Associations between modifiable risk factors and cognitive function: Joint modelling of longitudinal and survival data (Preprint)

BACKGROUND Stronger associations between modifiable risk factors and cognitive function have been found in younger adults than in older ones. OBJECTIVE This age pattern could be subject to mortality selection and non-ignorable missingness caused by dropouts due to death. It remains unclear whether and how these factors influence the associations between modifiable risk factors and cognitive function. METHODS Longitudinal data from Waves 1-4 (2011-2018) of the China Health and Retirement Longitudinal Study were used. Joint models of longitudinal and survival data were applied to examine the associations of socioeconomic disadvantage (rural-urban residence and education), lifestyle (smoking, alcohol drinking, and sleep duration), and health conditions (hypertension, dyslipidemia, diabetes, heart disease, stroke, and probable depression) with cognitive function and mortality. RESULTS A total of 9,562 participants aged 50 and older were included. Worse cognitive function was associated with being female (-1.669, 95% CI: -1.830, -1.511), low education (-2.672, 95% CI: -2.813, -2.530), rural residence (-1.204, 95% CI: -1.329, -1.074), stroke (-0.451, 95% CI: -0.857, -0.051), probable depression (-1.084, 95% CI: -1.226, -0.941), and current smoking (-0.284, 95% CI: -0.437, -0.133); while dyslipidemia, heart disease, and BMI of 24 and higher were associated with better cognitive function. These associations changed less than 5% when the longitudinal and survival data were modelled separately. An increase in cognitive function over age was associated with reduced mortality risk (HR: 0.418, 95% CI: 0.333, 0.537). The association between socioeconomic disadvantage and cognitive function was more evident in women than in men, while the effects of socioeconomic disadvantage and lifestyle increased with age. CONCLUSIONS Mortality selection and non-ignorable missingness caused by dropouts due to death had minimal effect on the associations between modifiable risk factors and cognitive function. The age pattern of the associations of socioeconomic disadvantage and lifestyle with cognitive function underscores the importance of tackling socioeconomic inequalities and targeting individuals of advanced age with socioeconomic disadvantage and an unfavorable lifestyle to prevent cognitive impairment and dementia.