Transient reduction of metabolic rate by food restriction.

It is widely held that restriction of food intake leads to a decrease in metabolic rate, and this effect has been implicated as the mechanism by which food restriction retards aging in laboratory rodents. Data on which this belief is based were obtained mostly from studies of basal metabolic rate after acute rather than chronic food restriction. In contrast we previously reported no effect of chronic food restriction on metabolic rate measured under usual living conditions. The present study examines changes in metabolic rate immediately after restriction of food so as to include the initial response to decreased intake of food. Two groups of specific pathogen-free Fischer 344 rats were housed under barrier conditions from 6 to 24 wk of age. Restricted rats were fed 60% of food consumed by rats eating ad libitum. Metabolic rate was measured indirectly by gas analysis with rats under normal daily living conditions (24MR) and basal metabolic rate (BMR) was also measured. Both 24MR and BMR decreased after food restriction, but this decrease was transient, so that within a few weeks metabolic rate of restricted rats was the same as that of rats fed ad libitum.