Low circulating levels of total 25-hydroxyvitamin D (25OHD) have been reported in obese people of different ethnicities in several countries. Low total 25OHD in obesity could be due to lower binding proteins (with normal free 25OHD), lower dietary intake or sunlight exposure, greater volume of distribution (pool size) or more rapid metabolic clearance.
The aims of this study were to determine if free 25OHD and 1,25-dihydroxyvitamin D (1,25(OH)2D) are also low in obese people, and the cause of low vitamin D in obesity.
We assessed 216 normal weight (BMI 18.525), overweight (BMI 2530) and obese (BMI>30) adults ages 2540 and 5575 in autumn/spring, with vitamin D measurements and questionnaires for sunlight exposure and dietary vitamin D. Metabolic clearance of 25OHD was assessed by half-life of an orally administered stable isotope in 112 participants in winter. In autumn/spring, total 25OHD (immunoassay and LC-MS/MS), free 25OHD (measured and calculated) and total 1,25(OH)2D (all adjusted for date of sample collection, age group and gender) were lower in obese people (ANOVA P<0.002), and negatively correlated with BMI (R2 total 25OHD 0.248; free 25OHD 0.296; 1,25(OH)2D 0.055, all P<0.05). The difference in 25OHD between normal weight and obese groups was greater in autumn/spring than in winter. Dietary vitamin D, sunlight exposure and 25OHD half-life did not differ by BMI group. We conclude that total and free 25OHD and 1,25(OH)2D are lower in obesity, and this is likely to be due to greater volume of distribution for vitamin D.
|Total 25OHD, nmol/l||Free 25OHD, pmol/l||Total 1,25(OH)2D, pmol/l|
|n=75||(46.7, 61.4)||(9.43, 11.98)||(87.1, 103.7)|
|n=61||(35.4, 46.6)||(6.54, 8.59)||(72.3, 87.1)|
|n=80||(34.0, 43.7)||(6.91, 8.76)||(72.3, 85.3)|
|Measurements by immunoassay in autumn/springGeometric mean (95% CI), ANOVA group differences all P<0.002|
17 May 2014 - 20 May 2014