Good Fat is BAT!
◑ Beige, Brite, Brown or White: If you thought all fat was white and wobbly, think again. Some fat depots are colored brown because they are rich in mitochondria– powerhouses of energy loaded with iron-containing cytochrome proteins (bottom panel in image). Beige/Brite fat cells are in between white and brown fat. Human infants have stores of this brown adipose tissue (BAT), up to 5% of their body weight, between their shoulder blades and in their neck, colored green in the MRI scan (top image). Hibernating animals stock up on brown fat too. Why? Because brown fat generates heat.
◑ The Heat is On: Our mitochondria are factories that extract energy from food and store it in the form of a proton (Hydrogen ion) gradient across their borders. Like water cascading down a fall turns a turbine to generate electricity, this gradient of protons can run downhill through the ATP synthase, propelling it like a motor to capture energy from food by making a chemical compound called ATP. But if these protons ran downhill without doing any work, their potential energy would be lost as heat. The mitochondria in brown fat do just that. They make an uncoupling protein that short circuits the proton gradient to generate heat instead of ATP. This form of thermogenesis is important to newborns and hibernating animals who can’t use the shiver reflex to give off heat from their muscles.
◑ Better with BAT: Since brown fat burns calories, more BAT could counter obesity, cardiovascular disease and Type 2 diabetes. But adult humans lose most brown fat during adolescence. Fortunately, new studies show that we do have some, mostly deep inside our neck (lower image). One approach to increasing BAT is exposure to cold! Brrr … if that sounds uncomfortable, stem cells may boost your BAT. Curiously, brown fat cells share a common lineage, not with white fat cells, but with muscle cells. Recent research has revealed the presence of adult stem cells that can be coaxed into active BAT. The hope is to induce these cells to form calorie-burning brown fat in humans. Now that’s a healthier browning than a tan!
◑ Images and Refs: (1) Evidence for two types of brown adipose tissue in humans.
(2) How brown is brown fat? Depends where you look. Nedergaard and Cannon Nature Medicine 19, 540–541 (2013)