Nutrition influences reproduction and lifespan; however, the optimal balance of macronutrients may differ between these endpoints but well controlled experimental studies have not yet clarified these optima in mammals. Using a geometrically based 3D experimental design (“Geometric Framework”) we evaluated the effects of protein (P), carbohydrate(C) and fat (F) in diets of a wide-range of compositions on lifespan and reproductive potential in male and female mice (n=86 and 97, respectively). Animals were confined to one of 25 fixed diets ad libitum with gamete stock assessed at 15 months. Ovarian follicle number was driven primarily by F intake (p=0.006). Females that consumed a low P:F diet (1:18 ratio) had the most average follicles per section but fewer average corpora lutea (CL) per section. Greatest CL number per section, on the other hand, was driven mostly by C intake (p<0.001) and were found in mice on a high P:C diet (ratio 1:2.5). Data in males were more complex. Maximum sperm counts occurred at C intakes of 20 kJ/d and fell away as intake went over or under this target. Median lifespan, however, was maximised when males and females ate a low P:C diet (ratios 1:13 and 1:11, respectively). This is consistent with data in invertebrates that show a low P:C ratio increases longevity while a higher P:C ratio optimises reproductive parameters. This first evidence from a comprehensive controlled lifespan study of dietary macronutrient composition in mammals suggest that nutritional interventions may have important but previously unrecognised public health implications for diet and the extension of healthy reproductive and overall human lifespan.