There is a fat that burns calories instead of storing them. It was thought to belong to infants only; we now know that adults keep some, and that it switches on in the cold. A reference review published in 2026 sets out what we really know.
A fat packed with mitochondria
White fat stores energy. Brown fat spends it: its cells are full of mitochondria fitted with a special protein, UCP1, which short-circuits energy production to produce heat. This wealth of mitochondria, and their iron, is what gives it its colour.
In adults it sits in the neck, along the spine, above the collarbones. Its amount varies enormously from one person to another, and it decreases with age and excess weight.
What switches it on
Cold remains the most powerful trigger: moderate exposure, a cool room, a lukewarm rather than hot shower, is enough to start it up. Diet and certain hormones, including thyroid hormones, modulate it too. Researchers have developed increasingly precise imaging methods to measure its heat production live.
What it changes, and what we do not yet know
The author of the review, from the University of Sherbrooke, is cautious: the contribution of brown fat to total energy expenditure remains modest in adults. But people whose brown fat is active have, consistently, less abdominal fat and a lower cardiometabolic risk. The open question is whether activating brown fat protects, or whether protected people simply have more active brown fat. The answers will come from ongoing studies.
Key points
- Brown fat spends energy as heat thanks to its mitochondria.
- Moderate cold is its main activator.
- Its role in weight is modest, but its activity goes hand in hand with better metabolic health.
- Do not confuse moderate exposure with extreme cold: hormesis is about the dose.
Sources
- Blondin DP. Brown adipose tissue thermogenesis. Physiology, 2026. https://doi.org/10.1152/physiol.00002.2026
- Liu H et al. The role and mechanism of UPRmt in adipocytes. Adipocyte, 2026. https://doi.org/10.1080/21623945.2026.2651605
