Repeated measures of physical activity before dementia diagnosis in community-dwelling older adults: a longitudinal study.

Joint modelling showed an overall association between physical activity and incident dementia across the study period.

• Hazard ratio of 0.78 (95% CI 0.69–0.88; p<0.0001) for physical activity and incident dementia using joint modelling integrating a linear mixed-effects model and a Cox model.
• 972 older adults were included with a mean age of 80.5 years (SD 7.3).
• 745 (77%) of 972 participants were female and 227 (23%) were male.
• 286 (29%) of 972 participants developed dementia, clinically diagnosed as Alzheimer's disease.

Baseline physical activity was not associated with dementia risk more than 7 years after baseline.

• HR of 1.00 (95% CI 0.69–1.45; p=0.99) for baseline physical activity and dementia risk beyond 7 years.
• This finding indicates that early physical activity measures have no detectable predictive value for dementia diagnosis in the distant future.
• Physical activity was measured biennially using multiday wrist-wearing sensor recordings.

Physical activity measured at year 6 was significantly associated with dementia risk.

• HR of 0.55 (95% CI 0.37–0.80; p=0.0021) for physical activity at year 6 and incident dementia.
• This contrasts with the null finding at baseline, suggesting that the association strengthens as measurement approaches dementia diagnosis.
• Model accuracy was described as stronger for physical activity closer to dementia diagnosis.

In a time-varying effects model, repeated physical activity measures were not associated with incident dementia except during the last 2 years before dementia diagnosis.

• A time-varying effects model was used to retrospectively examine the association of dementia with repeated measures of physical activity before dementia diagnosis.
• The association between physical activity and incident dementia was only statistically significant in the 2-year window preceding dementia diagnosis.
• Participants had a mean of 4.9 (SD 2.6) biennial physical activity measurements.

The study population was community-dwelling older adults recruited from retirement centres and subsidised long-term care facilities in northeastern Illinois.

• The Rush Memory and Aging Project (MAP) served as the source cohort.
• Mean age of participants was 80.5 years (SD 7.3).
• Physical activity level was extracted from biennial multiday wrist-wearing sensor recordings.
• Dementia diagnosis was based on neuropsychological test scores, clinical history, and examination.

The authors conclude that physical activity may be a modifiable protective factor for dementia, but the timing of beneficial effects relative to dementia onset requires further investigation.

• The narrowing of the significant association to the last 2 years before diagnosis raises questions about whether reduced physical activity is a prodromal feature of dementia rather than a causal risk factor.
• The authors state that 'further work is needed to define the timing of the beneficial effects of physical activity relative to the onset of dementia.'
• The study was funded by the National Institutes of Health.