What actually works

Caveat on this rating: Confidence in these numbers is low by the authors' own grading: 1 of 218 studies met the Cochrane criteria for low risk of bias, and CINeMA confidence was low for walking or jogging and very low for every other modality. The credible intervals for the five modalities overlap heavily, so the order above is not an established ranking and must not be read as one. The authors name the blinding problem themselves and call for future trials to blind participants and staff. Results did appear robust to publication bias. A correction was published (BMJ 2024;385:q1024) and is logged as unverified - the BMJ site refuses automated fetch - but nothing quoted here depends on a precise ranking. Effect sizes drawn from different literatures are not directly comparable, so this row deliberately does not set these numbers against antidepressant trial results.

Caveat on this rating: Observational. The correlations are small - 0.05 within persons, 0.08 between - and the authors state plainly that the practical effect is comparable to other everyday activities. People who move more may differ from people who move less in many ways this design cannot adjust away. The association runs in both directions, so "exercise lifts mood" is only half of what was measured: feeling better also predicts moving next. Considerable heterogeneity across individuals, only partly explained by sociodemographic moderators, means the average says little about any one person. The negative association with calmness is a real finding rather than a nuance to skip - activity tracked energetic arousal most strongly, so someone seeking calm may not find it here.

Caveat on this rating: Read this as absence of evidence, not evidence of absence - the body says so and the authors insist on it: "Contrary to expectations, nature-based interventions did not significantly outperform controls. However, this finding should be interpreted cautiously... studies in this group were small, at moderate-to-high risk of bias and conceptually diverse... This conceptual heterogeneity probably diluted the pooled effect estimate and limits interpretability." The abstract adds that the evidence "was limited by conceptual and methodological heterogeneity". Across the 183 trials risk of bias was "frequently moderate to high" and funnel asymmetry suggested publication bias, though sensitivity analyses held. Separately: the field's most-cited experiment, Bratman et al. 2015 PNAS (38 people), did not hold exercise constant - its nature route climbed 155m and its urban route 4m over the same 5.3km - and its brain finding has never been replicated.

Caveat on this rating: Strong design, still not causal, and the authors say so: "Despite the strong longitudinal design of our study, our risk estimates fundamentally only show correlations." Three limits they name: residential choice may be genetically confounded; unmeasured socioeconomic factors such as green-space quality and crime rates may remain; and NDVI "provides no information about the use of green space", so a child who only sees a park scores the same as one who plays in it daily. Our own added critique: there is no sibling or within-family analysis, so families who move somewhere greener are never compared with their own relatives. The 55% figure is the paper's "up to" across various disorders, not a single headline estimate.

Caveat on this rating: THE WIDELY-QUOTED VERSION OF THIS FINDING IS RETRACTED. Gu et al. 2019 (Br J Psychiatry 215(2):456-467, doi 10.1192/bjp.2018.295), the source of the much-repeated "19% higher depression risk per 10 ug/m3 PM2.5", was retracted - confirmed three ways on 2026-08-28: the PubMed record for PMID 30719959 is flagged RETRACTED ARTICLE; a separate retraction notice exists at PMID 32349797 / doi 10.1192/bjp.2020.87; and Crossref carries a type-retraction update-to relation from both the publisher and Retraction Watch. The published notice gives no reason we could read, so none is asserted here. Do not use the 19% figure; 1.102 [1.023-1.189] is the surviving estimate. It rests on five observational studies with, in the authors' words, "heterogeneous outcome definitions, exposure assessment, and residual confounding" - and poverty tracks bad air, so the inversion is about consistency, not about proof.

Caveat on this rating: The famous number is far weaker than its fame. The threshold was not pre-specified - it emerged from cutting the data into 60-minute blocks - and the curve is not a staircase: adjusted odds for good health run 1.59 [1.31-1.92] at 120-179 minutes, then FALL to 1.44 at 180-239, rise to 1.55, and fall again to 1.33 at 300+, with overlapping confidence intervals throughout. The data cannot distinguish two hours from five. The authors call the threshold and peak "more as a starting points for discussion and further investigation, than clearly established findings", report that nature time explained "approx. 1%" of variance, and state they "are unable to rule out the possibility that the association is, at least in part, due to healthier, happier people spending more time in nature."

Caveat on this rating: One city, 112 people, one week of measurement, analysed cross-sectionally - this is a signal to take seriously, not a settled result, and the badge reflects that. The direction is what matters: the authors write that these findings "contrast many studies showing positive relationships between time outdoors and mental health and highlight the need for research with diverse populations and better measures of outdoor conditions and intentionality of greenspace contact." Reverse causation is unexcluded here too: people already anxious may move differently. The reading this row supports is that "outdoors" is not a single exposure, not that being outdoors is harmful.

Caveat on this rating: These dropout figures come from trial conditions - supervised, scheduled, usually free - which is not the same as starting alone, and the review's own finding that professional supervision lowered dropout says so. The 18.1% is trim-and-fill adjusted for publication bias; the unadjusted rate differs. Trial participants had volunteered for an exercise study, so they were more willing than depressed people in general by selection. The finding that higher baseline depression predicted greater dropout is an association from meta-regression across studies, not a prediction about any individual. Behavioural activation is adjacent to exercise but not the same thing: it schedules valued and rewarding activity of any kind, delivered by a trained worker, and no trial found here tests exercise as behavioural activation against behavioural activation as usual. COBRA compared it with CBT, not with exercise.

Caveat on this rating: The 2011 trial carries a published rebuttal in the same journal: Coen, Lawlor and Kenny, "Failure to demonstrate that memory improvement is due either to aerobic exercise or increased hippocampal volume", PNAS 2011;108(18):E89, author reply E90, doi 10.1073/pnas.1102593108. A 2% change in a subregion sits near the limit of what structural MRI resolves reliably in 120 people, and the anterior hippocampus is a smaller compartment measured more noisily than total volume. The meta-analysis found the left-hippocampus effect only, and attributed it post-hoc to prevented decline - a post-hoc subgroup result, weaker than the headline sounds. Neither study enrolled depressed people or measured mood, so neither supports any claim about exercise and emotional health. Animal models show hippocampal growth consistently; the human evidence is called equivocal by these authors.

Caveat on this rating: Observational, not a trial: 15 prospective cohorts, so causation is not established. The standing objection is reverse causation - early, undiagnosed depression reduces activity, so low activity may be an early symptom rather than a cause. The 3-year minimum follow-up mitigates this but does not remove it. Heterogeneity was large and significant (I2 = 74%), and physical activity was self-reported in the contributing cohorts, which over-estimates it. The paper's 11.5% population attributable fraction assumes a causal relationship the design cannot establish. The 45-minute, three-to-five-times figure comes from Chekroud 2018 - a cross-sectional survey of self-reported poor mental health days in 1,237,194 US adults, not a trial, so it cannot establish that any type or dose is best; its senior author is affiliated with Spring Health, a commercial mental-health company, and Microsoft provided cloud computing.

Caveat on this rating: The "as effective as antidepressants" claim cannot be made from this review and is refused here. Its head-to-head comparisons are underpowered: exercise vs medication is 5 trials and 330 people (SMD -0.11, 95% CI -0.33 to 0.10, low certainty); exercise vs psychotherapy is 10 trials and 414 people (SMD 0.03, -0.16 to 0.23, moderate certainty). Absence of a detected difference in small trials is not evidence of equivalence - those intervals are wide enough to hold a meaningful advantage either way. A real non-inferiority trial: COBRA (Richards, Lancet 2016) randomised 440 people against a pre-specified 1.9 PHQ-9 margin. Nobody has done that for exercise versus antidepressants. Also recorded here: adverse events included musculoskeletal injuries and depression among those exercising. The 2013 version put the robust-trial subset at -0.18 and non-significant; the 2026 update puts it at -0.46. The effect attenuates under good methods; it no longer disappears.