Various possible benefits have been attributed to the dietary isoflavone genistein. In common with resveratrol, which may act via sirtuin 1 (Sirt1), these benefits include protection from diet-induced obesity. We thus aimed to determine if genistein affected adipose tissue physiology and/or Sirt1 expression using a cell culture model.NIH3T3-L1 cells were differentiated into adipocytes (from 48 h post-confluence, taken as day 0) in the presence or absence of genistein. Gene expression was measured by RT-qPCR, and data expressed as mean ± SEM for n=3-6 relative to control. Mitochondrial oxygen consumption was measured using the Seahorse Bioscience XF system and expressed as mean ± SEM for n=10. Statistical analysis of data was by one-way ANOVA followed by Dunnett’s test for RT-qPCR data and by Student’s t-test for data on mitochondrial oxygen consumption. A low concentration of genistein (10 µM) and/or shorter exposure (days 0-12) promoted differentiation to white adipocytes, indicated by large fat droplets and increased expression of adipocyte marker genes Acaca (acetyl Co-A carboxylase-α) (1±0.092; 1.88±0.007; P<0.002), Fasn (fatty acid synthase) (1±0.057; 2.2019±0.135; P<0.0001), Fabp4 (fatty acid binding protein 4) (1±0.056; 2.56±0.37; P<0.007) Lipe (hormone sensitive lipase) (1±0.043; 2.43±0.212; P<0.0012), Retn (resistin) (1±0.074; 2.36±0.34; P<0.008), Rarres2 (chemerin) (1±0.022; 2.56±0.27; P<0.002)). However, at higher concentrations of genistein (50-100 µM) and/or after longer exposure (days 0-12) cells had smaller fat droplets and lower expression of these genes, coupled with increased expression of Sirt1 (1±0.037; 2.113±0.065; P<0.00004) and of genes characteristic of brown adipocytes (Ucp1 (uncoupling protein 1) (1±0.10; 1.63±0.17; P<0.009), Tnfrsf9 (tumor necrosis factor receptor superfamily member 9) or (CD-137) (1±0.06; 1.37±0.021; P<0.0021), and Cebpb (CCAAT/enhancer binding protein-β) (1±0.056; 2.78±0.11; P<0.0.00007)). In addition, basal and proportion of uncoupled mitochondrial oxygen consumption were higher in cells treated with genistein than in control cells (253±11 versus 305±7 pmol/min/mg; 0.54±0.04 versus 0.87±0.06, respectively; P<0.001) consistent with a switch in metabolic phenotype towards brown or beige adipocytes.Dietary genistein may thus protect against obesity by encouraging the development of brown or beige, rather than white, adipose tissue, possibly through a mechanism involving Sirt1.