Theoretical reliability models of system failure in aging considered in this book chapter lead to the following conclusions:
1. Redundancy is a key notion for understanding aging and the systemic nature of aging in particular. Systems, which are redundant in numbers of irreplaceable elements, do deteriorate over time (fail more often with age), even if they are built of nonaging elements. The positive effect of systems' redundancy is damage tolerance, which decreases mortality and increases lifespan. However damage tolerance makes it possible for damage to be tolerated and accumulated over time, thus producing the aging phenomenon.
2. An apparent aging rate or expression of aging (measured as age differences in failure rates, including death rates) is higher for systems with higher redundancy levels (all other things being equal). This is an important issue, because it helps to put a correct perspective on fascinating observations of negligible senescence (no apparent aging) observed in the wild and at extreme old ages. Reliability theory explains that some cases of negligible senescence may have a trivial mechanism (lack of redundancies in the system being exposed to challenging environment) and, therefore, will not help to uncover ''the secrets of negligible senescence.'' The studies of negligible senescence make sense, however, when the death rates are also demonstrated to be negligible.
3. Reliability theory also persuades a re-evaluation of the old belief that aging is somehow related to limited economic or evolutionary investments in systems longevity. The theory provides a completely opposite perspective on this issue—aging is a direct consequence of investments into systems reliability and durability through enhanced redundancy. This is a significant statement, because it helps to understand why the expression of aging (differences in failure rates between the younger and the older age groups) may be actually more profound in more complicated redundant systems, designed for higher durability.
4. During the life course the organisms are exhausting the reserve numbers of their cells (Gosden, 1985; Herndon et al., 2002), losing reserve capacity (Bortz, 2002; Sehl and Yates, 2001), and this redundancy depletion explains the observed ''compensation law of mortality'' (mortality convergence at older ages) as well as the observed late-life mortality deceleration, leveling-off, and mortality plateaus.
5. Living organisms seem to be formed with a high load of initial damage, and therefore their lifespan and aging patterns may be sensitive to early-life conditions that determine this initial damage load during early development. The idea of early-life programming of aging and longevity may have important practical implications for developing early-life interventions promoting health and longevity.
The theory also suggests that aging research should not be limited to the studies of qualitative changes (like age changes in gene expression), because changes in quantity (numbers of cells and other functional elements) could be an important driving force of aging process. In other words, aging may be largely driven by a process of redundancy loss.
The reliability theory predicts that a system may deteriorate with age even if it is built from nonaging elements with constant failure rate. The key issue here is the system's redundancy for irreplaceable elements, which is responsible for the aging phenomenon. In other words, each particular step of system destruction/deterioration may seem to be apparently random (no aging, just occasional failure by chance), but if a system failure requires a sequence of several such steps (not just a single step of destruction), then the system as a whole may have an aging behavior.
Why is this important? Because the significance of beneficial health-promoting interventions is often undermined by claims that these interventions are not proven to delay the process of aging itself, but instead simply delay or "cover-up" some particular manifestations of aging.
In contrast to these pessimistic views, reliability theory says that there may be no specific underlying elementary "aging process itself''—instead aging may be largely a property of a redundant system as a whole, because it has a network of destruction pathways, each being associated with particular manifestations of aging (types of failure). Therefore, we should not be discouraged by only partial success of each particular intervention, but instead we can appreciate an idea that we do have so many opportunities to oppose aging in numerous different ways.
Thus, the efforts to understand the routes and the early stages of age-related degenerative diseases should not be discarded as irrelevant to understanding the "true biological aging.'' On the contrary, the attempts to build an intellectual firewall between biogerontological research and clinical medicine are counterproductive. After all, the main reason why people are really concerned about aging is because it is related to health deterioration and increased morbidity. The most important pathways of age changes are those that make older people sick and frail (Bortz, 2002).
Reliability theory suggests general answers to both the "why" and the "how" questions about aging. It explains "why" aging occurs by identifying the key determinant of aging behavior—system redundancy in numbers of irreplaceable elements. Reliability theory also explains "how" aging occurs, by focusing on the process of redundancy loss over time as the major mechanism of aging.
Ageing is a complex phenomenon (Sehl and Yates, 2001), and a holistic approach using reliability theory may help to analyze, understand, and perhaps to control it. We suggest, therefore, adding theoretical reliability models of system failure in aging to the arsenal of methodological approaches for the studying of human aging.
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For centuries, ever since the legendary Ponce de Leon went searching for the elusive Fountain of Youth, people have been looking for ways to slow down the aging process. Medical science has made great strides in keeping people alive longer by preventing and curing disease, and helping people to live healthier lives. Average life expectancy keeps increasing, and most of us can look forward to the chance to live much longer lives than our ancestors.