Spermidine supplementation and protein restriction protect from organismal and brain aging independently

Brain aging and cognitive decline are significant biomedical and societal concerns. Both dietary restriction, such as limiting protein intake, and fasting, which restricts the timing of food consumption, have been proposed as strategies to delay aspects of aging. Recent studies suggest that intermittent fasting effects are mediated by the endogenous polyamine spermidine. Spermidine supplementation promotes mitochondrial integrity and functionality in aging brains by supporting hypusination of the translational initiation factor eIF5A. However, how molecular mechanisms underlying fasting mimicking interventions and protein restriction converge remain unclear, yet biomedically relevant. In this study, we combined low- and high-protein diets (2% versus 12% yeast in food) with spermidine supplementation in aging <i>Drosophila</i> fruit flies. Effective hypusination was essential for normal life expectancy on both 2% and 12% yeast diets. Spermidine supplementation increased longevity, protected against age-related locomotion decline on both diets and improved memory scores in older flies regardless of protein intake. Notably, spermidine did not reduce the positive effects of the 12% protein diet on fecundity. Our findings suggest that while both protein restriction and spermidine supplementation improve brain mitochondrial function, they largely operate through distinct mechanisms in modulating <i>Drosophila</i> brain aging. These results offer a basis for potential synergistic lifestyle interventions targeting age-related brain decline.