Collectively , these outcomes support a volumetric dilution of vitamin D. The slopes of the respective regression lines , however , indicate a smaller increase in 25OHD than would be expected from a direct mobilization of stores into the circulation .
Hence , sequestration of 25OHD and its conversion to inactive metabolites would also play a role .
The purpose of this review was to confirm a volumetric dilution of vitamin D in obesity .
It was based on the hypothesis that weight loss , particularly fat loss , would increase serum 25-hydroxyvitamin D ( 25OHD ) in the obese .
Rising serum 25-hydroxy-vitamin D levels after weight loss in obese women correlate with improvement in insulin resistance.
OBJECTIVE The objective of the study was to examine changes of 25-hydroxy-vitamin D ( 25OHD ) and PTH blood levels 4 and 20 wk after low-calorie diet-induced weight loss . METHODS Forty-four obese women [ aged 40.6 + /- 11.4 yr , body mass index ( BMI ) 36.7 + /- 4.9 kg/m(2 ) ] and 25 controls ( BMI 22.9 + /- 1.5 kg/m(2 ) ) were examined .  Anthropometric and cardiometabolic parameters and 25OHD and PTH levels  were determined at baseline and 4 and 20 wk after a low-calorie diet . RESULTS At baseline , 25OHD levels were lower in obese compared with control subjects ( 17 + /- 6.0 vs. 23.8 + /- 8.7 ng/ml , P < 0.001 ) , whereas no differences were found in PTH levels . In all women , a negative correlation was found between 25OHD levels and body weight ( BW ) ( r -0.32 , P < 0.001 ) , BMI ( r -0.37 , P < 0.001 ) , waist circumference ( r -0.26 , P < 0.05 ) , and percent fat mass ( r -0.38 , P = 0.001 ) as determined by bioelectrical impedance analysis . The 4-wk low-calorie diet ( n = 37 ) reduced BW and led to significant improvements in the homeostasis model assessment ( HOMA ) index and lipid levels . The 20-wk low-calorie diet ( n = 26 ) result ed in reduction of BW and BMI by 10 % , HOMA index ( 4.7 + /- 3.8 vs. 3.10 + /- 1.7 , P < 0.01 ) , and lipids levels ( except high density lipoprotein cholesterol ) and increase in 25OHD ( 15.4 + /- 6.0 vs. 18.3 + /- 5.1 ng/ml , P < 0.05 ) , compared with baseline . PTH levels were unchanged . The increase of 25OHD levels was associated with the reduction of insulin levels and HOMA index ( r -0.43 , P < 0.05 ) . CONCLUSIONS Blood 25OHD levels were low in obese women and correlated inversely with severity measures of obesity . Weight loss of 10 % after low-calorie diet increased 25OHD levels , and this increase was mainly associated with improvement of insulin resistance
Collectively , these outcomes support a volumetric dilution of vitamin D. The slopes of the respective regression lines , however , indicate a smaller increase in 25OHD than would be expected from a direct mobilization of stores into the circulation .
Hence , sequestration of 25OHD and its conversion to inactive metabolites would also play a role .
The purpose of this review was to confirm a volumetric dilution of vitamin D in obesity .
It was based on the hypothesis that weight loss , particularly fat loss , would increase serum 25-hydroxyvitamin D ( 25OHD ) in the obese .