2.2. Study Design
The study duration was 10 weeks. This study was conducted during the winter months, between January and April, in Germany. The UV index was continuously below 3 for the entire 10 weeks. The daily UV index was provided by the Federal Office for Radiation Protection (BfS) and was measured using an array spectroradiometer.The first blood sample of each athlete was obtained in a one-week period before the start of this study. Afterward, the participants were randomized using matched-pair randomization based on baseline 25(OH)D concentrations. Athletes with established vitamin D inadequacy (25(OH)D3 level < 30 ng/mL) were randomly assigned to one of three groups: individualized supplementation (INDV), standardized supplementation (STAD), and a control group (CON). Athletes with a sufficient vitamin D status (25(OH)D3 level ≥ 30 ng/mL) (SUF) did not receive any supplementation but were also monitored until the end of this study.Vitamin D3 supplements were obtained from RheinNutrition (R(h)ein Nutrition UG & Co. KG, Cologne, Germany). All capsules provided were from the same batch and comprised cellulose capsules containing 1000 IU of cholecalciferol. The manufacturer reported a maximum allowable variation in vitamin D content of 5%.The individualized supplementation regimen was based on the formula described by Groningen et al. [The resulting total dose was divided into daily doses of 4000 IU, corresponding to four capsules daily. After the calculated time provided to reach the goal status of 40 ng/mL (t