FNDC5 and congestive heart failure: In the multivariate regression analysis with age, irisin, indoxyl sulfate, congestive heart failure, higher indoxyl sulfate concentrations, and advanced age were associated with the risk of low handgrip strength (OR for age: 6.728, 95% CI: 1.418–31.33, p = 0.016; OR for indoxyl sulfate: 8.525, 95% CI: 1.807–40.207, p = 0.007).