Trends in multimorbidity among US adults and the correlation between physical activity and mortality: Prospective cohort study from 1999 to 2018.

Among the 39,940 participants (weighted average age: 46.84; 50.86% female), the age-standardized prevalence of multimorbidity rose significantly, from 15.87% (95% confidence interval [CI] = 14.86%-16.87%) in 1999 to 2000 to 19.96% (95% CI = 17.61%-22.32%) in 2017 to 2018.

The population's multimorbidity detection rate was 25.22%.

The all-cause mortality risks for multimorbidity were: dual-multimorbidity (hazard ratio [HR] = 1.97, 95% CI = 1.76-2.20), triple-multimorbidity (HR = 2.69, 95% CI = 2.35-3.08), quadruple-multimorbidity (HR = 3.22, 95% CI = 2.59-3.99). "Diabetes mellitus + cardiovascular disease + respiratory diseases" had the highest mortality risk (HR = 4.37, 95% CI = 2.67-7.16).