There are two main types of adipose tissue in human physiology. These are designated white and brown adipose tissue (BAT). Discussion of their different physiological roles is important in any exploration of weight gain and fat loss. White adipose tissue is the most well known sort of fat. If anyone has cooked and eaten a joint of red meat they will have seen the white adipose tissue of the animal around the meat. The distinctive white colouration can be seen. The fat is humans is very similar to this and quantitatively white adipose tissue makes up most of the fat we carry on our frames. The white nature of this tissue relates to its low concentration of mitochondria, because it is metabolically not very active. The physiological function of white adipose tissue is mainly as a store of energy, but white fat does fulfil other roles such as acting as a shock absorbed to delicate structures such as internal organs and joints. When we gain weight, it is the white adipose tissue that becomes engorged with triglycerides.
Brown adipose tissue is much less common in humans. Babies possess a fair amount of brown adipose tissue, but as we age the amount we carry seems to diminish. The presence of brown adipose tissue in babies confirms the conclusions of scientific studies, that brown adipose tissue is a source of heat. The brown colour of the tissue is related to the presence of a high number of mitochondria in the cells of brown adipose tissue, and this makes the cells very metabolically active. Brown adipose tissue is able to perform a special metabolic trick called uncoupling, which it achieves with the help of special uncoupling proteins. In normal cells when glucose or triglycerides are oxidised, a high energy compound called ATP is produced and this adds to the energy content of the cell. Only the workings of the cell can remove this energy. However, uncoupling proteins uncouple the glucose and triglycerides from the production of ATP, and instead divert the resulting energy to produce heat.
This uncoupling of energy has two important consequences in the body. The first is that the cells can manufacture heat from stored energy and in this way can increase the temperature of the body. Small animals that have relatively small surface areas use this trick to maintain warmth, and as a result small mammals have high amounts of brown adipose tissue. Babies too have a small surface area and so require brown adipose tissue. The adult human has less need for this process because they have a larger surface area and also because they are able to move themselves from the cold to the warmth. Adaptive thermogenesis describes the ability of brown adipose tissue quantity to upregulate following chronic exposure to cold. Another consequence of the ability of brown adipose tissue to uncouple energy production is the fact that this allows the wasting of energy. Brown adipose tissue can be stimulated through the release of hormones such as adrenaline, to waste energy, and the consequence of this is a reduction in body fat.