Direct Measurement of Adipose Thermogenesis by Isothermal Microcalorimetry

Direct measurement of thermogenic heat production remains a major challenge in adipose biology. Isothermal microcalorimetry offers a label-free approach to quantify metabolic heat output, yet key parameters governing its application to adipose tissue remain poorly defined. Here, we systematically evaluate the use of the CalScreener isothermal microcalorimetry platform to quantify thermogenic heat production across multiple adipose models, including adipocyte spheroids, freshly isolated adipocytes, and intact adipose tissue explants. Heat production scaled with spheroid size within a defined range and increased linearly with spheroid number per well, demonstrating the quantitative sensitivity of the calorimetric measurements. Pharmacological modulation of mitochondrial respiration in cultured primary beige adipocytes demonstrated that oxidative phosphorylation is a major driver of the calorimetric heat signal, including heat generation associated with mitochondrial proton leak. Freshly isolated adipocytes and intact adipose tissue exhibited depot-specific thermogenic activity and retained responsiveness to β-adrenergic stimulation ex vivo. Across adipose depots, intact tissue explants revealed unexpected differences in thermogenic heat production that were not fully reflected by thermogenic gene expression, highlighting divergence between molecular and functional readouts. Intact adipose tissue maintained measurable thermogenic heat production following extended ex vivo handling in nutrient-containing medium, such that tissues collected across a prolonged harvest window exhibited comparable calorimetric activity, enabling batch analysis of large experimental cohorts. Microcalorimetry further resolved regional differences in thermogenic heat production within the inguinal adipose depot following cold exposure. Together, these findings define key experimental considerations for applying isothermal microcalorimetry to adipose biology and demonstrate its utility for directly quantifying thermogenic metabolism in cells and intact tissues.