12 January - 20 June 2016

Effect of restricted diet on lifespan

26 Mar 14

That limiting nutrient intake via dietary restriction appears to reduce fecundity and lengthen lifespan is a phenomenon that has been documented in the laboratory for a wide range of organisms. A new paper in BioEssays claims to rebut previous hypotheses about the evolutionary implications of this fact, with potential application to human health.

According to the authors, Margo I Adler and Russell Bonduriansky from the University of New SOuth Wales, previous suggestions that dietary restriction stimulates resources to be diverted from reproduction to maintenance and repair of the body are unsupported by the evidence. They claim that a longer lifespan at the expense of reproduction is unlikely to be evolutionarily favourable in the wild, noting that significant differences in what tends to cause death in wild and laboratory animals must be taken into account when trying to explain the effect of dietary restriction. For example, they note that when previous studies have observed extension of lifespan for laboratory animals under dietary restriction, this has often been due to reduced incidence of cancer. The effect of lower cancer incidence on lifespan is likely, however, to be a laboratory artefact, as wild animals are far more likely to die as a result of predation or infection.

Furthermore, the authors argue, dietary restriction is also likely to reduce animals' ability to survive environmental challenges in the wild. They may well be more susceptible to cold, to pathogens and, due to impaired immune response, more likely to die from infection. Significantly, humans undertaking a restricted diet have been demonstrated to show greater sensitivity to cold.

The alternative hypothesis put forth by Adler and Bonduriansky separates the effects of dietary restriction on reproduction and on lifespan, rather than positing a reallocation of resource from one set of activities to the other. They suggest that under a normal diet, evolution selects for an inhibition of cellular recycling mechanisms such as programmed cell death and the breakdown of unnecessary cellular components, as they reduce cell growth rate and thus reproductive rate. Under dietary restriction, it is beneficial for these processes to be up-regulated so as to make more efficient use of scarce resources; and the reduced reproductive rate also results. In the laboratory environment, the additional effect of increased lifespan is also seen, but the authors are sceptical of whether this would translate to a wild environment, where effects have not thus far been studied.

Nevertheless, they suggest that the effect on lifespan in a benign environment such as the laboratory may be relevant to human health even if not to animals in wild environments, and call for further research.

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