Abstract
Aging is associated with neuroendocrine, immune, and inflammatory changes that may increase vulnerability to physiological stress. This systematic review synthesized evidence on whether controlled direct exposure to forests, parks, gardens, or other green spaces is associated with changes in biospecimen-based stress-related biomarkers in middle-aged and older adults. PubMed and Scopus were searched from inception through August 8, 2026. Eligible studies delivered direct green-space exposure and measured at least one biomarker reflecting neuroendocrine, autonomic, inflammatory, or immune function. Findings were synthesized qualitatively, and risk of bias was assessed using design-appropriate tools. Thirty-eight studies reported in 42 articles, comprising 1,518 participants, met inclusion criteria. Exposures ranged from brief single visits to multi-day residential programs. Cortisol was the most frequently assessed biomarker. It showed the most consistent pattern of decrease particularly in saliva. Inflammatory, immune, and autonomic markers showed more variable responses. Several controlled comparisons suggested that the natural environment may contribute to physiological responses beyond physical activity alone. Due to some risk of bias and the diversity of design, more studies are needed in this space to quantify these physiological changes.