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/m2] and 25 controls (BMI 22.9 ± 1.5 kg/m2) were examined. Anthropometric and cardiometabolic parameters and 25ΟΗD and PTH levels were determined at baseline and 4 and 20 wk after a low-calorie diet. Results: At baseline, 25ΟΗD 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 25ΟΗD 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) resulted 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 25ΟΗD (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 25ΟΗD levels were low in obese women and correlated inversely with severity measures of obesity. Weight loss of 10% after low-calorie diet increased 25ΟΗD levels, and this increase was mainly associated with improvement of insulin resistance.