Association of metabolic syndrome severity with cognitive decline among Chinese older adults: evidence from two prospective cohort studies

Introduction The relationship between metabolic syndrome (MetS) severity and cognitive decline is not well understood, largely because the traditional binary diagnosis of MetS is unable to capture its continuous nature. Methods This longitudinal study included 1,191 participants aged ≥ 60 years from the Hubei Memory and Aging Cohort Study (HMACS, 2016–2024) and 2,281 participants from the China Health and Retirement Longitudinal Study (CHARLS, 2011–2020). Annual rates of cognitive decline were calculated using standardized z-scores. An age-sex-ethnicity-specific MetS score model was used to calculate the MetS score for assessing MetS severity. The Cumulative MetS score was defined as (baseline MetS score + final MetS score)/2 × follow-up duration. The association between baseline and cumulative MetS score and annual rates of cognitive decline was evaluated using linear mixed models. Results from the two cohorts were pooled through random-effects meta-analysis. Sensitivity analyses included re-clustering of MetS trajectories and exclusion of participants with younger than 65 years and baseline cognitive impairment. Results A higher baseline MetS score was associated with a faster rate of global cognitive decline [ β HMACS = –0.052 (–0.073, –0.031); β CHARLS = –0.028 (–0.041, –0.015); pooled β = –0.038 (–0.062, –0.015) z-score/year]. Participants in the highest quartile (Q4) of cumulative MetS score showed a more rapid annual decline in memory compared with those in the lowest quartile (Q1) [ β HMACS = –0.100 (–0.16, –0.041); β CHARLS = –0.118 (–0.149, –0.086); pooled β = –0.114 (–0.142, –0.086) z-score/year]. These associations remained stable across all sensitivity analyses. Discussion Greater MetS severity is linked to a faster rate of cognitive decline in Chinese older adults, underscoring the urgent need for early detection of MetS and targeted interventions to mitigate the risk of cognitive deterioration.