Drosophila melanogaster is ideal for studying lifespan modulated by dietary restriction (DR) and oxidative stress, and also for screening prolongevity compounds. It is critical to measure food intake in the aforementioned studies. Current methods, however, overlook the amount of the food excreted out of the flies as feces or deposited in eggs. Here we describe a feeding method using a radioactive tracer to measure gender-specific food intake, retention and excretion in response to DR and oxidative stress to account for all the ingested food. Flies were fed a full, restricted or paraquat-containing diet. The radioactivity values of the food in fly bodies, feces and eggs were measured separately after a 24-hr feeding. Food intake was calculated as the sum of these measurements. We found that most of the tracer in the ingested food was retained in the fly bodies and <8% of the tracer was excreted out of the flies as feces and eggs in the case of females during a 24-hr feeding. Under a DR condition, flies increased food intake in volume to compensate for the reduction of calorie content in the diet and also slightly increased excretion. Under an oxidative stress condition, flies reduced both food intake and excretion. Under all the tested dietary conditions, males ingested and excreted 3- to 5-fold less food than females. This study describes an accurate method to measure food intake and provides a basis to further investigate prandial response to DR and prolongevity interve ntions in invertebrates.
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