This paper investigates the effect of the consumption of food on mortality–in particular the startling but real possibility that an reduction in calories can lead to a increase in longevity. In perhaps the simplest model, one that trades off survival and fertility, it may be evolutionarily optimal for a permanent reduction in the food supply to cause such an increase in longevity. However, it is impossible to account for all the data with a model in which fertility is a function of the food input alone. We propose, therefore, a more flexible model of the relationship between food and longevity, incorporating both metabolic by-products and infectious disease. In circumstances where there is little infectious disease, the only effect of decreased food is to decrease the production of metabolic by-products, so individuals have increased life expectancy. On the other hand, in circumstances where infectious disease is prevalent, decreased food intake increases mortality because the resulting reduction in immune function increases the impact of the disease. These predictions are shown to be consistent with an empirical model based on income per capita and calorie intake per capita across modern nations. Keywords.–Dietary restriction, longevity, aging, mortality, metabolic by-products, infectious disease. 2
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