This study investigates the relationship between diet, nutrient intake, lifespan, and reproduction in *Drosophila melanogaster* using advanced nutritional research techniques. The authors found that caloric restriction (CR) was not responsible for extending lifespan in their experimental flies. Instead, response surfaces for lifespan and fecundity were maximized at different protein-carbohydrate (P:C) ratios. Longevity was highest at a P:C ratio of 1:16, while egg-laying rate was maximized at 1:2. Lifetime egg production, a measure of fitness, was maximized at an intermediate P:C ratio of 1:4. Flies regulated their intake to maximize lifetime egg production, indicating a role for both direct costs of reproduction and other negative consequences of high protein intake. The study reconciles conflicting findings in the literature and provides a framework for studying aging in various organisms.This study investigates the relationship between diet, nutrient intake, lifespan, and reproduction in *Drosophila melanogaster* using advanced nutritional research techniques. The authors found that caloric restriction (CR) was not responsible for extending lifespan in their experimental flies. Instead, response surfaces for lifespan and fecundity were maximized at different protein-carbohydrate (P:C) ratios. Longevity was highest at a P:C ratio of 1:16, while egg-laying rate was maximized at 1:2. Lifetime egg production, a measure of fitness, was maximized at an intermediate P:C ratio of 1:4. Flies regulated their intake to maximize lifetime egg production, indicating a role for both direct costs of reproduction and other negative consequences of high protein intake. The study reconciles conflicting findings in the literature and provides a framework for studying aging in various organisms.