We evaluated serum irisin and betatrophin responses to 9 h of passive heat exposure (40°C, 9% relative humidity) in 19 young adults (19–31 years) and 37 older adults (61–78 years), including those with hypertension (HTN) and type 2 diabetes (T2D). This evidence concerns the gene ANGPTL8 and type 2 diabetes mellitus.