Metabolism: The role of brown fat

There are two main types of adipose tissue in the body: white and brown fat. Whereas white fat is found throughout the body, brown fat, which is involved in temperature regulation, is only present in a few specific locations. In rodents, like most mammals, brown fat is located in the subscapular region and in the areas surrounding the aorta; it is ubiquitous in both newborns and adults. In humans, brown fat was originally thought to be prevalent only in newborns, but newer studies indicate that brown fat exists in the neck and upper chest of adult humans. In fact, it has also been discovered that the amount of brown adipose tissue in adult humans varies with an individual’s body mass index (BMI).

To understand the function of brown fat, scientists took adult rats under anesthesia and inserted thermal probes in the midbrain, colon, and interscapular fat. The scientists then proceeded to measure the rats’ body temperature at these various locations while the rats were exposed to external temperatures of 31°C and 20°C. The scientists also recorded the amount of oxygen the adult rats consumed during the experiment. The results are detailed in Figure 1.

Figure 1 Change in inner body temperature of rat colon, brain, and subscapular fat under 11°C external temperature change; and, the amount of consumed oxygen as a function of time

Another team of scientists discovered that the brown color of brown fat was due to the fact that the brown adipose cells were filled with mitochondria. And upon further investigation, the brown adipose mitochondria were discovered to contain a unique protein called UCP, which stands for UnCoupling Protein. UCP is a mitochondrial inner membrane protein that facilitates the transport of hydrogen ions across the membrane. The rate of diffusion of you $H^{+}$‍ ions across UCP depends only on the relative concentration of $H^{+}$‍ ions.