Association of joint trajectories of dietary diversity and physical activity with cognitive function in older adults: a prospective cohort study

Background: Cognitive impairment represents a major public health challenge affecting older adults' health and quality of life. Dietary diversity and physical activity are important modifiable lifestyle factors; however, previous studies have pre-dominantly relied on single time-point assessments, neglecting longitudinal behavioral changes. This study investigated the association between joint longitudinal trajectory patterns of dietary diversity and physical activity and cognitive function in older adults. Methods: Using data from four waves (2008-2018) of the Chinese Longitudinal Healthy Longevity Survey, we included 1,826 older adults aged ≥65 years with normal baseline cognitive function. Latent class growth analysis identified distinct trajectory patterns of dietary diversity score (DDS) and physical activity (PA) from 2008 to 2014, which were cross-classified into joint trajectory groups. Cognitive impairment (education-stratified MMSE cutoffs) and continuous MMSE scores in 2018 served as outcomes. Modified Poisson regression and multivariable linear regression examined associations between joint trajectories and cognitive function. Results: = 1.41, 95% CI: 1.03-1.93) showed significantly elevated cognitive impairment risks. For continuous MMSE scores, the "High DDS and Low PA" group (β = -1.12, 95% CI:-1.94 to -0.30) and "Low DDS and Low PA" group (β = -1.20, 95% CI: -2.04 to -0.36) demonstrated significantly poorer cognitive function. Sensitivity analyses confirmed these findings. Conclusions: Maintaining high dietary diversity with regular physical activity was associated with optimal cognitive function. Physical inactivity emerged as an independent risk factor for cognitive decline that favorable dietary habits could not fully offset. These findings support integrated cognitive health strategies combining dietary intervention with physical activity promotion.