Fat and aging: a tale of two tissues.

The thrifty aged hypothesis (TAH) holds that aging is a "saving program", a set of strategies all of them focused to reduce energy consumption in the post-reproductive age. Aged adults remove fewer energy resources from the environment, and the remaining fuel can be profitably used by offspring. Thus, from an evolutionary point of view, the variable submitted to natural selection would be the amount of energy removed by post-reproductive individuals from the environment, favoring "reduced energy extraction" (REE) strategies. Senescence is characterized by a set of metabolic changes, and one of them is a deep modification in adipose tissue. A pattern of progressive loss of subcutaneous (SF) and gain of visceral fat (VF) throughout life has been demonstrated. The evolutionary reasons for this change have not been explored. No one has questioned why energy is deposited in VF. It appears that VF is not the best place for storing fat. The objective of this theoretical paper is to analyze the relationship between aging and adipose tissue. Furthermore, an evolutionary explanation is proposed. Fat redistribution could be thought of as a consequence of the TAH. While SF responds to the equilibrium between energy needs and fuel supply, VF acts in a dysfunctional manner. VF is not coordinated with pathways that regulate the state of fat depots. Hence, the purpose of the changes observed upon aging is to limit the effectiveness of those pathways that force the organism to obtain energy from the environment. In summary, adipose tissue modifications promote REE strategies.