The effect of vitamin D supplementation on serum 25OHD in thin and obese women

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 (400-4800 IU/d) the increase in serum 25OH D levels was compared in women with a normal body mass index to obese women. The response to the low doses of vitamin D (400-800 IU/d) was significantly less than that of the medium (1600-2400 IU/d) and high doses groups (3200-4800 IU) (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 (2) 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 2 develop much higher levels of serum 25OHD after vitamin D supplementation compared to those with BMI of >25 kg/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 <25 kg/m ²) 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'.