INTRODUCTION
The importance of maintaining optimal vitamin D levels is now well established, both for the general population and for athletes of different performance levels.[1] This is largely due to its well-documented effects on protein synthesis, immune and cardiovascular function, its beneficial effects on bone health, and its role in modulating inflammatory responses.[2,3] For athletes, the key effects of vitamin D are primarily associated with its impact on the musculoskeletal system and muscle tissue, which may indirectly influence athletic performance.[4-6] Despite significant research interest in this topic, vitamin D deficiency remains highly prevalent among athletes and is likely comparable to that observed in the general population.[7]
Numerous studies have highlighted the widespread prevalence of vitamin D deficiency in athletes, with young athletes being particularly vulnerable in this regard.[8-10] Other risk factors for vitamin D deficiency in athletes include residing in regions above 40 degrees north latitude, insufficient outdoor exposure outside training periods and a diet low in vitamin D, and indoor training.[7,11-13]
Research findings consistently show a high prevalence of vitamin D deficiency in such cohorts of athletes,[9,10,14] this highlights the need for timely prevention of vitamin D deficiency and, when necessary, its rapid and safe correction. Therefore, studying the prevalence of vitamin D deficiency and its correction through dietary supplements containing cholecalciferol in young athletes from high-risk groups is of practical interest.
While vitamin D deficiency is well-studied in general populations and adult athletes, there is limited research specifically examining correction protocols in young athletes residing in high latitudes (>55° N). Most existing studies have focused on daily supplementation or single mega-doses, with few randomized controlled trials investigating weekly dosing protocols in adolescent athletes. Our study aims to fill this gap by evaluating the effectiveness and safety of a weekly 15,000 IU cholecalciferol supplementation protocol in young soccer players permanently residing above 55° N latitude.
The novelty of this work lies in the specific population studied (young soccer players training at northern latitudes, where the combination of geographical location and training intensity creates unique vitamin D requirements), the first comprehensive assessment of vitamin D deficiency prevalence in this specific demographic during winter months at these extreme northern latitudes, the evaluation of supplementation efficacy within this distinct environmental and physiological context, where both natural vitamin D synthesis and athletic performance considerations intersect. We believe these elements contribute meaningful new insights to the existing literature, particularly given the scarcity of research focusing on young athletes in these extreme geographical conditions.
METHODS
1. Subjects
The study involved 49 male soccer players from an elite soccer academy who lived permanently in a region above 55 degrees north latitude. Based on an initial (baseline) blood sample taken in February 2024, all participants were found to be vitamin D deficient, with 25-hydroxy-vitamin D (25[OH]D) levels below 20 ng/mL.[15] Initially, all players in the academy (N=209) underwent vitamin D testing. From this larger cohort, all athletes who were identified as vitamin D deficient (N=49) were included in the current study. To ensure homogeneity in age characteristics and minimize age-related factors that could influence the results, participants of the same age were randomized into two groups: an experimental group (N=25; mean age,13.0±2.78 years) and a control group (N=24; mean age, 12.3±3.14 years).
All participants followed identical study and training schedules, with a daily routine that included academic classes until 15:00, followed by evening training sessions of at least 1.5 hr.
2. Inclusion criteria
- Permanent residence in a region above 55 degrees north latitude;
- Diagnosed vitamin D deficiency (25[OH]D levels below 20 ng/mL) based on baseline blood sampling;
- Absence of injuries or conditions that could interfere with training or disrupt the training process;
- Informed consent from participants and their legal representatives to participate in the study.
3. Exclusion criteria
- Withdrawal from the study at any stage by participants or their legal representatives;
- Development of side effects related to the study procedures;
- Missing more than one weekly dose of vitamin D.
The randomization was performed using a stratified approach based on two key parameters: age and baseline vitamin D levels. This strategy was implemented to ensure comparable groups in terms of both age characteristics and initial 25(OH)D status, which are crucial factors that could influence the study outcomes. As a result, we achieved well-balanced groups with similar mean age (13.0±2.78 vs. 12.3±3.14 years; P=0.425) and baseline vitamin D levels (15.59±2.66 vs. 15.56±2.30 ng/mL; P=0.971).
The experimental group received 15,000 IU of cholecalciferol weekly for six weeks (total dose, 90,000 IU), while the control group received no supplements. However, one participant in the control group was excluded from the study after it was discovered that he had been taking vitamin D supplements independently. Therefore, the final analysis included 25 participants in the experimental group and 23 in the control group.
A second blood sample was taken for both groups one week after the experimental group had completed their cholecalciferol supplementation.
4. Supplements
Participants in the experimental group received 15,000 IU of cholecalciferol weekly for six weeks. Vitamin D was administered orally and distributed and taken on the same day of the week and at the same time for all participants. This dosing regimen was based on the Endocrine Society consensus guidelines recommendation of 2,000 IU daily, but was adapted to a weekly administration to enhance compliance among young athletes.[16] Each participant took three capsules of 5,000 IU vitamin D3 (cholecalciferol), supplied by NOW Foods (USA), with a sufficient amount of water.
5. Sunlight exposure
The participants followed a structured daily routine that significantly limited their sun exposure. They spent mornings commuting to school, followed by indoor academic classes until 15:00. After classes, they traveled to training facilities while wearing winter clothing that minimized skin exposure. Training sessions were conducted indoors for at least 1.5 hr, after which they commuted home, typically after sunset. This schedule, combined with the geographical location, resulted in minimal direct sunlight exposure during the study period.
6. Laboratory testing
(1) Baseline blood sampling
A single blood sample was taken from all participants during the routine pre-season medical examination in February 2024. Blood was drawn from the cubital vein between 8:00 and 10:00 am in the fasting state by an experienced specialist. All athletes were instructed to rest the day before blood sampling.
Total serum 25(OH)D was analyzed by liquid chromatography-mass spectrometry on an Agilent 1,200 liquid chromatograph (Agilent Technologies, Palo Alto, CA, USA) coupled to an AB Sciex 3200 MD mass detector (Sciex, Framingham, MA, USA). The 25(OH)D concentration was measured using the total 25(OH)D level, which is currently considered the most appropriate to reflect the body's vitamin D stores.[17,18]
The classification used to determine 25(OH)D status was as follows: deficiency was defined as a 25(OH)D level below 20 ng/mL, insufficiency as a level between 20 and 30 ng/mL, and normal as a level above 30 ng/mL.[15] This classification has been used repeatedly in studies involving athletes, with results published in high-impact journals.[19-21]
(2) Second blood sampling (control)
In May 2024, a single blood sample was taken from all participants. Blood sampling was performed in a fasting state between 8:00 and 10:00 AM from the cubital vein by an experienced specialist. All athletes were instructed to rest the day before blood sampling.
The analysis of total 25(OH)D levels in serum was conducted using the same equipment and laboratory as in February 2024 for the baseline sampling. To evaluate the impact of the vitamin D deficiency correction protocol on calcium and phosphorus metabolism, serum levels of calcium (mmol/L), ionized calcium (mmol/L), parathyroid hormone (PTH; pg/mL), and phosphorus (mg/L) were also measured. These parameters were analyzed using the same laboratory equipment.
(3) Monitoring for side effects
The academy's physician conducted daily medical examinations and interviews with each athlete during training sessions. These daily evaluations included systematic clinical examinations and specific questions about gastrointestinal function, overall well-being, and any changes in physical condition or health status. Additionally, parents were actively involved in monitoring their children's health status at home and were instructed to report any concerns immediately. To facilitate immediate communication, we established a dedicated WhatsApp group where parents and athletes could report concerns or ask questions about their well-being at any time, ensuring round-the-clock monitoring capability. The academy's physician maintained detailed records of all reported symptoms or concerns, regardless of their perceived connection to supplementation. The absence of adverse events was not merely a passive observation but the result of systematic daily monitoring through these established protocols.
(4) Statistical methods
Statistical analysis was conducted using Python libraries, including pandas for data manipulation, SciPy and statsmodels for statistical testing, seaborn and matplotlib for visualization, and openpyxl for Excel operations. The significance level was set at a P-value less than 0.05. Mean values and standard deviations were calculated for age, 25(OH)D levels (in February and May), calcium, PTH, and phosphorus. The Shapiro-Wilk test was used to assess the normality of data distributions. Independent t-tests were performed to compare biochemical parameters between the experimental and control groups for normally distributed data. The Kruskal-Wallis test was used for data that did not follow a normal distribution. Cohen's d was calculated to determine the effect size, and power analysis was conducted to assess statistical power. Pearson correlation coefficients were computed to explore relationships between 25(OH)D levels and other biochemical parameters. The study was conducted using available subjects who met the inclusion criteria, without a prior sample size calculation. A post hoc power analysis was performed after study completion to evaluate the statistical power of the conducted analyses.
(5) Ethical approval
The study was conducted in accordance with the Declaration of Helsinki. All participants and their legal representatives signed voluntary informed consent forms and were informed about the potential risks of participating in the study. The protocol of the study was approved by the [removed for the blind review].
RESULTS
Baseline mean 25(OH)D levels at the first blood sample were similar between the two groups 15.59±2.66 ng/mL in the experimental group and 15.56±2.30 ng/mL in the control group (P=0.971). After completion of the correction course, the mean 25(OH)D level in the experimental group increased to 30.25±5.17 ng/mL, while in the control group it reached 20.59±5.56 ng/mL. The change in mean 25(OH)D levels in the experimental group was statistically more significant (P<0.001) (Table 1).
The 25(OH)D status changed in 36 of the 48 study participants. In the experimental group, only two participants (8.00%) remained deficient, whereas in the control group 10 participants (43.47%) were still deficient (Table 2).
There were no significant effects of the correction course on calcium-phosphorus metabolism across the analyzed parameters. None of the variables showed statistically significant differences in either the experimental or control groups.
In the experimental group, the mean level of ionized calcium (Ca+) was 1.21±0.02 mmol/L, and total blood calcium (Ca) was 2.39±0.07 mmol/L, with no substantial differences compared to the control group (Ca+, P=0.684; Ca, P=0.556). The mean level of PTH in the experimental group was 39.44±16.90 pg/mL, and phosphorus (P) was 262.61±13.72 mg/L. In the control group, the corresponding values were 48.61±20.02 pg/mL for PTH and 274.17±18.01 mg/L for P, with P-values of 0.514 and 0.325, respectively (Table 3).
The effect size evaluation (Cohen's d) and power analysis for each parameter are presented in Table 4. These analyses demonstrated that cholecalciferol supplementation had a large and statistically robust effect on serum 25(OH) D levels, as indicated by Cohen's d values of 1.802 (post-intervention) and 1.694 (change from baseline to post-intervention), both with a power of 1.000. In contrast, the effect sizes for calcium, phosphorus, and PTH were small to negligible (Cohen’s d <0.5), with low power values, indicating no meaningful changes in these parameters.
DISCUSSION
The study demonstrated that administering 15,000 IU of cholecalciferol weekly for six weeks is an effective method for correcting vitamin D deficiency in young athletes. Numerous studies have been conducted to evaluate the effectiveness of various approaches to correcting vitamin D deficiency.[22-24] The most common methods of correction are oral dietary supplements with various forms of vitamin D,[22,23,25] but injectable forms are also used,[26,27] strategies related to food intake [28,29] and related to insolation patterns.[30,31]
According to current consensus data, the most common treatment options for severe vit-amin D deficiency in children aged 1 to 18 years include the use of cholecalciferol (D3) supple-ments at doses of 800 to 2,000 IU daily for 4 to 12 weeks,[32] 50,000 IU weekly for 12 weeks and 100,000 IU every two weeks for 12 weeks.[33] For example, the Endocrine Society rec-ommends that patients aged 1 to 18 years with vitamin D deficiency take 2,000 IU of vitamin D2 or D3 daily for at least six weeks, or 50,000 IU of vitamin D2 once a week for the same pe-riod.[16] A systematic review by Kearns et al. [34], summarizing guidelines from leading profes-sional societies, found that a single oral dose of 300,000 IU was the most effective method for correcting vitamin D deficiency in adults. The cholecalciferol was chosen because it has been demonstrated to be more effective than ergocalciferol (D2) in correcting vitamin D deficiency and insufficiency.[35] The correction protocol used in this study is therefore in line with existing consensus guidelines. Its efficacy can be described as high and comparable to the results of the studies by Bezrati et al. [36], Micah and Robert-McComb [37], involving young athletes (8-18 years). These studies employed the following correction protocols for vitamin D deficiency, re-spectively: a one-time oral dose of 200,000 IU and a daily oral dose of 4,000 IU for eight weeks.[36,37]
When correcting vitamin D deficiency (as with the treatment of any pathological condition), it is essential to ensure the safety of the intervention and minimize the risk of side effects. The most common side effects of vitamin D supplementation include hypercalcemia, hypercalciuria, dehydration, and disturbances in the digestive (nausea, vomiting, abdominal pain, constipation) and nervous systems (confusion).[38,39] It should be noted, however, that the development of toxic effects may develop only when high doses (e.g., 30,000-50,000 IU per day) are administered over several months [38,40] or unintentional one-time administration at doses ranging from 240,000 to 4,500,000 IU.[38,40,41] The incidence of such effects is minimal and has never led to life-threatening complications. According to a study by Lee et al. [40] involving 73,779 unique patients, the incidence of these conditions during vitamin D supplementation was 0.00005% - with four reported cases, including constipation, weight loss, abdominal pain, and nausea.
In current study, no pathological conditions, including those mentioned above, were reported by participants as being associated with supplementation. These findings are consistent with studies involving young athletes, which also demonstrated no side effects when correcting vitamin D deficiency using doses of 4,000 IU daily for eight weeks or a single dose of 200,000 IU.[36,37]
Vitamin D supplementation can also affect a range of hematological parameters of phosphorus-calcium metabolism, including free calcium, ionized calcium, phosphorus, and PTH.[38,39] Therefore, monitoring these parameters is crucial for assessing the safety of vitamin D correction protocols.
This study demonstrated that the correction protocol used did not lead to changes in calcium-phosphorus metabolism parameters that could be considered pathological. Levels of calcium, ionized calcium, phosphorus, and PTH in the experimental and control groups showed no statistically significant differences (P=0.556, 0.684, 0.325, and 0.514, respectively) and remained within reference ranges in all cases.
It is worth noting that the control group also showed a modest increase in 25(OH)D levels from February to May (15.56±2.30 to 20.59±5.56 ng/mL). This increase likely reflects the natural seasonal variation in vitamin D synthesis, as the study period coincided with increasing outdoor training time and sun exposure during spring months. However, despite this natural increase, only one participant in the control group reached sufficient vitamin D levels (>30 ng/mL), highlighting that natural synthesis alone may be insufficient for achieving optimal vitamin D status in young athletes at this latitude, even with regular outdoor training.
While short-term supplementation successfully elevates vitamin D levels, some studies have shown that maintaining optimal status requires ongoing attention. Close et al. [42] demonstrated that 6 to 12 weeks of supplementation effectively raised serum 25(OH)D concentrations above 50 nmol/L in athletes. However, research by He et al. [43] indicates that maintaining adequate vitamin D levels, particularly for optimal immune function, requires a more sustained approach. Their findings suggest that athletes should receive a daily maintenance dose of approximately 2,000 IU (1,000 IU from diet and 1,000 IU from supplements) throughout the year, especially during winter months when sunlight exposure is minimal.
1. Limitation
The study has several limitations. The first limitation is that measurements of calcium, ionized calcium, PTH and phosphorus levels were conducted only during the second blood sampling. This approach was chosen considering the specifics of working with adolescent athletes and the need to maintain an optimal balance between obtaining necessary data and minimizing invasive procedures in this age group. While post-intervention measurements demonstrated the safety of the protocol with all values within reference ranges, the absence of baseline data limits our complete understanding of how these markers changed throughout the study period.
The second limitation is the administration of a fixed cholecalciferol dose (15,000 IU weekly for six weeks). This limits the ability to perform a more detailed analysis of possible dose-dependent effects.
Another limitation of our study is the lack of long-term follow-up data after the 6-week supplementation period. While we demonstrated successful elevation of vitamin D levels during the intervention, the optimal maintenance protocol requires further consideration.
2. Future research direction
Future research directions could address several important aspects. First, conducting larger studies with baseline assessments of participants' blood levels of free and ionized calcium, PTH, and phosphorus before intervention would provide a more accurate understanding of baseline mineral metabolism and allow detailed tracking of changes in response to cholecalciferol supplementation.
Second, studies using different doses of cholecalciferol are recommended to investigate possible dose-dependent effects. This could involve establishing several parallel groups with different supplementation protocols to identify the most effective and safe dosages for correcting vitamin D deficiency.
In addition, future research should consider the feasibility of long-term monitoring of calcium, phosphorus, and PTH levels to assess not only short-term but also long-term effects of the intervention.
Finally, the inclusion of a wider range of biomarkers of mineral metabolism, such as magnesium or alkaline phosphatase, could provide deeper insights into the physiological changes associated with vitamin D supplementation.
In conclusion, a six-week course of 15,000 IU cholecalciferol administered weekly effectively and safely corrects 25(OH)D deficiency in young soccer players residing permanently in regions above 55 degrees north latitude, with minimal impact from spring outdoor training.
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