Calorie restriction and aging: A life-history analysis

The disposable soma theory suggests that aging occurs because natural selec tion favors a strategy in which fewer resources are invested in somatic mai ntenance than are necessary for indefinite survival. However, laboratory ro dents on calorie-restricted diets have extended life spans and retarded agi ng. One hypothesis is that this is an adaptive response involving a shift o f resources during short periods of famine away from reproduction and towar d increased somatic maintenance. The potential benefit is that the animal g ains an increased chance of survival with a reduced intrinsic rate of senes cence, thereby permitting reproductive value to be preserved for when the f amine is over. We describe a mathematical life-history model of dynamic resource allocatio n that tests this idea. Senescence is modeled as a change in state that dep ends on the resources allocated to maintenance. Individuals are assumed to allocate the available resources to maximize the total number of descendant s. The model shows that the evolutionary hypothesis is plausible and identi fies two factors, both likely to exist, that favor this conclusion. These f actors are that survival of juveniles is reduced during periods of famine a nd that the organism needs to pay an energetic ''overhead'' before any litt er of offspring can be produced. If neither of these conditions holds, ther e is no evolutionary advantage to be gained from switching extra resources to maintenance. The model provides a basis to evaluate whether the life-ext ending effects of calorie-restriction might apply in other species, includi ng humans.