The potential for dementia prevention in Japan: a population attributable fraction calculation for 14 modifiable risk factors and estimates of the impact of risk factor reductions

Background: As dementia prevalence increases globally, preventive strategies targeting modifiable risk factors have become increasingly important. In Japan, with its super-ageing society, dementia is the leading cause of increased disability-adjusted life years among older adults. This study quantified the contribution of 14 potentially modifiable risk factors for dementia in older adults using Japan-specific prevalence data. Methods: We calculated population attributable fractions (PAFs) and potential impact fractions (PIFs) using recent publicly available prevalence data from national surveys and cohort studies in Japan, and relative risks and communality weights from the 2024 Lancet Commission report on dementia. We then modelled how 10% and 20% reductions in each risk factor would affect national dementia prevalence. Findings: The weighted combined PAF for all 14 risk factors was 38.9%, indicating that nearly 4 in 10 dementia cases in Japan might be preventable. Hearing loss (6.7%), physical inactivity (6.0%), and high LDL cholesterol (4.5%) were the largest contributors. Reducing all risk factors by 10% could prevent ∼208,000 dementia cases; reducing them by 20% could prevent ∼407,000 cases. Interpretation: Dementia preventive efforts in Japan should prioritise hearing care, physical activity, and metabolic health. Japan-specific data confirmed that hearing loss is a leading contributor to dementia, underscoring the urgency to increase public awareness and access to hearing interventions. Funding: The Royal Danish Embassy in Japan, Danish Ministry of Foreign Affairs, Danish Ministry of Health, and Japan Agency for Medical Research and Development funded this study.