The Oxygen Imperative: Cardiorespiratory Fitness, Dose-Dependent Exercise Thresholds, and Longevity—A Narrative Review

Background: The relationship between physical exercise and human longevity constitutes one of the most consequential intersections in contemporary preventive medicine. Although international guidelines recommend 150 min of moderate-intensity exercise weekly, growing evidence suggests that the architecture of optimal exercise is far more complex, encompassing dose, modality, timing across the lifespan, and the paradox risks imposed by extreme endurance. Methods: We included in this narrative review landmark cohort studies, randomized controlled trials, meta-analyses, and expert physiological frameworks published in high-impact cardiovascular, sports medicine, and longevity journals from 1966 to 2024. Results: Cardiorespiratory fitness (CRF), indexed by maximal oxygen uptake (VO2 max), demonstrates the strongest and most linear dose–response relationship with all-cause mortality identified in preventive medicine, with every 1 metabolic equivalent of task (MET) increment associated with a 12–15% reduction in mortality risk. The optimal dose of vigorous-intensity exercise follows a J-shaped dose–response curve: 3–5 sessions per week generating 1–2.4 h of vigorous activity is associated with the lowest all-cause mortality risk in large prospective cohorts, whereas chronic extreme endurance exercise incurs measurable atrial remodeling, patchy myocardial fibrosis, and a 5.3-fold increase in the risk of atrial fibrillation. The importance of exercise types shifts profoundly across the lifespan, transitioning from aerobic capacity effort in the third decade to resistance training in the seventh decade and neuromuscular stability in the eighth. Based on our interpretation of the available evidence, we propose a structured, personalized four-step exercise pathway integrating CRF assessment, lifespan-adapted prescription, lifestyle co-interventions, and periodic reassessment. Conclusions: Among currently available lifestyle interventions, regular exercise is consistently associated with some of the largest and most reproducible reductions in all-cause and cardiovascular mortality observed in prospective cohort data, while remaining accessible and cost-effective.