Now, he says, researchers can focus on whether we can harness brown fat — perhaps by spurring its activity with drugs — to help overweight patients burn off unsightly and unhealthy white fat.

The pan must not be crowded. Enough air space must be left between each piece of food or it will steam rather than brown, and its juices will escape and burn in the pan.

But people living and working in consistently warmer indoor temperatures, the researchers argue, have less brown fat and therefore burn less energy.