Modern urban environments are increasingly disconnected from human evolutionary biology, contributing to a dementia cliff where rising rates of cognitive decline threaten global healthcare sustainability. This commentary argues that Protected Areas (PAs) are not merely recreational spaces but essential neurological infrastructure. Physical exercise triggers the release of the myokine Irisin, which crosses the blood-brain barrier to stimulate Brain-Derived Neurotrophic Factor (BDNF) in the hippocampus. However, in high-stress urban or gym settings, elevated cortisol levels act as a biochemical brake, suppressing this neurogenic potential. Natural environments provide a green multiplier effect: forest phytoncides and fractal visual patterns lower systemic cortisol, unlocking the brain to receive maximum BDNF dividends. Furthermore, navigating the stochastic, uneven terrain of natural trails demands intense spatial mapping and proprioceptive engagement, physically enlarging hippocampal volume. By delaying the clinical onset of neurodegeneration by three to five years, universal access to green exercise offers a superior return on investment (ROI) compared to traditional pharmacological interventions. Integrating health and environmental policies is a biological and economic necessity to ensure long-term cognitive resilience in an ageing population.

Green exercise, Irisin-BDNF axis, Neurogenesis, Dementia cliff, Protected areas, Cortisol suppression