There is increasing epidemiological evidence linking sub-optimal vitamin D status with overweight and obesity. Although increasing BMI and adiposity have also been negatively associated with the change in vitamin D status following supplementation, results have been equivocal. The aim of this randomised, placebo-controlled study was to investigate the associations between anthropometric measures of adiposity and the wintertime serum 25-hydroxycholecalciferol (25(OH)D) response to 15 μg cholecalciferol per d in healthy young and older Irish adults. A total of 110 young adults (20–40 years) and 102 older adults ( ≥ 64 years) completed the 22-week intervention with >85 % compliance. The change in 25(OH)D from baseline was calculated. Anthropometric measures of adiposity taken at baseline included height, weight and waist circumference (WC), along with skinfold thickness measurements to estimate fat mass (FM). FM was subsequently expressed as FM (kg), FM (%), FM index (FMI (FM kg/height m2)) and as a percentage ratio to fat-free mass (FFM). In older adults, vitamin D status was inversely associated with BMI (kg/m2), WC (cm), FM (kg and %), FMI (kg/m2) and FM:FFM (%) at baseline (r − 0·33, − 0·36, − 0·33, − 0·30, − 0·33 and − 0·27, respectively, all P values < 0·01). BMI in older adults was also negatively associated with the change in 25(OH)D following supplementation (β − 1·27, CI − 2·37, − 0·16, P = 0·026); however, no such associations were apparent in younger adults. Results suggest that adiposity may need to be taken into account when determining an adequate wintertime dietary vitamin D intake for healthy older adults residing at higher latitudes.