A meta-analysis of 74 studies and 4,227,297 participants, followed for a median ten years, examined physical activity and dementia by domain rather than in aggregate. High overall activity was associated with less all-cause dementia (RR 0.80), Alzheimer's disease (0.79) and vascular dementia (0.71).
The domain breakdown is where it becomes interesting. Leisure-time activity was associated with reduced risk (RR 0.76, interval 0.65 to 0.88) and household activity likewise (0.85). Occupational physical activity went the other way — RR 1.20, interval 1.01 to 1.42 — as did commuting activity, at 1.10. Dose-response relationships were non-linear for both leisure and occupational activity.
This is the physical activity paradox, now in dementia. The plausible explanations are that occupational activity is sustained, involuntary and offers no recovery, and that it travels with lower income, shift work and worse cardiovascular risk. What the study cannot do is separate those, and it says so: 93% of studies came from high-income countries, heterogeneity was very high, and certainty of evidence was rated very low to low across every domain.
- Observational throughout — reverse causation from prodromal dementia is not excluded
- The occupational finding rests on five studies, the commuting one on two
- Heterogeneity was extreme (I² 97% for all-cause dementia); pooled estimates are rough
- Certainty of evidence was rated very low to low by the authors themselves
- The message is about which activity to recommend, not whether to recommend it
The statistics, in plain English
An I² of 97% means almost all the variation between studies is real difference rather than chance, so a single pooled number conceals more than it reveals — the summary estimate is a rough average of genuinely different findings. The occupational result, RR 1.20 with an interval of 1.01 to 1.42, only just excludes 1.0 and rests on five studies; treat it as a hypothesis worth testing, not an established harm.
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