Forest walks, phytoncides & the science of anxiety relief

Causal links between woodland walking, cortisol, prefrontal cortex function, and insulin resistance

What the trees are producing: phytoncides

The compounds produced by leaves and fresh forest air away from buildings have a precise name: phytoncides — volatile organic compounds (VOCs) released by trees as a biological defence against bacteria, fungi, and insects. Trees produce phytoncides through specialised cells in their leaves, bark, and wood, releasing them into the air as a fine mist. The most common compounds include alpha-pinene, beta-pinene, limonene, and camphene — with coniferous trees like pine, cedar, and cypress being particularly prolific producers. (Study 10)

When inhaled, phytoncides enter the nasal passages and are detected by the olfactory system, which is directly wired to the limbic system — the ancient part of the brain governing emotion, memory, and instinct. This is not a metaphor but a direct neuroanatomical pathway from nostril to emotional brain, bypassing the analytical cortex entirely. (Study 10)

The cortisol evidence: quantified and replicated

Cortisol reduction from forest walking is among the most replicated findings in environmental health research. In all but two included studies across a systematic review and meta-analysis of 22 studies, cortisol levels were significantly lower after forest intervention compared with control groups, or a significant pre-post reduction was reported in the forest groups. (Study 1)

The numbers are striking. In a 2024 clinical study measuring multiple physiological parameters, salivary cortisol dropped from 5.2 μg/dL before nature exposure to 2.77 μg/dL after — a near halving. (Study 2)

Phytoncides are a specific driver of this effect. In a randomised controlled trial, phytoncide exposure significantly decreased epinephrine by 5.29% and cortisol by 24.94%, while parasympathetic nerve activity increased — confirming that phytoncide inhalation directly suppresses the sympathetic nervous system and activates the rest-and-recover parasympathetic pathway. (Study 3)

Forest bathing reduces stress hormones including urinary adrenaline, noradrenaline, and salivary and serum cortisol, while simultaneously increasing parasympathetic nerve activity and reducing sympathetic nerve activity — stabilising the autonomic nervous system balance. (Study 4)

The prefrontal cortex: the brain science is specific

The prefrontal cortex (PFC) is the brain's executive and emotional regulation centre. When chronically over-activated by stress and anxiety it drives rumination, hypervigilance, and the persistent sense of dread that characterises anxiety disorders. The forest walk directly deactivates it.

Field experiments across 24 Japanese forests found that walking or viewing trees reduced cortisol levels in saliva. The lead investigator had previously found that haemoglobin levels in the left prefrontal cortex — associated with stress and apprehension — decrease during strolls among the trees. Walks taken inside a laboratory did not produce this effect. (Study 5)

Using near-infrared spectroscopy (NIRS), total haemoglobin and oxyhaemoglobin concentrations were both significantly lower in the forest environment than the urban environment, directly demonstrating measurable prefrontal cortex deactivation. Feelings of "comfortable", "natural", and "soothed" were simultaneously significantly higher. (Study 6)

Walking in forests compared to urban environments decreases cerebral blood flow in the prefrontal cortex, reduces blood pressure and pulse rate, increases parasympathetic nerve activity, suppresses sympathetic nerve activity, and decreases salivary cortisol. (Study 9)

The 90-minute walk: directly studied at that duration

The observation about long walks producing significant anxiety relief is backed by a landmark study at exactly that duration, published in the Proceedings of the National Academy of Sciences. Participants who walked for 90 minutes in a natural environment showed significantly reduced rumination — the repetitive negative thought pattern that is a core feature of anxiety — and decreased activity in the subgenual prefrontal cortex, compared to those who walked in an urban environment. (Study 7)

Decreased functional connectivity between the anterior cingulate cortex and amygdala is a predictor of anxiety. Nature experience specifically reduced activity in precisely this region, suggesting that nature walks are directly modulating the neural circuitry of anxiety at the level of brain structure and function — not merely producing a subjective feeling of calm. (Study 7)

In a separate fMRI study, amygdala activity — in the brain's primary threat-detection and fear centre — decreased significantly after a one-hour nature walk in women, providing further direct neural evidence for anxiety circuit modulation. (Study 8)

In Profile of Mood States testing, forest bathing reduces scores for anxiety, depression, anger, fatigue, and confusion, while increasing vigour. (Study 4)

The insulin resistance connection: cortisol is the bridge

For someone with anxiety and possible insulin resistance, the forest walk becomes genuinely therapeutic rather than merely pleasant — because cortisol is a direct driver of insulin resistance.

Cortisol is a glucocorticoid whose primary metabolic function is to raise blood glucose for the fight-or-flight response. Chronically elevated cortisol therefore chronically elevates blood glucose, which chronically elevates insulin, which progressively impairs insulin receptor sensitivity. Circadian disruption, chronic stress, and dysregulated cortisol secretion directly promote insulin resistance and leptin resistance. (Study 11)

Every walk that halves salivary cortisol is simultaneously reducing the chronic glucose-raising signal that underpins insulin resistance. The forest walk is not just a mental health intervention — it is directly metabolically therapeutic.

There is direct forest walking evidence for blood glucose specifically. Forest walking has been documented to decrease blood glucose levels in patients with non-insulin-dependent diabetes — confirming the metabolic pathway from forest exposure to improved insulin function. (Study 12)

Why the effects last and compound

Research by Professor Qing Li shows that phytoncide exposure reduces blood pressure and heart rate, reduces symptoms of anxiety, depression, and fatigue, improves sleep, increases energy and memory, and boosts anti-cancer proteins — with effects from a forest visit lasting more than 30 days after a single trip. (Study 4)

The reason repeated long walks produce cumulative benefit is that each one is resetting the HPA (Hypothalamic-Pituitary-Adrenal) axis — the stress regulation system — down toward its baseline. For a chronically anxious person whose HPA axis has been running in a heightened state for months or years, each 90-minute forest walk is a dose of neurological and hormonal recalibration that their urban or indoor life cannot provide.

Previously sedentary adults who walked one hour per day for six months showed measurable enlargement of the hippocampal formation — the brain's memory and emotional regulation structure — confirming that regular walking in nature produces durable structural brain changes, not merely transient mood effects. (Study 11)

Summary: what a 90-minute woodland walk delivers

• Phytoncide inhalation via the olfactory-limbic pathway — directly modulating emotion and stress centres (Study 10)
• Near-halving of salivary cortisol — replicated across 22 studies in the meta-analysis (Study 1) (Study 2)
• Measurable prefrontal cortex deactivation — confirmed by NIRS and fMRI across multiple studies (Study 6) (Study 9)
• Reduced subgenual PFC activity and rumination — the 90-minute walk studied directly at Stanford University (Study 7)
• Reduced amygdala reactivity — the anxiety and fear circuit directly downregulated (Study 8)
• Autonomic shift from sympathetic to parasympathetic dominance (Study 3) (Study 4)
• Direct blood glucose reduction — relevant to insulin resistance (Study 12)
• Effects lasting up to 30 days from a single forest exposure (Study 4)

For a client with anxiety and insulin resistance, this is one of the most evidence-rich, zero-side-effect, zero-cost interventions available. It is not complementary to clinical care — in the context of cortisol-driven insulin resistance, it is mechanistically clinical.

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Studies referenced

For Radiant Health | forradianthealth.com