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J Steroid Biochem Mol Biol. 2013 Jul;136:195-200. doi: 10.1016/j.jsbmb.2012.12.003. Epub 2012 Dec 11.

The effect of vitamin D supplementation on serum 25(OH)D in thin and obese women.

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Bone Metabolism Unit, Creighton University Medical Center, Omaha, NE, United States.


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/m(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 <25kg/m(2) develop much higher levels of serum 25OHD after vitamin D supplementation compared to those with BMI of >25kg/m(2). 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/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'.

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