Is brown fat really different from regular fat, and can I activate it?

Brown fat is fundamentally different from ordinary white fat. White fat stores energy as fat droplets. Brown fat burns energy directly as heat, via a protein (UCP1, sometimes called the 'uncoupling protein') that short-circuits energy production in the cell. The result: heat instead of stored calories. This mechanism is well supported by evidence.

The good news: you can activate brown fat yourself. Cold exposure is by far the most effectively proven method. In research conducted at 16 degrees Celsius, brown fat activity was measurable in 96% of participants, whereas at room temperature almost nothing happened. Repeated cold exposure can also increase the amount of brown fat in your body. Think cold showers or spending time in a cool room, although the ideal 'dose' has not been precisely established.

Adults do not have the classic brown fat found in babies, but primarily so-called beige fat cells: ordinary white fat cells that can temporarily take on brown fat properties. Cold, exercise and certain nutrients stimulate this switching process. Exercise also has an indirect effect: muscles produce a hormone during exertion that prompts white fat cells to behave in the same way. This has largely been demonstrated in mice, however, and is not clearly established in humans.

Capsaicin (the compound in chilli peppers) appears in the research literature as a possible brown fat stimulant, but the effect in humans is limited and there are no approved supplements or medications that deploy this safely. No medication is yet available that effectively activates brown fat, despite extensive laboratory research. Finally, people with excess weight demonstrably have less active brown fat, but whether that is a cause or a consequence of the excess weight remains unclear.