Association of smoke exposure with cognitive function trajectories among middle and old-aged adults: evidence from the China Health and Retirement Longitudinal Study.

Background: The prevalence of cognitive impairment among middle-aged and older adults remains high. While it has been proven that cigarette smoke exposure is associated with cognitive impairment, limited research has examined its relationship with the cognitive function trajectories of middle-aged and older adults. Methods: We included data on 5084 middle-aged and older adults from the China Health and Retirement Longitudinal Study (CHARLS), version D, which covers the latest surveys from 2011 to 2018. In the CHARLS, cognitive function was measured by the Chinese version of the Mini-Mental State Examination (MMSE). Individuals exposed to cigarette smoke were categorised into four levels: non-smokers, second-hand smokers, former smokers, and current smokers. We used the latent growth mixture model (LGMM) to identify the potential heterogeneity of cognitive trajectories, and an unordered multilevel logistic regression to explore the relationship between baseline cigarette smoke exposure and cognitive function trajectories. Results: We identified three cognitive trajectory groups: slow decline group (6.2%), stable group (84.6%), and rapid decline group (9.1%). After controlling for other variables, we found that current smokers were 1.429 times more likely to develop into the rapid decline group than non-smokers (odds ratio (OR) = 1.429; 95% confidence interval (CI) = 1.086-1.881). As we continued to include demographic factors as covariates, currents smokers were 1.454 times more likely to develop into the rapid decline group than non-smokers (OR = 1.454; 95% CI = 1.052-2.01). After we included social activities, drinking and health factors as covariates, current smokers were 1.414 times more likely to develop into the rapid decline group than non-smokers (OR = 1.414; 95% CI = 1.015-1.97). This meant that current smoking remained an independent risk factor for decline trajectories, even after accounting for demographics, social activities, and health factors, suggesting that smoking has a robust association with functional or health decline. Conclusions: The developmental trajectories of cognitive function among middle-aged and older adults are heterogeneous. We found that not smoking was a protective factor for cognitive function. This warrants further attention to the risk of cigarette smoking, which is a modifiable risk factor, and the subsequent adoption of interventions for smokers in order to slow down cognitive impairment and reduce its social and economic burden in the future.