Brown adipose tissue is so-named because it appears brown or red and it has this distinctive color because brown adipocytes (brown fat cells) are not big clear balloons of fat like white adipocytes, but instead are a mixture of fat and lots of cellular "organelles"(little organs) called mitochondria that are in turn full of reddish iron. Furthermore, unlike white fat, brown fat is richly interwoven with blood vessels and nerves. All these differences reflect the vastly different role played by brown fat in the body. A microscopic photograph of brown fat is shown below and looks nothing like the large clear "bubbles" of white fat cells show here.
Unlike white adipose tissue which stores fats for the body to use as energy during starvation, brown fat stores fats in order to "burn" them up and produce heat when we are exposed to low temperatures. This is especially significant in newborns where up to four percent of an infant's body weight is brown fat (whereas adult humans only have several grams of the tissue). Because it produces heat directly from within its constituent cells, brown fat can actually feel hot and this heat can be imaged as a thermal "hot-spot" using an infra-red camera. Heat produced by brown fat allows babies to warm themselves without shivering.
Recent studies have shown in inverse correlation between the amount of brown fat in a person's body and that person's weight; the more brown fat, the lighter a person is, the less brown fat, the heavier. This suggests that brown fat may be one of the important factors controlling body weight. Brown Fat in Human Adults
Most adult brown fat is located around the shoulders and neck as shown in the photograph below. When totaled up, most adults have no more than one to three ounces of brown fat.
Brown fat has the ability to create astonishing amounts of heat very quickly and it does this through a protein called UCP-1 (uncoupling protein). This protein works in much the same way as a very old and long-banned weight loss drug called "dinitrophenol", by destroying the "proton gradient" (the power supply) of the mitochondrion. Normally the mitochondrion uses this gradient to make high energy molecules called ATP that can fuel cellular metabolism, but with UCP-1, they simply convert all the energy in fat into heat. It's a bit like the difference between using five gallons of gasoline to drive your car 150 miles on the one hand or simply lighting the gas on fire on the other hand. One way produces useful work (and a small amount of heat) and the other way produces no work and lots of heat.