Penultimate Explanations Mechanisms of Aging

How good are those reasons? The categorization of the reasons leads to the different mechanisms that are known to be involved in the aging process. There are several methods by which one can organize the different theories of aging. None of these systems is fully satisfactory, but the origins of the change and its level of action both appear to be reasonable and logical pegs from which to hang these descriptions. Here a dual classification scheme is employed in which one considers whether the theories suggest that their particular effects are exerted within all or most cells (intracellular theories) or whether they are exerted mostly on the structural components and/or regulatory mechanisms that link groups of different cells (intercellular theories). In addition, the following paragraphs will consider simultaneously whether the effects postulated by each theory are conjectured to take place accidentally (stochastic theories) or are the result of the hierarchical feedback cascades characteristic of the species (systemic theories). Table 1 lists fourteen major theories sorted out by this dual classification scheme, and Table 2 offers a very brief summary of each theory. The highlighted terms in both tables indicate those theories for which the empirical data support their playing a central and important role.

The experimental data also show that certain aging phenomena are observed in almost all species. For example, experimental organisms extend their life spans significantly if those organisms are maintained under a reduced food intake regime but under conditions that maintain good nutrition. This method, called caloric restriction (CR), has worked in almost all species tested. It also is generally accepted that longevity is inversely related to early adult fecundity or reproduction. Elevated resistance to oxidative stress is observed commonly in many longevity mutants.

Interestingly, all these phenomena appear to be interrelated. For example, experimental organisms maintained under CR conditions have higher levels of antioxidant defense system (ADS) activities and lower levels of fecundity compared with controls. In addition, the mild dwarfism noted in CR-raised animals also is observed in mutants screened for longevity. Finally, it has been demonstrated

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Staying Young

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