Analysis of UK Biobank participants reveals a precise threshold where vigorous exercise shifts from cognitive protection to harm. Peak brain benefits occur at 1,000-1,300 MET-minutes weekly—roughly equivalent to five intense 45-minute sessions—while higher volumes trigger measurable cognitive decline through lactate-mediated neurological changes. This finding challenges the prevalent 'more is better' exercise paradigm and establishes the first quantified upper limit for vigorous activity's neuroprotective effects. The lactate mechanism represents a significant advance in exercise neuroscience, potentially explaining why elite endurance athletes sometimes report cognitive fatigue and concentration difficulties. Previous studies focused primarily on exercise deficiency rather than excess, making this dose-response curve particularly valuable for optimizing training protocols. The research methodology combining population data with mechanistic models and randomized trials provides robust evidence that could reshape exercise prescription guidelines. For health-conscious adults pursuing cognitive longevity, this suggests strategic periodization may be more beneficial than continuous high-intensity training, particularly as metabolic efficiency and lactate clearance decline with age.