Limb, sex, but not acute dietary capsaicin, modulate the near‐infrared spectroscopy‐vascular occlusion test estimate of muscle metabolism

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 (DeO 2 ) 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 DeO 2 (%/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 DeO 2 than the forearm (−2.31 ± 1.34 vs. −1.78 ± 1.22%/s, p = 0.007, η p 2 = 0.19). There was a significant effect of sex on DeO 2 ( p = 0.005, η p 2 = 0.203) with men exhibiting a lesser DeO 2 than women (−1.73 ± 1.03 vs. −2.36 ± 1.32%/s, respectively). This manifested in significant interactions of limb*capsaicin ( p = 0.001, η p 2 = 0.26) as well as limb*capsaicin*sex on DeO 2 ( p = 0.013, η p 2 = 0.16) being observed. Capsaicin does not clearly alter O 2 ‐dependent muscle metabolism, but there was apparent limb and sex specificity, interacting with capsaicin in this NIRS‐derived assessment.