To explore the relationship between body mass index (BMI) trajectories and cognitive impairment and cognitive function in the elderly in China.

Based on data from the China Health and Retirement Longitudinal Study (CHARLS) from 2005 to 2018, a sample of 6 423 elderly individuals was selected. Group-based trajectory modeling was employed to construct BMI trajectories, and Cox proportional hazards regression models were used to analyze the relationship between BMI trajectories and the risk of cognitive impairment. Generalized linear models were applied to examine the relationship between BMI trajectories and cognitive function.

Three BMI trajectory groups were identified among males: “Normal-Stable Group (57.59%)”, “Normal-Overweight Group (36.27%)”, and “Overweight-Stable Group (6.13%)”. Among females, three BMI trajectory groups were identified:“Normal-Stable Group (68.14%)”, “Normal-Overweight Group (26.70%)”, and “Overweight-Obese Group (5.16%)”. Using the Normal-Stable Group as reference, after adjusting for confounding factors, the risk of cognitive impairment for males in the Normal-Overweight Group and Overweight-Stable Group was [HR (95%CI)] 0.76 (0.64-0.91), P<0.01 and 0.65 (0.43-0.99), P<0.05,respectively. For females, the risk of cognitive impairment in the Normal-Overweight Group and Overweight-Obese Group was [HR(95%CI)] 0.74 (0.63-0.87), P<0.001 and 0.70 (0.50-0.99), P<0.05, respectively. Cognitive function scores for males in the Normal-Overweight Group and Overweight-Stable Group increased by 0.66 (0.42-0.89) points, P<0.001 and 0.78 (0.32-1.24) points, P<0.01,respectively. For females, cognitive function scores in the Normal-Overweight Group and Overweight-Obese Group increased by 1.19(0.91-1.46) points, P<0.001 and 1.44 (0.91-1.96) points, P<0.001, respectively.

There is heterogeneity in the BMI trajectories of the elderly in China, and the classification of BMI trajectories is significantly associated with cognitive impairment and cognitive function. Trajectories of increasing BMI are associated with a reduced risk of cognitive impairment, and cognitive function is generally higher in populations where BMI remains at normal levels.