This study aimed to clarify whether 1 year of vitamin D3 supplementation has a direct effect on body composition and physical fitness in healthy adults.Ninety-five participants randomly received either 420 IU vitamin D3 per day (n = 48) or placebo (n = 47) in a double-blind manner for 1 year. Lean body mass and percentage body fat were determined. Physical fitness including hand grip strength, leg extension power and cardiorespiratory fitness (CRF) were assessed. Serum 25-hydroxyvitamin D (25[OH]D) and 1,25-dihydroxyvitamin D (1,25[OH]2D) concentrations were assessed using ELISA kits.Serum 25(OH)D and (1,25[OH]2D) concentrations significantly increased by approximately 11.2 ± 9.2 ng/mL (pinteraction <0.001)and 7.0 ± 7.8 pg/mL (pinteraction <0.001) after 1 year of vitamin D3 supplementation respectively. Lean body mass significantly increased from 43.8 ± 9.6 to 44.3 ± 9.8 kg in vitamin D group, while no change was observed in placebo group (from 42.6 ± 8.9 to 42.4± 8.9 kg) after 1 year intervention. Furthermore, no treatment effects on other indicators of body composition and physical fitness were observed.One year of vitamin D supplementation effectively improves lean body mass, but not muscle strength and CRF in healthy adults.