Vitamin D and parathyroid hormone in insulin resistance of abdominal obesity: cause or effect?

The objective was to examine whether there were causal links between vitamin D status, parathyroid hormone, insulin resistance (IR)/insulin sensitivity (IS) and the metabolic syndrome (MS). A total of 72 Caucasian men and women, aged 55.7±7.57 years, with body mass index 33.4±4.02 kg/m2 and abdominal obesity, were assessed for IR/IS based on three commonly used indices before and after 12 weeks of supervised weight loss. During weight stability, though both lower intact parathyroid hormone (iPTH) and higher vitamin D were independently associated with greater IS/lower IR, this was consistent for iPTH across the surrogate measures tested. Higher iPTH, but not lower vitamin D, increased the risk of MS after adjustment for IR/IS. Weight loss resulted in significant reductions in percent fat (−2.83±2.20%), waist (−9.26±5.11 cm), improvements in all IS indices, reductions in MS and iPTH (−0.28±1.17 pmol/l), but no increase in vitamin D (+2.19±12.17 nmol/l). Following weight loss, ΔiPTH either predicted change in IR/IS or contributed to their variance by 4.1–8.9%. On adjustment for IR/IS, higher ΔiPTH did not significantly predict MS after weight loss, though the odds ratios for the effect were sizeable. The data are suggestive of an intrinsic inverse relationship between iPTH and IS in abdominally obese individuals, independent of vitamin D. There remains the possibility of a direct relationship between iPTH and MS.