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What Causes What
MECHANISM− DECREASES

Aerobic exercise → Depression

Higher Aerobic exercise decreases Depression.

Prescribed walking or jogging reduced depression scores in randomized trials of people with major depression. The programs looked similar to sertraline. Against a placebo pill, SMILE II's HAM-D gap did not reach significance. A year later, assigned arm no longer predicted mood.

Randomized trial · Moderate · N=202

Supported by an experiment or a tightly identified design. Read the caveats.

Mechanism

The trials assign a heart-rate range and a weekly minutes target. In SMILE that meant three sessions a week, walking or jogging at 70 to 85 percent of heart-rate reserve, thirty minutes after a warm-up. Yu used outdoor walking at the WHO minimums: 150 minutes of moderate work or 75 minutes of vigorous work. The authors do not have a settled pathway. Blumenthal's group names self-efficacy, a sense of mastery, and a break from rumination, and they list candidate biology: norepinephrine, the HPA axis, serotonin turnover. Supervised SMILE II patients gained more peak VO2 than home exercisers. Remission did not. A group class is a weak account of the mood change. Brush measured reward positivity and error-related negativity. Eight weeks of aerobic work beat stretching on symptoms. Both EEG markers stayed put. A large share of the SMILE II change also appeared on the placebo pill. Attention, weekly check-ins, and the hope of being in a trial sit in those numbers. Hoffman followed the SMILE II cohort for a year after the trial ended. Assigned arm no longer predicted HAM-D or remission. Minutes of moderate or vigorous exercise that people reported on their own sat with lower HAM-D scores up to about three hours a week. The authors say they cannot tell whether exercise kept mood down or better mood kept people exercising. A later SMILE II read found that high pretreatment state anxiety shrank the depression benefit of the active arms, especially exercise. Anxiety itself is not this page.

Caveats

SMILE randomized 156 adults aged 50 and over with DSM-IV major depression to aerobic exercise, sertraline, or both. No placebo. At 16 weeks the arms did not differ on HAM-D or BDI (P=.67). Medication moved faster at first. Babyak followed the same 156 to month 10. Remitted exercisers relapsed less than the medication arm (p=.01). Self-exercise in follow-up was observational (OR 0.49). SMILE II added a placebo pill and a home-exercise arm. 202 adults, mean age 52, 153 women. Remission at 16 weeks: supervised 45 percent, home 40, sertraline 47, placebo 31 (p=.057). HAM-D versus placebo p=.23. The authors put that contrast at about 0.20. Hoffman followed 172 of those 202 for a year after the trial. Assigned arm did not predict HAM-D (p=.73) or remission (p=.96). People who reported about 180 minutes of moderate or vigorous exercise had HAM-D scores 3.1 points lower than people who reported none. The authors say they cannot tell the direction. Blumenthal 2021 reread 148 SMILE II completers. High pretreatment state anxiety shrank the depression benefit of the active arms on HAM-D (p=.004) and BDI-II (p=.02), more so in exercisers. The STAI drop is anxiety, not this page. Brush is 66 young adults with a stretching control and is available here only as an abstract. Yu is 35 people with major depression on medication, against a waitlist, also available here only as an abstract. This is a journal entry, not medical advice and it is not a reason to stop a prescribed drug.

Effect

qualitative. SMILE II remission 45%/40% vs 31% placebo (p=.057); HAM-D p=.23. Hoffman 1-year: assignment ns; Godin 180 vs 0 min -3.1 HAM-D (observational). Brush gs=0.66. Yu walking vs waitlist p<.001.

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Lifestyle & environmentAerobic exercisePsychology & behaviorDepression

Named confounders

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What Causes What. “Aerobic exercise → Depression.” https://whatcauseswhat.org/edges/e-exercise-depression (atlas updated 2026-09-02).

https://whatcauseswhat.org/edges/e-exercise-depression

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