Drosophila diet restriction in practice: do flies consume fewer nutrients?

Although many studies of Drosophila melanogaster report an effect of diet upon lifespan, it is assumed rather than know that adults maintained on restricted diet acquire fewer nutrients. Diet is restricted in practice by feeding flies a medium where nutrients are diluted and some early reports suggest that flies on diluted medium compensate with increased food intake to consume equal calories on all diets. Here we measure the feeding rate of adult Drosophila upon a yeast-restricted diet that increases survival and reduces fecundity. We directly assessed food intake from the volume of consumed dye-marked medium and from the quantity of marked fecal pellets. Females were longest lived on diet with intermediate yeast concentration but food intake was greatest on diet with abundant yeast. Rather than compensatory feeding upon diluted diet, females increased food intake on the diet with elevated nutrient concentration. Consequently, females on diluted yeast-limited media will consume fewer calories as well as less yeast. To understand the importance of specific nutrients relative to calories as mediators of Drosophila aging, we must directly measure food intake and control for the feeding stimulation of nutrients.