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Young male jockeys compromise bone health by engaging in caloric restriction and high volumes of physical activity during periods of musculoskeletal growth and development. The aim of this randomised, double-blinded, placebo-controlled trial was to establish whether calcium and vitamin D supplementation would improve bone properties of young male jockeys. We conducted a 6-month trial with two groups of weight-, height- and age-matched apprentice male jockeys (age = 20.2 ± 3.2 yrs). Participants were supplemented with 800 mg of calcium and 400 IU of vitamin D (S, n = 8) or a placebo (cellulose) (P, n = 9) daily for 6-months. Baseline calcium intake was (669.7 ± 274.3 (S) vs 790.4 ± 423.9 (P) and vitamin D 64.6 ± 19.5 (S) vs 81.2 ± 24.4 (P) with no statistical differences. Peripheral quantitative computed tomography (pQCT) measured ultra-distal (4%) and proximal (66%) tibial bone properties at baseline and 6 months. Blood-borne markers of bone turnover, P1NP and CTX and vitamin D concentration were assessed. After co-varying for height, weight and baseline bone measurements, the supplemented group displayed greater post-intervention bone properties at the 66% proximal site with cortical content (mg mm) 6.6% greater (p < 0.001), cortical area (mm2) 5.9% larger (p < 0.001), cortical density (mg cm2) 1.3% greater (p = 0.001), and total area (mm2) 4% larger (p = 0.003). No other between group differences in bone variables were observed. Blood analysis indicated higher vitamin D levels (18.1%, p = 0.014) and lower CTx (ng/L) (− 24.8%, p = 0.011) in the supplemented group with no differences observed in P1NP. This is the first randomised controlled trial to examine the efficacy of calcium and vitamin D supplementation in improving bone properties in a highly vulnerable, young athletic, weight-restricted population. Results using pQCT indicate beneficial effects of supplementation on bone properties in as little as six months. Although the study size is small, this intervention appears promising as a strategy for improving bone health in young athletes in weight-restricted sports.

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