Theories of Aging

Aging is a complex process that is not yet fully understood, but that affects every tissue in the body and leads to deterioration of bodily function over time. Current theories of aging were developed from the idea of Weismann who proposed the theory of programmed death to explain why aging evolved as a price paid for the propagation of the species at the expense of the individual, and more importantly, suggested that there was a limitation to the number of somatic cell divisions [6]. This idea was later experimentally confirmed by Hayflick and Moorhead [87], and over the years, has been developed along with numerous theories of aging. These theories can be divided into two categories: programmed and damage theories of aging. Programmed theories of aging include: (1) programmed longevity—this is associated with increasing programmed genetic instability [7]; (2) endocrine theory—biological clocks regulate aging through the action of hormones [8]; (3) immunological theory—the immune system is programmed to decline as the organism ages, thereby increasing vulnerability to pathogens and gradually leading to death [9]. Damage theories include: (1) wear and tear theory—the idea that parts of the body gradually wear out after many years of use which results in aging [6] ; (2) rate of living theory—the greater an organism's basal rate of oxygen metabolism, the shorter is its life span [ 10] ; (3) cross-linking theory—the accumulation of cross-linking proteins damages cells and tissues, resulting in slowing down of bodily processes and aging [ 6 [ ; (4) DNA damage theory—DNA damage accumulation occurs over time, thus resulting in aging; (5) telomere theory—shortening of the telomere ends occurs following multiple cellular replications, resulting in loss of DNA until a critical level is reached, leading to cell death [11]; and (6) free radical theory or reactive oxygen species (ROS)—highly reactive free radicals cause damage to macromolecular components (such as nucleic acids, lipids, sugars, and proteins) of cells and accumulation of such damage leads to gradual dysfunction and aging [12]. We will discuss below three of the most prevalent theories of aging, i.e., the telomere theory, the immune theory, and the free radical/ROS theory.

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