Moreover, the impact of ageing needs to be considered as this is accompanied with a “natural” decline in brown fat mass, which could underpin the onset of type 2 diabetes.39 Critically, more sophisticated assessments (including the potential use of glucose tracers) to accurately assess glucose uptake by brown adipose tissue and of UCP1, both in vivo and in vitro, are required to enable a more accurate partitioning of its thermogenic and non-thermogenic functions. This evidence concerns the gene UCP1 and type 2 diabetes mellitus.