Multimorbidity, the coexistence of multiple chronic diseases within one individual, is becoming increasingly prevalent, particularly among older adults. Multimorbidity has posed substantial challenges to healthcare systems worldwide. However, there is limited evidence on the impact of multimorbidity on health outcomes; especially longitudinal research is lacking. The objective of this thesis was to investigate the association between multimorbidity and health outcomes over time by conducting three studies. Study one, a large population-based prospective cohort study of 14,573 patients examined the impact of multimorbidity on health-related quality of life (HRQoL) following total hip arthroplasty and total knee arthroplasty. A linear mixed-effects model was used to test the effects of multimorbidity and the number of chronic conditions on improvements in HRQoL. Findings show that multimorbidity adversely affected improvements in HRQoL. A higher number of chronic conditions was associated with increasing reductions in HRQoL improvements. Study two, a systematic review, assessed the longitudinal association between multimorbidity and depressive symptoms among older adults. PUBMED, EMBASE and PSYCINFO were systematically searched to identify longitudinal studies among adults aged 50 or above, published in English up to December 2020. A total of 20 studies including 57,349 participants were identified. Findings show that most studies detected a positive association between multimorbidity and more depressive symptoms over time. Study three, another large population-based prospective cohort study, analyzed three-year follow-up data of 16,919 community-dwelling older adults aged 65 and older in the Canadian Longitudinal Study of Aging (CLSA). Multivariate logistic regression models with analytic weights were built to examine the association between multimorbidity and depressive symptoms over time, and investigate the role of social support in this relationship. Results show that participants with multimorbidity had 1.3 times the odds of having depressive symptoms three years later, compared to those without multimorbidity. Social support served as a protective factor, buffering the negative impact of multimorbidity on depressive symptoms.
In conclusion, multimorbidity adversely affects HRQoL and mental health; social support may buffer the negative impact of multimorbidity on depression among older adults. These findings provide new and important evidence that can inform clinical practice, policy, and future research regarding the management of multimorbidity.