Annual Vitamin D Status of World-Class British Swimmers Following a Standardised Supplementation Protocol for Three Years

<b>Background/objectives:</b> British swimmers are at a heightened risk of vitamin D deficiency (serum 25-hydroxyvitamin D (25(OH)D): <50 nmol∙L^-1) as their large indoor training volumes often restrict sunlight exposure, especially during the winter when daylight hours are reduced in the United Kingdom. Previous research has recommended supplementation with 4000 IU∙day^-1 vitamin D₃ from October to March to offset vitamin D losses. However, no current study has analysed this approach over multiple seasons to assess if this is an appropriate strategy. <b>Methods:</b> Using a quasi-experimental design, twenty-nine world-class British swimmers (aged 16-30 years) provided a 10 mL venous blood sample (fasted) as part of their routine haematological screening in the September of three consecutive years (2018, 2019, and 2020). Serum 25(OH)D was determined by radioimmunoassay, and this result determined the length of the standardised vitamin D3 protocol (<30 nmol∙L^-1: 4000 IU∙day^-1 from September to March; 30-79 nmol∙L^-1: 4000 IU∙day^-1 from October to March; >75 nmol∙L^-1: no supplementation). <b>Results:</b> Mean serum 25(OH)D concentrations increased each year (2018: 76.4 ± 28.4 nmol∙L^-1, 2019: 91.5 ± 24.8 nmol∙L^-1, 2020: 115.0 ± 36.6 nmol∙L^-1, <i>p</i> < 0.001), which coincided with the eradication of vitamin D deficiency after one season (prevalence, 2018: 10%, 2019: 0%, 2020: 0%). In September 2020, 35% of swimmers had a serum 25(OH)D > 125 nmol∙L^-1, although it is currently debated whether this is a concern or a benefit for athletic populations. <b>Conclusions:</b> Supplementing with 4000 IU∙day^-1 of vitamin D₃ throughout the winter can increase the vitamin D status of swimmers. However, more frequent testing may be required to ensure that serum 25(OH)D remains within the sufficient range across the season (75-125 nmol∙L^-1).