Objectives: Chronic diseases are important predictors of self-rated health (SRH). This study investigated whether multimorbidity has a synergistic or cumulative impact on SRH. Moderation by gender and age was examined.
Methods: Data originated from the Longitudinal Aging Study Amsterdam (N=2046, aged 57-98 years). We assessed the presence of lung disease, cardiac disease, peripheral atherosclerosis, stroke, diabetes mellitus, arthritis, and cancer. SRH was measured with the question "How is your health in general?" including 5 response categories. Generalized ordered probit models were applied; possible synergism was examined by testing for nonlinearity of the association.
Results: The association between multimorbidity and SRH was nonlinear in that the effect of having a single disease was larger than the added effects of co-occurring diseases. However, from the second disease onward, each additional co-occurring disease caused cumulative declines in SRH. Only in the oldest old (85+), the impact of a single disease was similar to that of co-occurring diseases. Results were similar for men and women.
Discussion: Our findings help to improve understanding of the impact multimorbidity has on SRH: Having a single disease increases the chance of poor health more than each co-occurring disease, indicating some overlap between diseases or adaptation to declining health.