Changes in prevalence and incidence of dementia and risk factors for dementia: an analysis from cohort studies.
Level 3 - non-randomized controlled study
Systematic review of non-randomized population-based cohort studies
PubMed 38942556 · doi:10.1016/S2468-2667(24)00120-8
What was done
Authors conducted an umbrella-style systematic review by searching PubMed through March 28, 2024, for systematic reviews of cohort studies assessing temporal changes in dementia prevalence or incidence. Primary studies were selected if they used population-based samples, validated diagnostic criteria, consistent methods across at least two time points in the same location, and reported age-standardized rates. Summary-level risk factor data were used to calculate population attributable fractions (PAFs) across time points and link them to changes in dementia rates.
What was found
From 5 systematic reviews, 27 primary papers were analyzed: 13 (48%) reported dementia prevalence changes, 10 (37%) reported incidence changes, and 4 (15%) reported both. Studies in Europe (n=5) and the USA (n=5) consistently demonstrated declining dementia incidence over time, whereas one study in Japan showed increases in both incidence and prevalence, and one study in Nigeria showed stable incidence. Across cohorts, PAFs for low education and smoking generally decreased over time, while PAFs for obesity, hypertension, and diabetes generally increased. The Framingham study (1997–2013) was the only cohort with sufficient data to directly link risk factor shifts to dementia trends, showing that reduced PAFs for low education and smoking were associated with falling dementia incidence. Specific quantitative effect estimates were not reported in the abstract.
Why it matters
These findings suggest that population-level improvements in formal education and smoking cessation in high-income countries have contributed to declining dementia incidence, supporting the potential of macro-level public health policies to reduce dementia risk.
Limits
The evidence is heavily concentrated in North America and Europe, with negligible representation from low- and middle-income countries (only single studies from Japan and Nigeria). Only one cohort had sufficient data to formally link risk factor changes to dementia incidence trends. The analysis relies on summary-level observational data subject to residual confounding.
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