This was the first study to have monitored the annual change in serum 25(OH)D concentrations in world-class British swimmers. Moreover, this study investigated how this cohort responded to an individualised 4000 IU∙day− 1 vitamin D3 protocol. Within one year of supplementation, vitamin D deficiency (< 50 nmol∙L− 1) was completely eradicated from within the group. Moreover, mean serum 25(OH)D increased on yearly basis until almost all swimmers (93%) displayed a sufficient vitamin D status (> 75 nmol∙L− 1) by the 2020 timepoint. However, a limitation was that serum 25(OH)D was measured in the September of each year, therefore whether this protocol offset deficiency over the winter months remains unclear. In the final year of observations, 35% of the cohort had ‘high, but reportedly safe’ (126–250 nmol∙L− 1) 25(OH) concentrations, which suggests that that toxicity may occur when utilising this strategy for over two years. More frequent haematological screening is therefore encouraged in order to identify appropriate supplementation approaches for each individual.
In September 2018, world-class British swimmers had a group mean 25(OH)D of 76.4 ± 28.4 nmol∙L− 1, suggesting that the majority of this cohort had sufficient vitamin D concentrations [3, 6]. However, based on previous studies, an autumn 25(OH)D concentration above 122.5 nmol∙L− 1 may be necessary to sustain serum 25(OH)D > 75 nmol∙L− 1 throughout the winter months . Indeed, a multi-cultural cohort of footballers based in Great Britain failed to maintain their ‘sufficient’ status (August: 104.4 ± 21.1 nmol∙L− 1) even after 18 weeks of daily outdoor training (December: 51.0 ± 19.0 nmol∙L− 1 ). This is more concerning for swimmers when analysing individual data, which identified that September 25(OH)D levels were below 75 nmol∙L− 1 in two thirds of the current cohort. Given that this measurement was taken immediately following Britain’s peak sunlight hours (May to August), this would suggest that British swimmers could be at a high risk of deficiency during the winter without vitamin D3 supplementation. Without a winter measurement, the current study cannot directly confirm how 4000 IU∙day− 1 affected the serum 25(OH)D concentrations of world-class swimmers. Nonetheless, based on previous research, NCAA Division I swimmers maintained mean concentrations of 100 nmol∙L− 1 from October to March when using the same supplemental dose . Based on this evidence, it is purported that vitamin D status was maintained at a higher concentration in the winter before being increased by summer sunlight exposure. This would partly explain why incremental increases in serum 25(OH)D that were observed each year. Future studies are therefore required with increased sampling frequencies (i.e., September, December, April) to assess whether supplementation protocols ensure sufficient vitamin D concentrations across the entire season.
The increases in group mean 25(OH)D that occurred each year resulted in almost all participants (93%) achieving a ‘sufficient’ vitamin D status for the beginning of the 2020/21 season. Though this outcome is seemingly beneficial, a controversial finding was that ‘high’ 25(OH)D concentrations became more commonplace (> 125 nmol∙L− 1; 2018: 10%, 2020: 35%). Based on the Endocrine Society guidelines, there are no risks of toxicity (i.e., hypercalcemia) when 25(OH)D is < 375 nmol∙L− 1 . To ensure that this concentration is not exceeded, a safety threshold of < 250 nmol∙L− 1 is therefore recommended , which was not surpassed by any swimmer over research timeframe (peak recorded 25(OH)D: 193 nmol∙L− 1). Conversely, the UK National Health Service (NHS) use more conservative guidelines akin to the American Dietetic Association (ADA), whereby serum 25(OH)D concentrations ≥ 150 nmol∙L− 1 are associated with possible adverse effects . These alternative guidelines could place 24% of current swimmers at risk of symptoms, such as nausea, dehydration, and lethargy [26, 27]; though, this is uncertain since more intense dosing protocols (i.e., 10,000 IU∙day− 1 for 20 weeks) have previously been well tolerated in healthy adults [28, 29]. One explanation is that excessive vitamin D exposure increases the rate of vitamin D catabolism, subsequently creating a negative feedback loop that tightly regulates 25(OH)D < 250 nmol∙L− 1 [3, 30]. However, this study cannot support this notion since no side-effects of vitamin D toxicity were measured. Nonetheless, with no additional benefits to exceeding the ‘sufficient’ threshold , the current approach halted supplementation when serum 25(OH)D exceeded 125 nmol∙L− 1. It is therefore speculated that serum 25(OH)D levels would decline in ‘at risk’ swimmers, though this requires a follow up investigation.
At present, there are no serum 25(OH)D guidelines that are considered to be optimal for world-class athletes. Due to the various metabolic roles of vitamin D, however, it has been recommended that athletes should maintain a 25(OH)D ≥ 100 nmol∙L− 1 to ensure that adequate amounts are available to support musculoskeletal health . Moreover, sustaining serum 25(OH)D > 120 nmol∙L− 1 is associated with less frequent and severe upper respiratory tract infections (URTI) in endurance athletes during the winter . Unpublished data from within this group found that URTIs accounted for 50% off missed training sessions in the 2016–2017 swimming season, with 56% of the world-class cohort experiencing at least one URTI instance. Therefore, fewer URTI cases would increase the consistency of training intensity, subsequently enabling greater training-induced adaptations over time . Conversely, no direct ergogenic effects are expected with vitamin D3 supplementation unless it involves the correction of neuromuscular defects caused by deficiency [2, 3, 33]. This has recently been contested, however, since 5000 IU∙day− 1 vitamin D3 improved deadlift (vitamin D: +13.8%, placebo: +2.5%) and vertical jump (vitamin D: +13.5%, placebo: +2.1%) performance in collegiate swimmers . Interestingly, both groups started (vitamin D: 112.1 ± 8.2 nmol∙L− 1, placebo: 117.0 ± 15.4 nmol∙L− 1) and ended the 12-week supplement protocol with a sufficient vitamin D status (vitamin D: 131.3 ± 28.6 nmol∙L− 1, placebo: 81.3 ± 17.0 nmol∙L− 1), although the group that supplemented with vitamin D3 displayed a better maintenance of free testosterone levels and sex-hormone binding globulin over the research timeframe. Though unclear whether this would directly improve swimming performance, this collective evidence warrants further research to determine whether a 25(OH)D > 120 nmol∙L− 1 could support training adaptations whilst also maintaining health status.
Serum 25(OH)D is an appropriate marker of vitamin D status based on its long half-life and close association with vitamin D exposure (e.g., sunlight, food, supplementation) . Whilst this measure has been discussed with relation to possible effects on illness and performance, a limitation is that other vitamin D metabolites (i.e., free 25(OH)D) have greater responsibilities within bone health [34–36]. Swimmers are consistently reported to have lower bone mineral density (BMD) compared to other athletes due to the non-weight bearing nature of their sport [37–39], therefore monitoring free 25(OH)D alongside serum 25(OH)D may be an important addition to the health screening process. Groups of ethnically diverse swimmers also warrant the measurement of multiple vitamin D metabolites since differences exist in skin pigmentation (i.e., ultraviolet-B exposure) and genetic polymorphisms (i.e., vitamin D binding protein, vitamin D receptor phenotypes) [40–42]. Consequently, Black swimmers could be considered at risk of deficiency and prescribed higher doses of vitamin D3 despite having similar free 25(OH)D and higher BMD compared to White swimmers . Due to these limitations, world-class swimming programmes are encouraged to screen for multiple vitamin D markers to accurately determine the ‘true’ vitamin D status of their athletes. However, with no current guidelines for interpreting optimal free or serum 25(OH)D in athletes, swimmers should continue to strive towards maintaining concentrations > 75 nmol∙L− 1 until more research is available.