The human body contains different types of fat. Most people are familiar with white adipose tissue (WAT), the body’s most abundant form. Its primary purpose is to store excess energy from food, insulate organs, and provide a reserve fuel source. Too much white fat is associated with obesity.
In contrast, brown adipose tissue (BAT), or brown fat, is specialized to burn energy rather than store it. Its cells are smaller than white fat cells and are composed of numerous small lipid droplets instead of a single large one.
The Function of Brown Tissue
The main function of brown adipose tissue is to generate heat, a process known as non-shivering thermogenesis. This allows the body to regulate its temperature when exposed to cold conditions. Unlike the heat produced by shivering, BAT generates warmth by burning calories from fat and glucose.
The heat-producing capability of brown fat comes from its high concentration of mitochondria. In brown fat, these cellular “powerhouses” are exceptionally dense. The high iron content within these mitochondria gives the tissue its characteristic brown color, a contrast to white fat.
Within these mitochondria is a protein called uncoupling protein 1 (UCP1) that alters energy production. Normally, mitochondria produce ATP, the cell’s main energy currency. UCP1 short-circuits this process, causing energy from burning fatty acids to be released as heat instead of ATP.
The tissue is also highly vascularized, meaning it has a rich supply of capillaries. This network of blood vessels distributes the produced heat throughout the body. It also delivers the oxygen and fuel that mitochondria require to sustain thermogenesis.
Location and Prevalence in Humans
The amount and location of brown adipose tissue change significantly throughout a person’s life. In newborns, brown fat is abundant, making up about 2% to 5% of their body weight. This is because infants are more susceptible to heat loss and cannot shiver effectively to stay warm.
In infants, brown fat is concentrated in specific areas to protect from hypothermia. The largest deposits are found on the back between the shoulder blades. Smaller deposits are also situated around the neck, shoulders, and upper spine.
As a person ages, the amount of this fat naturally decreases. The prominent deposits seen in infants shrink and scatter during childhood and adolescence. By adulthood, the quantity of brown fat is much lower, though it remains metabolically active and was confirmed in adults via advanced imaging scans.
In adults, brown fat is found in smaller, dispersed pockets. The most common locations are deep in the neck around the collarbone, along the spine, and near the kidneys. These deposits still play a role in energy expenditure and temperature regulation.
Activating Brown Tissue
While the body’s stores of brown fat diminish with age, the remaining tissue can be stimulated to become more active. One of the most studied methods is exposure to cold temperatures. When the body senses cold, the nervous system prompts brown fat cells to begin thermogenesis just before the body resorts to shivering.
Practical ways to induce this response include taking cold showers, an ice bath, or lowering the thermostat in your home. This mild cold stress encourages brown fat to burn energy for heat, increasing the tissue’s metabolic rate.
Exercise is another way to influence thermogenic activity. Physical activity can stimulate the release of a hormone called irisin from muscle cells. This hormone encourages “browning,” where certain white fat cells develop characteristics similar to brown fat cells. These are often called “beige” or “brite” adipocytes.
These beige cells, found interspersed within white fat depots, are distinct from the brown fat found in infants. While they may have lower baseline levels of UCP1, they can significantly increase its expression when stimulated by exercise or cold exposure. Some research also suggests that certain compounds in foods, like capsaicin in chili peppers, may help stimulate brown fat activity.