The Maintenance Gap

Much of the mainstream aging research community has little interest in building therapies for aging, being focused on investigation only - though, fortunately, this situation is changing rapidly these days. The past stigma associated with public discussion of treating and ultimately preventing aging has largely evaporated within the scientific world.

Among those researchers who are interested in therapies for aging, most are focused on the slow boat of metabolic alteration: work that will have comparatively little pay-off even if successful, but which fits more readily into established research programs and the prejudices of research funding institutions.

The principal downside of metabolic alteration strategies, from my point of view, is that even if successful they cannot produce any significant longevity benefit in a person already old. All it can do is slow down aging by a modest amount - which isn't terribly useful those already aged and damaged. Even under the most optimistic estimates it will take another twenty years and many billions of dollars to see the evolution of a robust market in commercially available human metabolic enhancements to slow aging. It is a challenging field of research, and progress to date has been slow even in this era of rapid advances in biotechnology.

There is another disadvantage, which is illustrated by the different degrees to which life span is enhanced by similar strategies applied in mice versus humans. It is taken for granted in the literature, and thus probably not emphasized to the degree it should be, that an extension of life by 50% in mice based on some genetic or metabolic alteration - such as calorie restriction or growth hormone knockout - is probably not going to map to a similar extension of life in humans. If humans could achieve that sort of life extension through simply eating well and eating less or being growth hormone mutants, we'd have known about it by now. Consider Laron dwarfism, for example, or the generation after generation of practitioners of various degrees of calorie restriction that exist in many cultures.

With an eye to this second disadvantage, I'll point out an open access paper that considers the evolution of aging from the point of view of the maintenance gap. This is the gap between the cost of maintenance required to keep an organism from aging and the resources actually devoted to maintenance - both of which are subject to evolutionary selection pressures, which operate to maximize success in genetic propagation rather than the comfort or longevity of individual members of a species. The paper was published last year, but showed up in a recent issue of Biogerontology.

The maintenance gap: a new theoretical perspective on the evolution of aging

One of the prevailing theories of aging, the disposable soma theory, views aging as the result of the accumulation of damage through imperfect maintenance. Aging, then, is explained from an evolutionary perspective by asserting that this lack of maintenance exists because the required resources are better invested in reproduction. However, the amount of maintenance necessary to prevent aging, 'maintenance requirement' has so far been largely neglected and has certainly not been considered from an evolutionary perspective. To our knowledge we are the first to do so, and arrive at the conclusion that all maintenance requirement needs an evolutionary explanation.

Increases in maintenance requirement can only be selected for if these are linked with either higher fecundity or better capabilities to cope with environmental challenges to the integrity of the organism. Several observations are suggestive of the latter kind of trade-off, the existence of which leads to the inevitable conclusion that the level of maintenance requirement is in principle unbound. Even the allocation of all available resources to maintenance could be unable to stop aging in some organisms.

This has major implications for our understanding of the aging process on both the evolutionary and the mechanistic level. It means that the expected effect of measures to reallocate resources to maintenance from reproduction may be small in some species. We need to have an idea of how much maintenance is necessary in the first place. Our explorations of how natural selection is expected to act on the maintenance requirement provides the first step in understanding this.

The point to take away from this argument is that we should expect to find a broad variation between species in their response to similar forms of metabolic and genetic alteration aimed at extending life span. So far, that is what is seen, with we humans having the short end of the stick - though obviously there is an ocean of data yet to be obtained on this topic. On the whole, though, it seems like one more slowly building argument for the research community to focus on repair-based strategies for treating aging: build biotechnologies that are explicitly designed to repair forms of biological damage that existing repair systems either cannot handle or handle too slowly. SENS is the most obvious example, though I expect other, competing repair-focused visions to emerge in the years ahead as the SENS Foundation obtains further scientific support and promising research results.

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