Abstract
BACKGROUND: In observational studies, frailty has been strongly associated with mental disorders. However, the mechanisms underlying the association between frailty and mental disorders remain unclear. METHODS: We conducted a two-sample Mendelian randomization (MR) study to assess the causal relationship between frailty, as measured by the frailty index (FI), and ten common mental disorders. The datasets involved European ancestry individuals and included measurements of the FI (N = 175,226), schizophrenia (SCZ; N = 320,404), major depressive disorder (MDD; N = 143,265), bipolar disorder (N = 337,199), insomnia (N = 462,341), obsessive-compulsive disorder (N = 33,925), anxiety disorders (N = 463,010), autism spectrum disorder (N = 46,351), anorexia nervosa (N = 14,477), opioid-related mental and behavioral disorders (N = 215,650), and mental and behavioral disorders due to use of other stimulants including caffeine (N = 215,570). RESULTS: Two-sample MR analyses were performed using inverse variance weighting followed by various sensitivity and validation analyses. Genetically predicted SCZ (odds ratio [OR] = 1.019, 95% confidence interval [CI] 1.005-1.033) and MDD (OR = 1.211, 95% CI 1.092-1.343) had significant causal effects on FI. In the reverse MR analysis, we discovered that MDD was significantly and causally affected by FI (OR = 1.290, 95% CI 1.133-1.469). No causal links were identified between the FI and the other eight common mental disorders. In the Multivariable MR, the estimated MDD effect on FI is comparable to the univariate IVW estimate (OR = 1.298; 95% CI, 1.175 to 1.435), while the estimated SCZ effect on FI fails to be significant compared to the univariate estimate. The results of the sensitivity and validation analyses confirmed stabilization. CONCLUSIONS: Our study found evidence of a causal relationship between SCZ, MDD, and frailty and explored the underlying mechanisms.