Sports are crucial for adolescent development, fostering physical, mental, and social growth through discipline and teamwork. However, adolescent athletes face unique challenges, including rapid physical changes, musculoskeletal risks, and a strong competitive drive. Unhealthy behaviours in sports can have immediate and long-term health consequences, especially in high-level competitions. Thus, maintaining health is key to performance and injury prevention, making health assessments vital for evaluating the well-being of adolescent athletes (1).

Vitamin D is a crucial nutrient for health, fitness, and recovery in sports. While traditionally recognised for its role in maintaining calcium levels, it is now understood to function as a hormone influencing immune defense, protein synthesis, hormone synthesis, and other physiological processes (2).

Vitamin D is an important regulator of musculoskeletal strength, immune defense, and cardiovascular (CV) function. Beyond its traditional role in calcium homeostasis, it functions as a hormone that influences protein synthesis, inflammation, and neuroendocrine regulation (2). It inhibits renin biosynthesis (3) and downregulates proinflammatory T-helper (Th) cytokines, such as Th-1, including interleukins (IL) such as IL-6, IL-8, IL-12, interferon-γ, and tumour necrosis factor-α (TNF-α), while promoting anti-inflammatory Th-2 cytokines, such as IL-4, IL-5, and IL-10 (4). Vitamin D deficiency has been associated with metabolic syndrome, myocardial hypertrophy, and both systolic and diastolic dysfunction (5).

Epidemiological studies have confirmed an association between low serum 25-hydroxyvitamin D [25(OH)D] concentrations and CV diseases, including coronary artery disease, hypertension, and arrhythmias (6–8).

Vitamin D is primarily produced through sun exposure, with smaller amounts obtained from dietary sources such as oily fish, egg yolks, and dairy products. Its importance in sports nutrition is increasingly recognised, particularly because of the prevalence of vitamin D deficiency in various athletic activities, which is linked to reduced strength and endurance and a heightened risk of injury (9). The incidence of vitamin D deficiency is notably higher during infancy and adolescence than in later life stages, attributable to the rapid growth and development characteristics of these periods. Consequently, it is crucial to prevent vitamin D deficiency in athletes within these age groups because of its potential adverse effects on health (10). Although the association between vitamin D status and cardiac function has been investigated in adult athletes, there is a paucity of data concerning adolescent populations (5, 11, 12). This study aimed to assess longitudinal changes in vitamin D status among adolescent athletes and determine whether vitamin D deficiency correlates with alterations in electrocardiographic (ECG) parameters that may indicate early electrophysiological changes.

A cohort of 93 male adolescent athletes aged 10–14 years, with a mean age of 11.6 ± 1.15 years, underwent health assessments at baseline and after a one-year follow-up period. The study identified significant changes in various anthropometric, biochemical, and cardiovascular (CV) parameters.

This longitudinal assessment study examined the health status of adolescent athletes over a one-year follow-up period, focusing on changes identified through comprehensive physical examinations and diagnostic investigations. The findings highlight the significance of regular check-ups in maintaining the health of adolescent athletes and their crucial role in the early detection and prevention of potential health issues. The identification of minor alterations in overall health is particularly important, as these changes can impact both performance and future health of the athletes (15)..

Substantial increases in both weight and height suggest a normal growth trajectory, with the body mass index (BMI) remaining relatively stable despite these changes. The implementation of aerobic and resistance exercises likely contributed to the marked reduction in systolic blood pressure (SBP), indicating enhanced cardiovascular health and a positive training response (16, 17). Diastolic blood pressure (DBP), heart rate, and most laboratory indices showed no significant changes, suggesting stable health conditions (18). While variations in most electrocardiogram (ECG) variables were insignificant, a notable reduction in the T-wave axis was observed in the study. Although the 2° reduction in the T-wave axis reached statistical significance, it remained within the normal adolescent range (30–60°) and likely reflected benign physiological adaptation rather than pathology (19).

Our study showed a positive correlation between vitamin D levels and the QRS axis, as well as negative correlations between vitamin D levels and the PR interval and R voltage. In the same context, intense physical exercise, particularly in running sports, induces cardiovascular modifications that may mimic pathological conditions but actually reflect an organ that has successfully adapted to exercise. Human studies have suggested that vitamin D deficiency negatively affects the structure and function of the heart (11).

In our study, the observed significant reduction of 27% in vitamin D levels among adolescent athletes after one year is concerning, as it contradicts previous studies that reported that regular exercise increases vitamin D levels (20–22). This finding aligns with existing evidence indicating that vitamin D deficiency is prevalent in athletic populations (23–25). This deficiency, particularly noted in athletes, has been associated with increased health risks, as highlighted by de la Puente Yagüe et al., who described it as a global issue, especially problematic during winter when vitamin D synthesis is naturally reduced (12).

Vitamin D deficiency in athletes likely results from multiple factors, including limited ultraviolet-B exposure due to indoor training or practices during non-peak sunlight hours, inadequate dietary intake, and increased physiological demands that deplete the vitamin D stores. Diet, environmental factors, and lifestyle choices, such as avoiding sunlight, further influence vitamin D levels (26–28). For instance, 74.5% of adolescent “Taekwondo” athletes were found to have insufficient vitamin D levels (25). In another study, 70% of athletes were initially deficient but showed increased vitamin D levels after one year due to regular supplementation (29), an intervention not applied to the participants in our study.

Exercise affects vitamin D levels, as physical activity stimulates fat metabolism and releases vitamin D stored in adipose tissue (21, 22). However, the body's increased demand for vitamin D during high-intensity exercise can lead to greater utilization, as it is stored in skeletal muscles rather than remaining in circulation (30).

Vitamin D insufficiency has significant implications, as it can lead to an increased risk of stress fractures, impaired immune function, and delayed recovery, affecting athletic performance (12, 31). Furthermore, it disrupts the balance between calcium and parathyroid hormone (PTH), potentially affecting cardiovascular health (12). Our study demonstrated this through positive correlations between vitamin D and serum calcium and phosphorus levels, and negative correlations between vitamin D and PTH levels. This finding aligns with previous research, which highlighted that vitamin D enhances calcium and phosphate absorption, supports bone mineralisation, and stimulates osteocalcin production. PTH facilitates bone resorption via receptor activator of nuclear factor kappa-B ligand (RANKL) signalling. In addition to inhibiting PTH, vitamin D establishes a feedback loop that promotes bone development and maintains calcium-phosphate equilibrium (32).

Our findings indicate that ECG changes are correlated with vitamin D deficiency after a one-year follow-up period. Guzelcicek et al. (33) identified a correlation between reduced vitamin D levels and modifications in the index of cardioelectrophysiological balance, suggesting that insufficient vitamin D may predispose individuals to arrhythmic risks by influencing cellular repolarization (33). Additionally, Vanga et al. (34) explored the comprehensive role of vitamin D in cardiovascular health and proposed that vitamin D deficiency may exacerbate endothelial dysfunction, enhance inflammatory pathways, and impact cardiac remodelling, all of which could contribute to arrhythmogenesis and adverse cardiac outcomes (34). Previous studies, particularly those by Anees et al. and Canpolat et al., have predominantly focused on atrial arrhythmias linked to vitamin D insufficiency (35, 36). Canpolat et al. identified an elevation in P-wave dispersion among vitamin D-deficient individuals; however, no alterations were observed after vitamin D replacement therapy (35).

Numerous studies have investigated the relationship between vitamin D deficiency and ventricular arrhythmias, and have yielded significant findings. A study conducted by Tuliani et al. (37), which included 5,108 participants from the Third National Health and Nutrition Examination Survey (NHANES-III), identified a correlation between vitamin D insufficiency and notable electrocardiographic (ECG) abnormalities (37). Individuals with 25-OH vitamin D levels below 40 ng/mL exhibited major ECG abnormalities that were independently and significantly associated with an increased risk of long-term all-cause combined cardiovascular (CV) events and ischaemic heart disease (37). Vitamin D modulates calcium handling and cardiomyocyte excitability by regulating L-type Ca²⁺ channels, SERCA2a expression, and connexin-43 gap junction coupling. Vitamin D deficiency results in intracellular Ca²⁺ overload, prolonged repolarization, and heightened arrhythmogenic potential (8, 34). Mechanistically, deficiency may contribute to myocardial remodelling through overactivation of the renin–angiotensin–aldosterone system (RAAS), oxidative stress, and endothelial dysfunction. In athletes, insufficient vitamin D levels have been linked to diminished muscle strength, slower recovery rates, and increased injury rates (9). Given that adolescence is a period of rapid skeletal and cardiovascular development, vitamin D deficiency during this stage may have amplified effects (10).

Zhang et al. (21) conducted a comprehensive cross-sectional analysis utilising data from both the NHANES-III and the Atherosclerosis Risk in Communities (ARIC) cohorts, comprising 7,312 participants, and identified no association between serum 25-hydroxyvitamin D levels and QT interval duration, which is in contrast to our findings. Similarly, Tezcan and Tezcan (2025) reported no significant differences in demographic characteristics, laboratory parameters, or QRS-T angles between individuals with and without vitamin D deficiency.

The primary strength of this study lies in its prospective design, which facilitated monitoring of health changes in adolescent athletes over time. However, this study has several limitations. The sample consisted exclusively of adolescent male athletes, resulting in underrepresentation of females and adults. Additionally, the follow-up period was insufficient to evaluate the long-term effects of athletic training in adolescents. We did not possess quantitative data regarding the training schedule, sun exposure duration, and vitamin D levels in extravascular stores, which are acknowledged determinants of serum vitamin D (27, 28). Future research protocols should incorporate these covariates to enable multivariate adjustment. Confounding factors such as lifestyle and genetics were also uncontrolled. The subgroup analyses related to ECG changes were exploratory and constrained by the small cell counts. Thus, due to sparse data in several rhythm subcategories, Firth's penalized logistic regression was applied to mitigate small-sample bias.

These findings emphasise the need for targeted care in adolescent athletes, who may benefit from regular laboratory tests and adjustments due to the role of vitamin D in musculoskeletal and CV health. Declining vitamin D levels raise concerns regarding bone and immune health, highlighting the importance of tailored management and monitoring. Future research should include balanced gender samples and long-term studies and consider training, diet, and environmental factors. Evaluating interventions, such as vitamin D supplementation, with objective measures, such as advanced imaging, could further enhance our understanding of CV and musculoskeletal health in young athletes. Based on the observed 27% annual decline, we recommend annual vitamin D screening for adolescent athletes, particularly during winter or indoor training seasons. Preventive supplementation of 600–1000 IU daily as per the Endocrine Society Clinical Practice Guidelines (13) and biannual CV evaluation may optimise the musculoskeletal and cardiac health of these patients.

This study highlights the evolving health profiles of adolescent athletes and the need for customised health promotion strategies. Regular assessments are crucial, as alterations in weight, blood pressure, and declining vitamin D levels have been documented, indicating that early detection and intervention can address emerging health issues. Our findings revealed that vitamin D deficiency was associated with a higher prevalence of certain ECG changes, particularly T-wave inversion, although it was not proven to cause these changes. Proactive monitoring and supplementation of vitamin D, where necessary, is especially critical given its importance in bone health, immunity, and athletic performance. Future research should refine intervention methods and investigate optimal supplementation to better support the health and performance of young athletes.