Thermogenesis

Thermogenesis refers to the metabolic production of heat within an organism. This process is found in all warm-blooded animals, as well as in certain thermogenic plants, including the Eastern skunk cabbage, the Voodoo lily, and giant water lilies from the genus *Victoria*. The lodgepole pine dwarf mistletoe, *Arceuthobium americanum*, uses thermogenesis to explosively disperse its seeds. For homeothermic animals, thermoregulation is a key part of their resting metabolic rate, helping to keep body temperature stable within a narrow range regardless of whether the surrounding environment is hot or cold. The energy needed for thermogenesis comes from cellular respiration, where nutrients like glucose or fatty acids are oxidized to produce ATP.

Thermogenic processes are categorized by whether they involve voluntary muscle movement. Obligatory thermogenesis is the heat generated by essential metabolic functions that keep an organism alive at rest, including cellular work (such as active transport, cell division, and DNA replication) and organ work (like heart contractions, liver detoxification, and kidney filtration). Exercise activity thermogenesis (EAT) is heat from structured physical activity. Non-exercise activity thermogenesis (NEAT) covers energy used for spontaneous, non-exercise movements, such as walking, fidgeting, leisure activities, and maintaining posture. Diet-induced thermogenesis (DIT) is the energy spent processing food.

Shivering is one method animals use to raise their body temperature. When an animal shivers, nearly all the energy expended is released as heat, and while it doesn’t produce useful motion, it effectively warms the body. For instance, hibernating mammals like some bats and ground squirrels rely on shivering to raise their temperature when emerging from hibernation.

Non-shivering thermogenesis occurs in brown adipose tissue, or brown fat, which is present in almost all eutherians (pigs are the only known exception). This tissue contains a unique uncoupling protein called thermogenin (or UCP1). Its atomic structure, solved using cryogenic-electron microscopy, shows a typical fold of the SLC25 family. UCP1 is locked in a cytoplasmic-open state by guanosine triphosphate in a pH-dependent manner, which prevents proton leak. Normally, mitochondria use protons to drive ATP synthesis, but thermogenin uncouples this process, allowing mitochondria to burn fatty acids and oxygen to generate heat instead. High levels of free fatty acids within cells stimulate this uncoupling by promoting proton leak. Hormones like thyroid hormone and norepinephrine boost beta oxidation, activating non-shivering thermogenesis during cold exposure. In this process, free fatty acids from triacylglycerols remove purine inhibition (from ADP, GDP, and others) of thermogenin, causing protons to flow into the mitochondrial matrix and bypass ATP synthase. This uncouples oxidative phosphorylation, and the energy from the proton motive force is released as heat rather than stored as ATP. Heat can also be produced by leakage from the sodium-potassium pump and the calcium pump. Futile cycles—such as simultaneous lipogenesis and lipolysis, or glycolysis and gluconeogenesis—also contribute to thermogenesis, and these cycles can be influenced by activity and rest patterns, like the Summermatter cycle. Acetylcholine stimulates muscle to raise metabolic rate. Because thermogenesis has low energy demands, free fatty acids primarily rely on lipolysis for energy production. A comprehensive list of human and mouse genes that regulate cold-induced thermogenesis (CIT) in living animals or tissue samples is available in the CITGeneDB database.

The ability to perform thermogenesis in animals did not evolve from a single common ancestor. Instead, birds (avians) and placental mammals (eutherians) developed this capacity independently through separate evolutionary paths, making it an example of convergent evolution. Although both groups can generate heat, the underlying biological processes differ. Why both lineages evolved thermogenesis is still under study, with two main competing explanations. The “aerobic capacity” model suggests that natural selection favored individuals with higher resting metabolic rates, and as birds and eutherians increased their metabolic capacity, they gained the ability for endothermic thermogenesis. Researchers have linked high oxygen consumption with high resting metabolic rates, implying a direct correlation. This theory proposes that thermogenesis is not a direct adaptation for stable body temperature, but rather a by-product of natural selection for greater aerobic and metabolic capacity.