Brown adipose tissue was for decades considered to be present only in infants, who require it to maintain body temperature before they can shiver effectively. In 2009, three papers published simultaneously in the New England Journal of Medicine demonstrated conclusively, using PET-CT scanning, that metabolically active brown adipose tissue is present in a significant proportion of healthy adults, primarily in the supraclavicular and paravertebral regions. Cold exposure activates this tissue, driving thermogenic fat oxidation through a process called non-shivering thermogenesis.
How Brown Fat Burns Energy
Brown adipocytes contain large numbers of mitochondria and express uncoupling protein 1 (UCP1), a protein that allows protons to flow back across the inner mitochondrial membrane without producing ATP, dissipating the energy as heat instead. This makes brown fat a metabolically expensive tissue. A small amount of active brown fat can increase basal metabolic rate measurably. When cold-activated brown fat is working at full capacity in adults with significant depots, it can burn hundreds of kilocalories per day in addition to basal metabolic requirements.
Regular Cold Exposure and Metabolic Adaptation
Regular cold exposure, through winter swimming or habitual cold showers, increases brown adipose tissue volume and UCP1 expression over weeks to months. It also promotes the browning of white adipose tissue, a process called beiging, in which white fat cells acquire some of the thermogenic characteristics of brown fat. The metabolic consequences include improved insulin sensitivity, lower fasting glucose, and reduced visceral fat in some intervention studies, though the magnitude of these effects in humans at practical cold exposure levels remains an active research area.
Sources
- Cypess, A.M., et al. (2009). Identification and importance of brown adipose tissue in adult humans. New England Journal of Medicine, 360(15), 1509-1517.
- Virtanen, K.A., et al. (2009). Functional brown adipose tissue in healthy adults. New England Journal of Medicine, 360(15), 1518-1525.
- Hanssen, M.J.W., et al. (2016). Short-term cold acclimation recruits brown adipose tissue in obese humans. Diabetes, 65(5), 1179-1189.