▸ There was an inverse and independent relationship between serum 25OHD and body fat mass at baseline. ▸ There was no treatment effect of vitamin D or calcitriol on fat mass in randomized trials. ▸ The response to vitamin D treatment varies according to body weight and those with normal or low BMI should use smaller doses of vitamin D. Obese people are known to have lower serum 25OHD levels compared to non-obese people. It is not known whether it is due to storage of vitamin D in fat, inadequate input from sunlight, diet or other unknown factors. We examined the relationship at study baseline of serum 25OHD, PTH, 1,25(OH)2D with body composition measurements using dual energy X-ray absorptiometry. The results showed a significant inverse relation between total body fat mass and serum 25OHD (p<0.0001) and serum 1,25(OH2)D (p=034) and an independent positive correlation between serum PTH and total body fat mass (p<0.0001). In a randomized controlled study of seven doses of vitamin D (400–4800IU/d) the increase in serum 25OHD levels was compared in women with a normal body mass index to obese women. The response to the low doses of vitamin D (400–800IU/d) was significantly less than that of the medium (1600–2400IU/d) and high doses groups (3200–4800IU) (p<0.0001) in all BMI categories. The increase in serum 25OHD in the medium and high dose groups was not significantly different with increasing level of obesity. But thinner women with a normal BMI (<25kg/m2) showed a much higher response to vitamin D at any dose level compared to other BMI groups. There was no significant change in total body fat mass after treatment with vitamin D or calcitriol in our randomized trials. In summary, the response to vitamin D is dependent on body weight. Women with BMI <25kg/m2 develop much higher levels of serum 25OHD after vitamin D supplementation compared to those with BMI of >25kg/m2. The differences in serum 25OHD levels between normal and obese women may be due to differences in volume dilution. After vitamin D supplementation, all obese women reach adequate levels of serum 25OHD but normal women (BMI<25kg/m2) reach much higher levels of 25OHD and in this group smaller doses of vitamin D used should be used. This article is part of a Special Issue entitled ‘Vitamin D Workshop’.