The Dependence of Running Speed and Muscle Strength on the Serum Concentration of Vitamin D in Young Male Professional Football Players Residing in the Russian Federation

Abstract

Background: Vitamin D insufficiency is prevalent among athletes and it can negatively affect physical performance. At the same time, most of the available data were obtained from untrained individuals of various ages, and published studies performed in athletes lead to contradictory conclusions. Methods: This study examined the serum concentration of vitamin D and its effect on running speed and muscle power in 131 young football players (mean age 15.6±2.4 years). Results: Vitamin D levels were below reference in 42.8%, and above reference in 30.5% of the participants. A comparison of results of 5, 15 and 30 m sprint tests and the standing long jump test found no statistically significant differences between the two groups. Athletes from the vitamin D insufficient group were treated with 5,000 IU cholecalciferol supplement daily for 60 days. After the treatment, vitamin D concentration increased by 79.2%, and was within reference in 84% of the treated athletes. Testing was repeated after the end of treatment, and a statistically significant increase in the results of the 5, 15 and 30 m sprint tests was observed, while the results of the standing long jump test remained unchanged. Body height and body weight of the football players also increased. Conclusions: These findings indicate that there is likely no correlation between serum levels of vitamin D, muscle power and running speed in young professional football players, and the changes observed post-treatment may be caused by the changes in anthropometric parameters.

Keywords: vitamin D3; cholecalciferol; muscle power and speed; vitamin D deficiency; treatment for vitamin D deficiency; young football players

Full text

nutrients
Article
The Dependence of Running Speed and Muscle
Strength on the Serum Concentration of Vitamin D in
Young Male Professional Football Players Residing in
the Russian Federation
Eduard Bezuglov 1, Aleksandra Tikhonova 2, Anastasiya Zueva 1, Vladimir Khaitin 3,
Anastasiya Lyubushkina 4, Evgeny Achkasov 1, Zbigniew Wa´skiewicz 1,5 ,
Dagmara Gerasimuk 6, Aleksandra ˙
Zebrowska 7, Pantelis Theodoros Nikolaidis 8,9 ,
Thomas Rosemann 10 and Beat Knechtle 10, 11, *
1
Department of Sport Medicine and Medical Rehabilitation, Sechenov First Moscow State Medical University,
119435 Moscow, Russia
2Department of Sport Medicine and Medical Rehabilitation, Faculty of Continuing Professional Education,
Sechenov First Moscow State Medical University, 119435 Moscow, Russia
3FC Zenit Saint-Petersburg, 197341 Saint Petersburg, Russia
4«Smart Recovery» Sports Medicine Clinic, 121552 Moscow, Russia
5Institute of Sport Science, Jerzy Kukuczka Academy of Physical Education, 40-065 Katowice, Poland
6Department of Sports Training, Jerzy Kukuczka Academy of Physical Education, 40-065 Katowice, Poland
7Department of Physiological and Medical Sciences, Jerzy Kukuczka Academy of Physical Education,
40-065 Katowice, Poland
8Exercise Physiology Laboratory, 18450 Nikaia, Greece
9School of Health and Caring Sciences, University of West Attica, 11244 Athens, Greece
10 Institute of Primary Care, University of Zurich, 8091 Zurich, Switzerland
11 Medbase St. Gallen Am Vadianplatz, 9001 St. Gallen, Switzerland
*Correspondence: beat.knechtle@hispeed.ch; Tel.: +41-(0)-71-226-93-00
Received: 28 July 2019; Accepted: 16 August 2019; Published: 21 August 2019


Abstract:
Background: Vitamin D insufficiency is prevalent among athletes, and it can negatively affect
physical performance. At the same time, most of the available data were obtained from untrained
individuals of various ages, and published studies performed in athletes led to contradictory
conclusions. Methods: This cohort prospective study examined the serum concentration of
25-hydroxycalciferol (25(OH)D) and its association with running speed and muscle power in 131
young football players (mean age 15.6
±
2.4 years). Results: 25(OH)D levels were below reference in
42.8% (serum 25(OH)D <30 ng/mL) and above reference in 30.5% of the participants (serum 25(OH)D
61–130 ng/mL). A comparison of the results of 5, 15, and 30 m sprint tests and the standing long
jump test found no statistically significant differences between the two groups. Athletes from the
25(OH)D-insufficient group were treated with 5000 IU cholecalciferol supplement daily for 60 days.
After the treatment, the 25(OH)D concentration increased by 79.2% and was within reference in
84% of the treated athletes (serum 25(OH)D 30–60 ng/mL). Testing was repeated after the end of
treatment, and a statistically significant increase in the results of the 5, 15, and 30 m sprint tests
was observed (Cohen’s dwas 0.46, 0.33, and 0.34, respectively), while the results of the standing
long jump test remained unchanged. Body height, body weight, and lean body mass of the football
players also increased. Conclusions: These findings indicate that there is likely no correlation between
serum levels of 25(OH)D, muscle power, and running speed in young professional football players,
and the changes observed post-treatment might have been caused by changes in the anthropometric
parameters. During the study, all the anthropometric parameters changed, but the amount of lean
body mass only correlated with the results of the 5 m sprint.
Nutrients 2019,11, 1960; doi:10.3390/nu11091960 www.mdpi.com/journal/nutrients

Nutrients 2019,11, 1960 2 of 10
Keywords:
vitamin D
3
; cholecalciferol; muscle power and speed; vitamin D deficiency; treatment for
vitamin D deficiency; young football players
1. Introduction
Vitamin D plays a crucial role in phosphorus and calcium metabolism and thus affects the state
of bone tissue. Its active form, 1,25-dihydroxyvitamin D (1,25(OH)2D), increases the efficiency of
intestinal calcium absorption from 10%–15% to 30%–40% by interacting with the vitamin D receptor
and retinoid X receptor (VDR-RXR), thereby promoting the expression of an epithelial calcium channel
and a calcium-binding protein [
1
–
3
]. It has been estimated that 1,25(OH)2D also increases intestinal
phosphorus absorption from 50%–60% to approximately 80% [
4
,
5
]. It promotes resistance to certain
diseases and affects the immune system as well as maintains muscle tone and the structure of connective
tissue. It also regulates lipid and carbohydrate metabolism and the level of blood glucose [
6
–
8
]. Vitamin
D receptors can be found in a multitude of tissues, which explains its numerous effects outside the
skeletal system [9,10].
Vitamin D insufficiency is most prevalent in regions located to the north of the 35th parallel
north because sun rays enter the atmosphere at a shallower angle and disperse [
11
,
12
]. Most of the
vitamin D found in human body is synthesized when ultraviolet (UV) rays penetrate the open skin at a
specific angle. Two distinct forms of vitamin D exist: ergocalciferol (vitamin D
2
), which is primarily
obtained from plant-based food, and cholecalciferol (vitamin D
3
), which is synthesized when the body
is exposed to UV rays. The vitamin D
3
metabolite 25-hydroxycalciferol (25(OH)D) is an important
biomarker used in clinical settings to prevent and treat vitamin D deficiencies [13].
Vitamin D insufficiency is also very common in professional athletes, where it can reach 60%–90%,
according to a number of different authors [
14
–
17
]. Vitamin D deficiency presents an important
challenge in football, where it was observed in 64%–83% of football players from England, Spain,
and Poland [
18
,
19
]. However, as shown by Hamilton et al., it is most frequent in the Middle East,
where it was diagnosed in 84% of the 342 examined Qatari football players [20].
Vitamin D has been shown to affect muscle tissue, which serves as an important target site.
However, most studies confirming the link between vitamin D deficiency and muscle weakness were
performed on people of varying ages with no adequate training [
17
–
26
]. This link may be facilitated
through multiple pathways, which either directly affect muscle tissue or, possibly, alter endogenous
testosterone synthesis. Pilz et al. showed that vitamin D exhibits ergogenic potential and indirectly
enhances testosterone production, which can also affect the muscular system. It is possible that this is
achieved through inhibition of testosterone aromatization and enhanced binding of androgens, which in
turn leads to muscle hypertrophy and increase in strength [
27
]. Animal studies have shown that
vitamin D influences myostatin inhibition, regeneration, and muscle cell proliferation processes [
28
,
29
].
Notably, most of the existing studies exploring the association between serum vitamin D level
and muscle performance in athletes were performed on adults and yielded contradictory results.
A systematic review by Chiang et al. showed that the administration of vitamin D
3
supplements
achieved a statistically significant improvement in muscle performance, which was not obtained when
vitamin D
2
supplements were used [
30
]. van Hurst et al. noted that muscle strength and stamina
associated with vitamin D administration only occurred in athletes whose vitamin D levels were
initially low [
31
]. Farrokhyar et al. found no association between vitamin D supplementation, vitamin
D concentration, and various indicators of physical performance, including muscle strength [32].
Football players have also been the subjects of such studies. Koundourakis et al. observed a
positive correlation between vitamin D level and muscle performance in a cohort of Greek football
players [
33
]. A randomized study performed by Close et al. also showed the beneficial effects of
vitamin D on muscle strength and power, with athletes who had received 5000 IU vitamin D for eight
weeks significantly improving their results in sprint and vertical jump tests [
14
]. Hamilton et al., on the

Nutrients 2019,11, 1960 3 of 10
other hand, found no significant association between the level of 25(OH)D and muscle function [
20
]. In
a study by Jastrzebska et al., where 5000 IU vitamin D were administered to football players, most of
the changes in the indicators of physical performance were insignificant [34].
So far, there have been a few studies regarding the prevalence of vitamin D deficiency in young
athletes and its effect on their muscle performance. Brannstrom et al. observed 19 young female
football players and found no statistically significant correlations between vitamin D levels and most
of the indicators of muscle tissue performance [
35
]. A study by Fitzgerald et al. found that vitamin
D insufficiency was highly prevalent in a cohort of 53 Canadian junior hockey players. However,
no correlation between vitamin D levels and muscle strength was observed [36].
Thus, there is no consensus regarding the effect of serum concentration of vitamin D on running
speed and strength in athletes. At the same time, most of the existing studies were performed in adult
populations, with only occasional studies performed on young athletes. To the best of our knowledge,
there is no published research on this topic that has been performed in a cohort of young football
players, which increases the importance of studying the effects exerted by vitamin D insufficiency on
muscle tissue. Therefore, the present study examined the serum concentration of vitamin D (25(OH)D)
and its association with running effect and muscle power in young male football players.
2. Materials and Methods
The study was conducted from December 2018 to February 2019 at the Lokomed Medical Center
of Lokomotiv FC Moscow with the participation of the staffof the Department of Sports Medicine
and Medical Rehabilitation of the Sechenov First Moscow State Medical University. The protocol of
this study was approved by the official Local Ethics Committee of the Sechenov First Moscow State
University under the statement number 11–19 of 07/25/2019. All stages of the study complied with the
legislation of the Russian Federation. All participants in the study provided their informed consent.
Consent from the parents of all study participants under 18 years of age was obtained. Athletes who
were 18 years or older provided the consent form directly.
This study summarizes the data obtained from a cohort of 131 white male football players from
Football School Lokomotiv and FC Lokomotiv Moscow Youth team aged 12 to 23 years (mean age
15.6
±
2.4 years, mean height 172.2
±
9.9, mean weight 62.1
±
10.9, mean body fat % 15.6
±
3.7) who did
not have any contraindications for sports. The study included young football players who trained at
Football School Lokomotiv or FC Lokomotiv Moscow Youth team and permanently resided in Moscow,
a city located at latitude 55◦north.
2.1. Criteria for Exclusion from the Study
The criteria for exclusion from the study were as follows:
- The athlete received vitamin D supplements 30 days or less prior to first blood sampling.
-
The athlete suffered from acute respiratory viral infections or any other condition that resulted in
absence from three or more training sessions 30 days or less prior to the examination.
-
The athlete could not maintain daily contact with the medical personnel distributing vitamin
D3supplements.
- The athlete spent more than three days outside Moscow during the last three months.
- The athlete was expelled from the academy during the study.
- The athlete refused to take part in speed and power testing.
2.2. Laboratory Testing
Two blood samples were obtained: the first in December 2018 and the second 70 days later.
Overall, treatment was administered over 60 days with two five-day breaks after the end of the first
and the second months.

Nutrients 2019,11, 1960 4 of 10
Fasting blood samples were collected from the cubital vein in the morning. Two immunoassay
blood tests for vitamin D
3
(25(OH)D) were conducted using an
in vitro
reagent set for 25(OH)D produced
by Euroimmun AG (Germany) and a Mindray MR-96A microplate reader (China). In accordance
with the modern guidelines, vitamin D plasma concentrations below 30 ng/mL were considered
insufficient (with 21–29 ng/mL and <21 ng/mL used as diagnostic criteria for vitamin D insufficiency
and deficiency, respectively). Values within the range of 30–60 ng/mL were considered normal and
values >60 ng/mL were considered as simply higher than normal indicators. Based on the results of
the 25(OH)D blood test, groups of athletes with insufficient (group 1) and higher than normal (group
2) vitamin D were formed.
Body height and body weight measurements were obtained from all participants. In the groups
with insufficient or excessive vitamin D, muscle and body fat mass measurements were obtained
using bioimpedance analysis on the day following the first and the second blood sampling procedures.
The ABC-02 “MEDASS” (Russia) analyzer was used for bioimpedance analysis. The procedure was
performed in the morning before and after the treatment, with the patient in the fasting state. Running
speed and power tests, i.e., 5, 15, and 30 m sprint tests and a standing long jump test, were performed in
both groups. All the football players had previously repeatedly performed these tests at least 3–4 times
within 2–3 years and were well acquainted with the rules for their conduct. After the warm-up, the long
jump test was performed, followed by the sprint tests with 5 min breaks between each sprint. For the
sprints, a timing system produced by Brower Timing Systems (USA) was used. Standing long jump
distance was measured using a PLR 15 Digital Laser Measure produced by Bosch (Malaysia).
2.3. Description of the Sprint Tests
Each athlete started the sprint from a stationary standing position, with the leading (the nearest
leg) foot located 20 cm before the starting line. Two pairs of timing gates had been set up: the first at
the starting line and the second on the finish line. The athlete began the sprint at will. The results were
immediately transferred from the timing gates to the chronometer and saved.
2.4. Description of the Standing Long Jump Test
Each athlete started the test with the legs shoulder-width apart, feet parallel, and arms by their
side. Mid-flight, the athlete pulled their legs close to their body and extended them forward heels first,
landing on both feet simultaneously. The length of the jump was measured along the perpendicular
line from the point of push-offto the athlete’s heel upon landing. None of the athletes or personnel
performing the tests knew which group the athlete was in.
2.5. Supplementation with Vitamin D
After the initial testing, athletes belonging to group 1 began receiving vitamin D correction therapy.
Vitamin D deficiency and insufficiency were treated with 5000 IU oral cholecalciferol (SiS vitamin
D
3
5000 IU, United Kingdom) daily after breakfast. Treatment lasted for 60 days, with a 5-day break
after the 30th day of treatment, and was supervised daily by the medical staffof the Football School.
After the end of treatment, a second series of tests (i.e., 5, 15, and 30 m sprint tests and the standing long
jump test) was carried out in group 1, and a second set of bioimpedance measurements was obtained.
2.6. Statistical Analysis
IBM SPSS Statistics software v.23.0 (IBM, New York, NY, USA) was used for statistical analysis.
Descriptive statistics and Kolmogorov–Smirnov test was used to determine the normality of the
distribution. Student’s t-test for independent samples was used to compare weight, height, body mass
index (BMI), body fat mass percentage, and 15 m sprint in groups 1 and 2. Mann–Whitney test for
independent samples was used to compare lean body mass, 5 and 30 m sprints, and standing long
jump in groups 1 and 2. Student’s t-test for dependent samples was used to compare weight, height,
BMI, body fat mass percentage, and 30 m sprint before and after treatment. Wilcoxon signed-rank

Nutrients 2019,11, 1960 5 of 10
test was used to compare 5 and 15 m sprints, standing long jump, and lean body mass before and
after treatment.
3. Results
Vitamin 25(OH)D levels were below reference values in 42.8% (56) of the examined football
players. Vitamin 25(OH)D deficiency and insufficiency was observed in 19.9% (26) and 22.9% (30) of
the players, respectively. In 57.2% (75) of the examined young football players, vitamin D levels were
normal (in the range of 30–60 ng/mL in 26.7% (35) of players) or high (in the range of 61–130 ng/mL in
30.5% (40) of players) (Figure 1).
Nutrients 2019, 11, x FOR PEER REVIEW 5 of 10
compare weight, height, BMI, body fat mass percentage, and 30 m sprint before and after treatment.
Wilcoxon signed-rank test was used to compare 5 and 15 m sprints, standing long jump, and lean
body mass before and after treatment.
3. Results
Vitamin 25(OH)D levels were below reference values in 42.8% (56) of the examined football
players. Vitamin 25(OH)D deficiency and insufficiency was observed in 19.9% (26) and 22.9% (30)
of the players, respectively. In 57.2% (75) of the examined young football players, vitamin D levels
were normal (in the range of 30–60 ng/mL in 26.7% (35) of players) or high (in the range of 61–130
ng/mL in 30.5% (40) of players) (Figure 1).
Based on the results of the vitamin D blood test, groups with insufficient (serum vitamin D <30
ng/mL, group 1) and higher than normal (serum vitamin D >60 ng/mL, group 2) vitamin D were
formed. During the study, the array of tests (i.e., sprint tests and standing long jump test) was
completed in full by 25 football players from each of the groups. Group 1 was composed of 25
individuals (mean age 13.96 ± 1.4 years) with the mean serum vitamin 25(OH)D level of 20.7 ng/mL.
Group 2 was composed of 25 individuals (mean age 14.8 ± 1.6 years) with the mean serum vitamin
25(OH)D level of 84.5 ng/mL. Age, body mass, height, BMI, body fat mass, and body muscle mass
were comparable in both groups (Table 1).
Deficiency
Insufficiency
Reference values
Excess
Figure 1. Serum level of vitamin 25-hydroxycalciferol (25(OH)D) in young professional football
players permanently residing in Moscow (percentage).
19.9%
22.9%
26.7%
30.5%
Figure 1.
Serum level of vitamin 25-hydroxycalciferol (25(OH)D) in young professional football players
permanently residing in Moscow (percentage).
Based on the results of the vitamin D blood test, groups with insufficient (serum vitamin
D<30 ng/mL, group 1) and higher than normal (serum vitamin D >60 ng/mL, group 2) vitamin
D were formed. During the study, the array of tests (i.e., sprint tests and standing long jump test)
was completed in full by 25 football players from each of the groups. Group 1 was composed of
25 individuals (mean age 13.96
±
1.4 years) with the mean serum vitamin 25(OH)D level of 20.7 ng/mL.
Group 2 was composed of 25 individuals (mean age 14.8
±
1.6 years) with the mean serum vitamin
25(OH)D level of 84.5 ng/mL. Age, body mass, height, BMI, body fat mass, and body muscle mass
were comparable in both groups (Table 1).
Speed and power tests were conducted in both groups. No statistically significant difference was
found in the results of any of the tests (Table 2). After the 60 day vitamin D supplementation therapy
course finished, mean vitamin 25(OH)D concentration in members of group 1 increased by 79.2% (from
20.7 to 31.7 ng/mL, p<0.001), and reference values were achieved in 84% (21) of them. A repeated
measurement of height, weight, body composition, and performance in speed and strength tests was
carried out in group 1.

Nutrients 2019,11, 1960 6 of 10
Table 1.
Comparison of age, body weight (kg), body height (cm), body mass index (BMI), body fat
mass percentage, and lean body mass percentage (mean value ±SD) in groups 1 and 2.
Testing
Groups
Mean Age,
Years
Body Height,
cm
Body Weight,
kg
BMI,
kg/m2
Body Fat Mass,
%
Lean Body Mass,
%
Group 1 13.96 ±1.4 171.9 ±9.92 60.6 ±9.72 20.4 ±1.45 15.5 ±4.14 56.7 ±7.39
Group 2 14.8 ±1.6 172.6 ±10.07 63.5 ±11.94 20.9 ±2.05 15.6 ±3.38 58.4 ±1.95
p-Value 0.054 0.827 0.359 0.266 0.973 0.567
Table 2.
Comparison of the results obtained by athletes from groups 1 and 2 in 5, 15, and 30 m sprint
tests and the standing long jump test (mean value
±
SD). No statistically significant difference between
the groups was observed.
Testing Groups 5 m Sprint,
Seconds
15 m Sprint,
Seconds
30 m Sprint,
Seconds
Standing Long Jump,
Meters
Group 1 1.04 ±0.07 2.49 ±0.15 4.45 ±0.28 2.34 ±0.17
Group 2 1.06 ±0.19 2.46 ±0.16 4.38 ±0.26 2.38 ±0.18
p-value 0.682 0.382 0.413 0.347
A statistically significant improvement in sprint results was observed (Table 3), and there were
statistically significant increases in height, weight, and BMI (Table 4).
Table 3.
Results of 5, 15, and 30 m sprint tests and the standing long jump test in group 1 pre- and
post-treatment (mean value ±SD).
Testing Period 5 m Sprint,
Seconds
15 m Sprint,
Seconds
30 m Sprint,
Seconds
Standing Long Jump,
Meters
Pre-treatment 1.04 ±0.07 2.49 ±0.15 4.45 ±0.28 2.34 ±0.17
Post-treatment 1.01 ±0.06 2.44 ±0.15 4.35 ±0.31 2.36 ±0.19
p-value 0.018 0.001 0.016 0.330
Table 4.
Body height (cm), body weight (kg), BMI, body fat mass percentage, and lean body mass
percentage in group 1 pre- and post-treatment (mean value ±SD).
Testing Period Body Height,
cm
Body Weight,
kg BMI, kg/m2Body Fat Mass,
kg
Lean Body
Mass, kg
Pre-treatment 171.9 ±9.92 60.6 ±9.72 20.4 ±1.45 15.5 ±4.14 56.7 ±7.39
Post-treatment 173.3 ±8.89 62.6 ±9.66 20.7 ±1.62 16.1 ±4.3 58.18 ±1.48
p-value <0.001 <0.001 0.008 0.247 0.203
Thus, increasing blood level of 25(OH)D in the group of players with an initially insufficient level
was associated with a statistically significant increase in performance in 5, 15, and 30 m sprint tests.
These changes might be associated with not only an increased level of serum concentration (OH) D but
also with a change in anthropometric indicators. However, we did not reveal a correlation between
changes in height and weight and sprint rates. We studied the correlation between changes in lean
body mass and changes in sprints. The correlation of lean body mass and the result of the 5 m sprint
was the only result that was significant, i.e., the greater the change in lean body mass, the longer
the time taken for running 5 m. Furthermore, in the 5 m run, there was a tendency for a correlation
between changes in weight and height. Other indicators did not significantly correlate with sprint
results (Table 5).

Nutrients 2019,11, 1960 7 of 10
Table 5.
Correlations between the change in sprint performance and change in vitamin D and
correlations between the change in sprint performance and change in anthropometrics before and
after treatment.
Running Speed Tests Statistics Vitamin D Height BMI
Sprint 5 m Pearson correlation
p-value 0.077
0.714
−0.333
0.104
−0.252
0.225
Sprint 15 m Pearson correlation
p-value
−0.043
0.84
−0.219
0.292
−0.119
0.571
Sprint 30 m Pearson correlation
p-value 0.125
0.553 0.101
0.63
−0.219
0.292
Weight Lean body mass
Sprint 5 m
Spearman correlation
p-value
−0.369
0.07 0.389
0.05
Sprint 15 m
Spearman correlation
p-value
−0.257
0.215
−0.17
0.933
Sprint 30 m
Spearman correlation
p-value
−0.05
0.812 0.213
0.297
4. Discussion
The aim of the present study was to examine the serum concentration of vitamin D and its effect
on running speed and muscle power in young male football players. The most important finding was
that vitamin D concentration was below normal in a substantial share (42.8%) of youth professional
football players residing in Moscow (latitude 55.9
◦
north). However, previously published studies on
vitamin D in football players of various ages and sex have reported a higher prevalence of vitamin D
insufficiency and deficiency, even in regions with sufficient insolation [
17
,
33
–
35
]. The relatively low
prevalence observed in Russian youth football players during the winter season may be explained by
the lower workload compared to the workloads endured in summer and autumn and by the constant
supervision by both coaches and doctors. Excessive exercise has been reported as a possible reason for
a decrease in serum vitamin D level [37].
In the study, not all athletes with a low vitamin D content had the same response to vitamin
D supplementation, which could be due to several factors. One of them is probably the uneven
sensitivity of individuals to vitamin D, possibly due to molecular factors [
38
]. However, even a
sufficient concentration of vitamin D may not indicate the expected effect on various body functions as
these effects may depend on the content of other biologically active substances in the body, such as
magnesium [39].
So far, the results of research examining the association between serum vitamin D concentration
and muscle strength and running speed in athletes have been contradictory. A number of authors have
found no evidence of any significant impact on various indicators of physical performance. Brännström
et al. measured parameters of both speed and power in a cohort of 19 young female football players
from Sweden, a region where insolation is limited. They found no significant correlation between
these parameters, including jump and sprint performance, and vitamin D levels [
35
]. Similar results
were obtained by Jastrz˛ebska et al., who studied the effect exerted on various performance indicators
by increasing serum vitamin D in a cohort of trained football players. The athletes assigned to the
treatment group were administered 5000 IU vitamin D daily. No significant difference in performance
improvement was observed between the control group and treatment group, even though the serum
vitamin D level increased by 119.2% in the treatment group [
34
]. Fitzgerald et al. analyzed the blood
test results and performance in the vertical jump test (power) and Wingate test (anaerobic power and
capacity) in 53 young hockey players residing and training at latitude 44.9
◦
north. They observed a
statistically significant positive correlation between serum vitamin D concentration and the performance

Nutrients 2019,11, 1960 8 of 10
of these athletes [
40
]. Koundourakis et al. worked with a cohort of 67 professional Greek football
players and reported that serum levels of vitamin D were directly associated with the performance in
jump tests and in both the 10 and 20 m sprint tests. Their results showed that vitamin D levels were
significantly associated with muscle power and speed as well as sprint performance and VO
2
max in
professional football players regardless of their performance level [33].
In our study, no statistically significant difference was observed between muscle power and
running speed in groups with insufficient and excessive vitamin D despite the large gap in vitamin D
concentrations between the two groups (20.7 and 84.5 ng/mL, respectively). After 60 days of treatment,
a significant increase in both serum vitamin D levels and 5, 15, and 30 m sprint performance was
achieved in the initially insufficient group. The improvement in performance might be associated with
the increase in vitamin D levels. It should be noted, however, that the participants became taller and
heavier during the study, and this anthropometric change may have also been the cause of the observed
improvement in performance. Previous research on the association between increased vitamin D
levels and both speed and strength performance did not consider a potential change in anthropometric
parameters of the participants.
There is no “gold standard” that can exactly estimate the concentration of vitamin D biochemical
markers in the human body. Most often, a serum concentration of 25(OH)D, which is the sum of
25(OH)D
3
and 25(OH)D
2
, is used for this purpose, although a number of other biochemical agents
have been proposed in recent years [
38
]. At the same time, isotope dilution liquid chromatography
mass spectrometry can be considered the most accurate method for determining the biochemical
markers of vitamin D; however, current immunoassays have demonstrated acceptable performance [
41
].
This hypothesis is supported by the fact that the tests performed initially did not find any significant
difference between the performance of athletes from the insufficient and excessive vitamin D groups.
5. Conclusions
These findings indicate that there is likely no correlation between serum levels of 25(OH)D,
muscle power, and running speed in young professional football players, and the changes observed
post-treatment may have been caused by the changes in anthropometric parameters. During the study,
all the anthropometric parameters changed, but the amount of lean body mass only correlated with the
results of 5 m sprint.
Author Contributions:
Conceptualization, E.B. and V.K.; methodology, E.B. and A.T.; software, A.L.; validation,
E.B., A.T., and V.K.; formal analysis, A.Z.; investigation, E.B. and E.A.; resources, E.B., Z.W., A.˙
Z., and D.G.;
writing—original draft preparation, Z.W., A. ˙
Z., D.G., E.B., and V.K.; writing—review and editing, V.K., Z.W., A. ˙
Z.,
D.G., P.T.N., T.R., and B.K.; visualization, A.˙
Z.; supervision, E.B., Z.W., A. ˙
Z., and D.G.; project administration, E.B.,
E.A., and A.T.
Funding: This research received no external funding.
Acknowledgments:
We express our gratitude and deep appreciation to the FC Lokomotiv, Sports Medicine Clinic
«Smart Recovery» and Sechenov First Moscow State Medical University for supporting our research.
Conflicts of Interest: The authors declare no conflict of interest.
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