This study investigates the relationship between diet, nutrient intake, lifespan, and reproduction in Drosophila melanogaster, using a geometric framework to analyze the effects of protein and carbohydrate intake. The research challenges the traditional view that caloric restriction (CR) is responsible for extending lifespan, showing instead that lifespan is maximized at a protein-to-carbohydrate ratio of 1:16, while egg production is maximized at 1:2. Lifetime egg production, a key measure of fitness, is maximized at an intermediate ratio of 1:4. Flies offered a choice of complementary foods regulated their intake to maximize lifetime egg production, indicating that they can adjust their nutrient intake based on the nutritional content of available food sources. The study also highlights the trade-off between longevity and reproduction, showing that while higher protein intake increases egg production, it also reduces lifespan. This suggests that the direct costs of reproduction and the negative effects of high protein intake on longevity play a role in shaping life history traits. The findings are supported by a detailed analysis of the response surfaces for lifespan, egg production, and egg production rate, which reveal how these traits are influenced by different protein and carbohydrate intakes. The research provides a framework for understanding the complex interplay between nutrition, aging, and reproduction in Drosophila, and has implications for studying aging in other organisms. It also underscores the importance of considering the specific nutritional composition of diets rather than just caloric intake when examining the effects of diet on lifespan and reproduction. The study's use of a geometric framework allows for a more nuanced analysis of nutrient intake and its effects on life history traits, offering new insights into the mechanisms underlying aging and reproduction in Drosophila.This study investigates the relationship between diet, nutrient intake, lifespan, and reproduction in Drosophila melanogaster, using a geometric framework to analyze the effects of protein and carbohydrate intake. The research challenges the traditional view that caloric restriction (CR) is responsible for extending lifespan, showing instead that lifespan is maximized at a protein-to-carbohydrate ratio of 1:16, while egg production is maximized at 1:2. Lifetime egg production, a key measure of fitness, is maximized at an intermediate ratio of 1:4. Flies offered a choice of complementary foods regulated their intake to maximize lifetime egg production, indicating that they can adjust their nutrient intake based on the nutritional content of available food sources. The study also highlights the trade-off between longevity and reproduction, showing that while higher protein intake increases egg production, it also reduces lifespan. This suggests that the direct costs of reproduction and the negative effects of high protein intake on longevity play a role in shaping life history traits. The findings are supported by a detailed analysis of the response surfaces for lifespan, egg production, and egg production rate, which reveal how these traits are influenced by different protein and carbohydrate intakes. The research provides a framework for understanding the complex interplay between nutrition, aging, and reproduction in Drosophila, and has implications for studying aging in other organisms. It also underscores the importance of considering the specific nutritional composition of diets rather than just caloric intake when examining the effects of diet on lifespan and reproduction. The study's use of a geometric framework allows for a more nuanced analysis of nutrient intake and its effects on life history traits, offering new insights into the mechanisms underlying aging and reproduction in Drosophila.