Brown Fat: The Calorie-Burning Tissue You Did Not Know You Had

Most people are familiar with white adipose tissue — the fat that stores energy and accumulates with weight gain. Fewer know about brown adipose tissue (BAT), a metabolically distinct type of fat that burns calories to generate heat through a process called thermogenesis. Once thought to be present only in infants, BAT was confirmed in adult humans in 2009 and has since become one of the most active areas of obesity research.

What makes brown fat different?

Brown adipose tissue gets its colour from a high density of mitochondria containing iron-rich cytochromes. These mitochondria express a protein called uncoupling protein 1 (UCP1), which allows them to generate heat rather than ATP when they oxidise fuel. This means brown fat can burn glucose and fatty acids without producing usable energy — effectively wasting calories as heat.

A third type of fat cell — beige adipocytes — can emerge within white fat depots in response to cold exposure and certain hormonal signals. Beige cells express UCP1 and behave similarly to brown fat cells, a process called browning of white adipose tissue.

How much does brown fat contribute to energy expenditure?

In fully activated adult BAT, thermogenesis can increase resting energy expenditure by 5 to 20 per cent. A 2021 study published in Nature Medicine analysed PET-CT scans from over 52,000 patients and found that individuals with detectable BAT had significantly lower rates of type 2 diabetes, dyslipidaemia, hypertension, and coronary artery disease — independent of BMI. Those with active BAT also had lower body fat percentages and better metabolic profiles.

What activates brown fat?

Cold exposure is the most potent activator of BAT. Even mild cold — such as spending time in a cool environment at around 17 degrees Celsius — significantly increases BAT activity. Regular cold exposure over several weeks increases both the amount and activity of BAT.

Exercise stimulates the release of irisin from muscle, which promotes browning of white adipose tissue. A 2012 study in Nature identified irisin as a key mediator of exercise-induced metabolic benefits.

Diet also influences BAT. Capsaicin from chilli peppers, catechins from green tea, and certain dietary fibres have been shown in human studies to modestly increase BAT activity and thermogenesis.

Is brown fat a treatment target?

BAT activation is an active area of pharmaceutical research. Several compounds that stimulate BAT thermogenesis are in clinical development. While no approved treatments currently target BAT directly, the research underscores the importance of physical activity and avoiding a sedentary, thermally comfortable lifestyle for metabolic health.

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