Non-shivering thermogenesis is the process by which the body generates extra heat without shivering. Shivering is a way for the body to use muscular contractions to generate heat, but non-shivering thermogenesis uses a completely different mechanism to accomplish the same goal: a specialized fat-burning tissue called brown fat. Brown fat is brown rather than white because it's packed with mitochondria, the power plants of the cell. Under cold conditions, these mitochondria are activated, using a specialized molecular mechanism called uncoupling* to generate heat.
The mechanism of brown fat activation has been worked out fairly well in rodents, which rely heavily on non-shivering thermogenesis due to their small body size. Specialized areas of the hypothalamus in the brain sense body temperature (through sensors in the brain and body), body energy status (by measuring leptin and satiety signals), stress level, and probably other factors, and integrate this information to set brown fat activity. The hypothalamus does this by acting through the sympathetic nervous system, which heavily innervates brown fat. As an aside, this process works basically the same in humans, as far as we currently know. Those who claim that rodent models are irrelevant to humans are completely full of hot air**, as the high degree of conservation of the hypothalamus over 75 million years of evolution demonstrates.
Two new studies concurrently published in the Journal of Clinical Investigation last week demonstrate what I've suspected for a long time: brown fat can be 'trained' by cold exposure to be more active, and its activation by cold can reduce body fatness.
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