Adipose Tissue Resting Energy Expenditure and Expression of Genes Involved in Mitochondrial Function Are Higher in Women than in Men

CONTEXT: Men and women differ in body fat distribution and adipose tissue metabolism as well as in obesity comorbidities and their response to obesity treatment. OBJECTIVE: The objective of the study was a search for sex differences in adipose tissue function. DESIGN AND SETTING: This was an exploratory study performed at a university hospital. PARTICIPANTS AND MAIN OUTCOME MEASURES: Resting metabolic rate (RMR), body composition, and sc adipose tissue genome-wide expression were measured in the SOS Sib Pair study (n = 732). RESULTS: The relative contribution of fat mass to RMR and the metabolic rate per kilogram adipose tissue was higher in women than in men (P value for sex by fat mass interaction = .0019). Women had increased expression of genes involved in mitochondrial function, here referred to as a mitochondrial gene signature. Analysis of liver, muscle, and blood showed that the pronounced mitochondrial gene signature in women was specific for adipose tissue. Brown adipocytes are dense in mitochondria, and the expression of the brown adipocyte marker uncoupling protein 1 was 5-fold higher in women compared with men in the SOS Sib Pair Study (P = 7.43 × 10(−7)), and this was confirmed in a cross-sectional, population-based study (n = 83, 6-fold higher in women, P = .00256). CONCLUSIONS: The increased expression of the brown adipocyte marker uncoupling protein 1 in women indicates that the higher relative contribution of the fat mass to RMR in women is in part explained by an increased number of brown adipocytes.