The human body has two primary forms of fat: white and brown adipose tissue. White fat (WAT) stores excess energy from the food we consume. In contrast, brown fat, also known as brown adipose tissue (BAT), is a distinct type of fat that is activated by cold temperatures. Its function is to generate heat to help maintain the body’s core temperature, a process called thermogenesis, while white fat acts as an energy reserve, brown fat actively expends energy.
The Function of Brown Adipose Tissue
The function of brown fat is linked to its cellular structure. Brown fat cells are densely packed with mitochondria, which are the energy-producing centers of a cell. These mitochondria are rich in iron, which gives the tissue its characteristic brown color. Its heat-generating ability comes from a specific protein found in these mitochondria called uncoupling protein 1, or UCP1.
Normally, mitochondria convert energy from food into a molecule called ATP, which powers cellular activities. However, UCP1 creates a shortcut in this process. It uncouples the breakdown of fatty acids and glucose from ATP production, causing the energy to be released directly as heat instead. This mechanism allows the body to warm itself without muscle movement, a process referred to as non-shivering thermogenesis.
Location and Prevalence in the Body
The amount and location of brown fat in the human body change significantly throughout a person’s life. Infants are born with relatively large stores of brown fat, making up about 2% to 5% of their total body weight. These deposits are primarily found on the back, around the neck, and between the shoulders. This is because newborns have a larger surface area relative to their body volume and are unable to shiver effectively to stay warm, making brown fat’s heat production a necessary survival mechanism.
As a person ages, the amount of brown fat decreases. In adults, it is no longer found in large, centralized deposits but rather in smaller pockets scattered throughout the body. The most common locations for these residual brown fat stores are in the neck, around the collarbones, along the spinal cord, and near the kidneys and adrenal glands. Positron emission tomography (PET) scans have been instrumental in identifying these metabolically active areas in adults.
While all adults retain some brown fat, the exact quantity varies between individuals. Studies have shown that leaner individuals tend to have more active brown fat compared to those who are overweight. Although the total volume is much less than in infancy, the retained brown fat in adults remains functional and can be stimulated to produce heat.
Methods for Activating Brown Fat
The most well-documented method for activating brown fat is exposure to cold temperatures. When the body senses a drop in temperature, just before the point of shivering, it signals the sympathetic nervous system to release hormones like norepinephrine. This chemical messenger binds to receptors on brown fat cells, triggering the UCP1-mediated process of thermogenesis to generate heat and warm the body. Practical applications include taking cold showers, immersing in ice baths, or lowering the thermostat, as research suggests daily exposure to 66°F (19°C) for two hours can stimulate BAT activity.
Regular physical exercise is another recognized method for stimulating brown fat. Research indicates that exercise promotes the “browning” of white fat, where some white fat cells begin to develop the characteristics and functions of brown fat cells. This process is partly mediated by a protein called irisin, which is released from muscle during physical activity and signals white fat to behave more like brown fat. High-intensity interval training (HIIT) may be particularly effective at increasing irisin levels.
While research is still emerging, certain dietary components may also influence brown fat activity. Compounds like capsaicin, found in chili peppers, and catechins, present in green tea, have shown some potential in preliminary studies to stimulate brown fat. These substances appear to trigger similar pathways to cold exposure, encouraging the tissue to burn calories for heat. However, cold exposure remains the most established and effective method for robustly activating brown fat.