CR and famine

Famines are part of the natural state

The fact that population growth tends to be exponential - while it is not very realistic to think that resources can behave in the same way - naturally results in taking organisms to the edge of starvation.

On these grounds it seems reasonable to suggest that abundant food resources would have rarely been available in the past.

If they had regularly been available, predators would most likely have reproduced until the sheer numbers of their offspring depleted the available resources.

A number of people don't seem to accept this point.

They argue that the ability to leave a famine environment will restore an abundant food supply - apparently ignoring the fact that organisms are pressed into competition with their cousins all over the entire planet - e.g.:

Dietary restriction would probably not increase longevity in human beings and other species able to leave unsuitable environments.

- PMID 16628488

Seasonal famines

De Grey's speculations

In particular, he has argued:

The pattern of starvation that the weather imposes is suggested here to be of a sort that will tend to cause all terrestrial animals, even those as far apart phylogenetically as nematodes and mice, to possess the ability to live a similar maximum absolute (rather than proportional) amount longer when food is short than when it is plentiful.

- PMID 15711074

In particular I assumed that CR did not "retard aging" - and merely resulted in a transient, reversible reduction of mortality during the famine period.

I then simulated a lifespan with a series of seasonal CR episodes - on simulated organisms with different aging rates - to examine the effect of the length of their lifespans on the degree of life extension produced.

The results are as illustrated in the following graph:

This is a "survival curve" graph - a plot of % of organisms surviving against time.

Organisms with different natural rates of aging are represented by different colours.

The lines on the left represents the survival curve of well-fed controls, while the ones on the right shows the survival curve of organisms subjected to periodic resource shortages.

The simulation models mortality as increasing with age according to a Gompertzian function - and it transiently reduces mortality by a constant proportion during the famines.

From this graph it can be seen that the median lifespan (i.e. the time when 50% of the population is dead) increases with the lifespan of the organisms - so that the life extension is a function of the lifespan of the organism.

I consider the assumptions made by this model pretty pessimisitic ones. If CR does result in actually retarding aging (rather than transiently reducing mortality while the organism is on CR) the results would even more strongly indicate that CR's effect on life expectancy were a function of lifespan.

Atrophy of the CR response due to reduced selection?

I consider that it is reasonable to suggest that small organisms with fewer resource reserves are more likely to die as a result of seasonal famines than large creatures with sufficient fat reserves to last through the winter (and thus are more likely to have powerful conservation responses activated by CR).

Unfortunately, it does not appear to be very realistic to expect to be able to estimate to what extent the CR response has atrophied due to disuse on theoretical grounds alone.

No doubt fewer humans than mice starve during a typical winter - but quanitifying the result of such an effect on lifespan presents some significant challenges - and I doubt whether it can realistically be done from one's armchair.

Certainly, empirically, humans do still have a fairly significant response to CR - at least in terms of its short-term effects on their metabolism.

Rare resource shortages?

That would be a prediction of a model which treats resource shortages as a rare event - only occurring very rarely - so the chance of two resource shortages happening in any organism's lifespan are low.

However, to me that seems unbelievably unrealistic.

Much more likely is seasonal resource shortages occurring every single year.

The idea that resource shortages are rare events is in conflict with the notion that reproduction takes ecosystems to the edge of starvation. Food shortages would have been common in the past - not rare.

Other possible reasons for reduced life extension in long-lived organisms

For example, if long-lived organisms are the result of selecting for long lifespans, then it may be that the same systems CR uses to extend lifespan are already being activated naturally - and in that case the additional life extension due to CR might be reduced.

Life extension theory

Nor should CR be regarded as merely a response to seasonal food shortages - since food shortages can occur over longer time periods than a year - and are likely to have done so historically.

Essentially, the question of how organisms with different lifespans respond to CR is an empirical one - since it depends on the extent to which adaptations which are rarely activated atrophy - and theory alone offers few clues about that in this instance.