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Purpose: Resting metabolic rate (RMR) accounts for two-thirds of the total energy expenditure in sedentary individuals. After accounting for traditional factors, there still remains a considerable unexplained variance in RMR. There is a pandemic of obesity and metabolic syndrome (MetS) which coexists with a high prevalence of vitamin D insufficiency. The aim of this study was to evaluate the potential effects of vitamin D status, insulin sensitivity (IS) and the metabolic syndrome (MetS) on RMR in Australian adults., Methods: RMR, respiratory quotient (RQ), McAuley's insulin sensitivity index, fat mass (FM), fat-free mass (FFM) and vitamin D status were assessed in Australian adults. The presence of MetS was evaluated by current standard criteria. Predictors of RMR were examined through multiple linear regression based on stepwise and backward regression approaches with attention to multi-collinearity. All analyses were conducted on SPSS version 21., Results: One hundred and twenty-seven participants (45 men, 82 women), aged 53.4 ± 11.7 years and BMI 31.9 ± 5.2 kg/m(2), were included. Forty-one subjects were insufficient in vitamin D status (<50 nmol/L), and 75 participants had the MetS. A parsimonious regression model explained 85.8 % of RMR and was given by: RMR (kJ/d) = 1931 + 83.5 × FFM (kg) + 29.5 × FM (kg) + 5.65 × 25(OH)D (nmol/L) - 17.6 × age (years) - 57.51 × IS., Conclusion: Vitamin D status and IS are novel independent predictors of RMR in adults. Future studies could validate a causal role for these factors in human energy metabolism.

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/m(2) 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.