Problems with Disposable Soma Theory and Caloric Restriction

Contradictions Between
Disposable Soma Hypothesis and
Caloric Restriction Effects on Lifespan

It is well documented across widely separated taxa that animals live longer with caloric restriction.

Disposable Soma Hypothesis maintains that aging is the consequence of a compromise in the allocation of caloric energy, and from this postulate follows that caloric restriction should cause a reduced life span [Mitteldorf 2001]. "When less food energy is available, each of the demands on that energy must share the burden, making do with a reduced share of the smaller total. Allocation for repair and maintenance must be smaller, and if the Disposable Soma (DS) theory is correct then aging must proceed more rapidly. This is the opposite of what is observed. Reduced caloric intake reliably leads to slower aging and enhanced life span.

For many species, reproduction requires a substantial energy investment. It is a logical possibility that reproduction may be constrained to be either ‘on’ or ‘off’, with no in-between state. Then there will be a point in the caloric restriction curve when reproduction is abruptly shut down, and more energy becomes available on the far side of that line, before the decline in available energy inevitably continues. Shanley and Kirkwood attempted a DS model of aging in mice based on this effect, and reported [Shanley & Kirkwood 2000] the limited success of their model in an optimistic light. But in most versions of their model, there was no energy dividend for repair and maintenance, and in the one version showing a dividend, it appeared over a narrow range of caloric restriction, for lactating female mice only. In contrast, the CR data show that life span is extended linearly as calories are reduced over a broad range, and the experiments are generally performed comparing non-reproducing mice with other non-reproducing mice, male and female [Mitteldorf 2001]. Absent the energy of lactation, Shanley’s model predicts clearly that the CR mice should have curtailed life spans.

The idea of metabolic tradeoffs is so appealing that other authors [van Noordwijk 1986]; [de Jong & van Noordwijk 1992]; [de Jong 1993]; [Cichon 1997] have advocated versions based on allocation of some other resource than energy. Theoretical models of this mechanism work well in the abstract, but authors have lots of wiggle room because they are not constrained by real world data. The primary problem with these theories is that the scarce resource (if it is not energy) remains unspecified. Whatever it is, it must be essential to life, in short supply, and more of it would enable most living things both to live longer and to function better. Until such a resource is specified, the idea remains difficult to evaluate.

A direct and general test applicable to any of the theories of metabolic tradeoff was conducted by Ricklefs and Cadena [Ricklefs & Cadena 2007]. They cross-tabulated fertility and longevity for captive birds and mammals in zoos, and found a slight positive correlation between fertility and longevity. Similarly, many demographers have sought for evidence of a “cost of reproduction” in humans, and have found no relationship [Le Bourg et al. 1993]; [Muller et al. 2002] or a small positive association [Korpelainen 2000]; [Lycett et al. 2000] between fertility and longevity. One well-publicized study claimed to discern a cost of reproduction in a historic database of British nobility [Westendorp & Kirkwood 1998], but its methodology was compromised by use of an obscure and inappropriate statistical test [Mitteldorf 2009]. Standard linear correlation on the same database reveals a positive correlation [Mitteldorf 2009].

All versions of the DS theory rely on the notion that “perfect” repair would be very expensive. This idea was exploded by Vaupel [Vaupel et al. 2004] ..." [Mitteldorf, personal communication, in press]

In short, the effects of caloric restriction on lifespan falsify some important predictions of Disposable Soma Hypothesis.

On the contrary, the negative relation between caloric intake and lifespan is compatible with Wear and Tear Hypotheses and Stochastic Hypothesis because a reduced metabolism caused by caloric restriction should slacken aging.

Moreover, the effects of caloric restriction on lifespan are compatible with Adaptive Hypothesis because they could be explained as the consequence of the regulation of aging program in particular conditions.

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