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Abstract The downward slope during the near‐infrared spectroscopy (NIRS)‐vascular occlusion test (NIRS‐VOT) is purported as a simplified estimate of metabolism. Whether or not the NIRS‐VOT exhibits sex‐ or limb‐specificity or may be acutely altered remains to be elucidated. Thus, we investigated if there is limb‐ or sex specificity in tissue desaturation rates (DeO2) during a NIRS‐VOT, and if acute dietary capsaicin may alter this estimate of muscle metabolism. Young healthy men (n = 25, 21 ± 4 years) and women (n = 20, 20 ± 1 years) ingested either placebo or capsaicin, in a counterbalanced, single‐blind, crossover design after which a simplified NIRS‐VOT was conducted to determine the DeO2 (%/s), as an estimate of oxidative muscle metabolism, in both the forearm (flexors) and thigh (vastus lateralis). There was a significant limb effect with the quadriceps having a greater DeO2 than the forearm (−2.31 ± 1.34 vs. −1.78 ± 1.22%/s, p = 0.007, ηp2 = 0.19). There was a significant effect of sex on DeO2 (p = 0.005, ηp2 = 0.203) with men exhibiting a lesser DeO2 than women (−1.73 ± 1.03 vs. −2.36 ± 1.32%/s, respectively). This manifested in significant interactions of limb*capsaicin (p = 0.001, ηp2 = 0.26) as well as limb*capsaicin*sex on DeO2 (p = 0.013, ηp2 = 0.16) being observed. Capsaicin does not clearly alter O2‐dependent muscle metabolism, but there was apparent limb and sex specificity, interacting with capsaicin in this NIRS‐derived assessment.

Abstract Obesity, insulin resistance, and poor metabolic profile are hallmarks of a high‐fat diet (HFD), highlighting the need to understand underlying mechanisms. Therefore, we sought to determine the effect of succinic acid (SA) on metabolism in high‐fat diet (HFD)‐induced obesity. Animals were randomly assigned to either low‐fat diet (LFD) or a high‐fat diet (HFD). Mice consumed their respective diets for 4.5 months and then assigned to the following groups: (LFD)+vehicle, LFD + SA (0.75 mg/ml), HFD + vehicle, or HFD + SA. Body weight (BW), food, and water intake, were tracked weekly. After 6 weeks, insulin, glucose, and pyruvate tolerance tests were completed, and spontaneous physical activity was assessed. Epididymal white adipose tissue (EWAT) mass and in vitro measurements of oxidative skeletal muscle (soleus) respiration were obtained. Expectedly, the HFD increased BW and EWAT mass, and reduced glucose and insulin tolerance. SA significantly reduced EWAT mass, more so in HFD (p .05). A significant (p