thermogenesis

Thermogenesis is the body's way of making heat, much like a furnace converting fuel into warmth rather than work. Some heat is an unavoidable by-product of normal metabolism, but the body can also deliberately turn up the heat — to stay warm, to respond to a cold day, or even after a large meal.

Physiologists distinguish a few types. Obligatory thermogenesis is the baseline heat that comes from simply running cellular metabolism. Adaptive (facultative) thermogenesis is heat generated on demand, including shivering by muscle and, importantly, non-shivering thermogenesis in brown adipose tissue. There is also diet-induced thermogenesis, the energy spent digesting and processing food. Hormones set the dial: thyroid hormones raise the overall metabolic rate, while catecholamines (adrenaline and noradrenaline) acutely activate brown fat.

Thermogenesis matters for body weight because it is a way to dispose of excess energy as heat rather than store it as fat. This has made it an attractive but tricky drug target — older agents that forced thermogenesis were dangerous, so current research focuses on safely activating brown and beige fat.