Multimorbidity and frailty often concurrently occur among older adults.

To assess the reciprocal association between multimorbidity (condition count and patterns) and frailty and examine the mutual mediation effect of multimorbidity and frailty in their associations with mortality among Chinese older adults.

This nationwide population-based longitudinal study included 16,563 participants aged ≥65 years in the Chinese Longitudinal Healthy Longevity Survey who were surveyed in 2008 and followed up in 2011, 2014, and 2018. Frailty phenotype was assessed by the modified Fried criteria and vital status was ascertained from family members. Cross-lagged panel model (CLPM) was used to test bidirectional associations between multimorbidity and frailty. The direct and indirect effects of multimorbidity and frailty on mortality were evaluated using the combined CLPM with survival analysis.

Three multimorbidity patterns were identified: cardiometabolic diseases, cognitive-sensory disorder, and arthritis-digestive-respiratory diseases. The number of chronic conditions and cognitive-sensory disease pattern showed bidirectional associations with frailty across waves (range for β: 0.046−0.109; all P < 0.001), while cardiometabolic and arthritis-digestive-respiratory patterns unidirectionally predicted frailty change. Furthermore, frailty mediated 23%–27% of the association between multimorbidity and mortality. Only the number of conditions and cognitive-sensory disease pattern were significant mediators in the association between frailty and mortality, with the proportion of mediation ranging 4%–12%.

Multimorbidity measures including condition count and cognitive-sensory disease pattern are bi-directionally associated with frailty in older adults. These multimorbidity measures and frailty partially mediated each other’s association with mortality, with frailty acting as a more prominent pathway in the association between multimorbidity and mortality.