ABSTRACT
The TAG stored in WAT is therefore the body’s main long-term repository for storing energy that is in excess of requirements and the most important fuel store for survival during starvation. Because of the high energy content of TAG (9 kcal/g) and its hydrophobicity, the storage of energy as TAG is highly efcient: 1 kg of WAT contains only 100 g of water but 800 g of TAG and, thus, about 7000 kcal of energy. In theory, a typical body fat mass of 15 kg would provide enough energy for 50-60 days of total starvation, and this is in agreement with the survival limit of initially normalweight adults under famine conditions. Obese subjects, by virtue of their increased fat mass, can survive starvation for much longer, over 120 days in some cases. This highlights the crucial importance of fat storage as a survival advantage during most of human evolution, during which famine has been a powerful selection force. Until relatively recently, the ability to store excess energy safely as TAG in adipocytes must have conferred enormous survival benets; however, in an energy-replete environment the ability to promote TAG
storage encourages the spread of obesity. The processes of fat deposition and mobilization are regulated by integrated endocrine and neural mechanisms, which cooperate to keep fat mass relatively constant under habitual conditions. This is achieved in part by the adipocyte’s ability to signal the size of the body’s TAG stores to the brain, through leptin and other mechanisms (see Section 20.3).
