training during the study, it is not clear whether the improvement in muscle strength was a result of vitamin D–training interaction, or rather an acute effect of single bolus dose of vitamin D. In addition, the treatment group, despite relatively low prevalence of vitamin D deficiency (36%), had significantly lower presupplementation 25(OH)D levels than control group, with possibly greater potential to improve (38). In another study, Wyon et al. (37) investigated the effect of vitamin D (2,000 IU daily for 4 months) on muscle strength in 24 elite ballet dancers and found a significantly improved isometric quadriceps strength and vertical jump height in the supplemented group compared with control group. The dancers did not continue any direct strength, speed, or power training. A main limitation of this study is the lack of the presupplementation and postsupplementation 25(OH)D concentrations.
One of the measures used in the current study, a mean change score, represents only the overall effect of the intervention. Many interesting individual effects (individual responses) may be hidden in the mean change. Therefore, we also checked whether some players in the supplemented group responded better (or worse) to vitamin D than other players within that group. Individual responses can be predicted by comparing standard deviations of the individual change scores between the supplemented and the placebo groups (21). The larger the variation in the change scores the greater the diversity of responses. As the variations in the mean changes for all measured variables in both groups were similar, there was no evidence of individual response in the supplemented group.
Supplementation of 5,000 IU daily has been used in our study, which is the half of the upper limit of recommended intake for adults (19–70 years) according to the Endocrine Society (27). It has also been suggested that the body uses on average 3,000–5,000 IU of vitamin D daily (1). A special care has been taken to avoid unsupervised access to any kind of dietary supplements. Adherence to diet and restrictions was satisfactory as judged by the near constant level of 25(OH)D in the control group throughout the study (Figure 1).
Vitamin D deficiency is widespread among both athletic and nonathletic populations (17), and in athletes, its prevalence varies from 11 to 83% (9,27). The experiment has been conducted during winter months (January to March). Pludowski et al. (28) found that in Central and Western Europe countries, wintertime 25(OH)D concentrations in adults aged 20–60 years ranged between 27.5 nmol·L−1 (Poland) and 50 nmo·L−1 (Austria). This finding is not surprising, as during winter months, UVB radiation is nonexistent at latitudes greater than 35° (27).
Several limitations in our study have to be pointed out. The first refers to the optimum vitamin D concentration (and supplementation dose) for athletes with respect to their neuromuscular function. There is no consensus on optimal serum 25(OH)D level It has been suggested that the 25(OH)D concentration >100 nmol·L−1 may be needed to induce a physiological response within skeletal muscle and enhance muscle performance, especially in young athletes (9). Visser et al. (34) found that the risk of sarcopenia with 25(OH)D < 25 nmol·L−1 was over 2 times higher compared with 25(OH)D level above 50 nmol·L−1, after adjustment for several covariates. Cross-sectional studies in the elderly showed that no further improvements in neuromuscular performances were evident above the level of 100–125 nmol·L−1 (7). According to von Hurst and Beck (23), the optimal intake and serum level of 25(OH)D in athletic populations have yet to be determined. In addition, the optimum level of 25(OH)D (whatever it is) might have been obtained too late during the study. Indeed, near the middle of experiment, only one subject in the supplemented group achieved the 25(OH)D level above 100 nmol·L−1. Although 80% of the subjects in the supplemented group had 25(OH)D greater than 50 nmol·L−1 in the middle of the experiment compared with none at the baseline (data not shown), it might have been insufficient for training–vitamin D interaction to occur. Possibly, the longer supplementation period and perhaps a higher dose of vitamin D are needed to induce such interaction.
In addition, no formal dietary assessment of the participants' intake of vitamin D before study has been undertaken. A documented insufficient intake of vitamin D would be a reasonable basis for issuing nutrition recommendations, especially in view of lack of the positive effect of supplementary vitamin D in our study.
Vitamin or other sports supplementation is common among adult or young sportsmen training. However, coaches often act individually and do not always seek specialist's advice. There are high expectations regarding even slightest improvement in sports performance in different disciplines. However, there is still no sufficient evidence to support vitamin D supplementation for improving performance or training adaptation and to recommend the coaches the adequate doses for age, type of sport, or training load, in particular. To date, only a few intervention studies investigating the effect of vitamin D on physical performance or training adaptations were undertaken. Moreover, the results of these studies are equivocal. Therefore, given the current level of evidence, we believe that the recommendation to use vitamin D supplements in all athletes to improve performance or training gains would be premature. However, there is a high rate of vitamin D deficiency in athletes, especially indoor athletes or during winter months at unfavorable latitude (>35–37°). To avoid a seasonal decrease in 25(OH)D level or to obtain optimal vitamin D levels (>100 nmol·L−1), the combination of higher dietary intake of vitamin D and supplementation (at least 2,000 IU/day until optimal levels are met, and then 1,000–2,000 IU/day, for maintenance) may be necessary (27).
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