The Gut-Heart-Kidney Axis in Heart Failure: Trimethylamine N-Oxide and Beyond-A State-of-the-Art Review.

Trimethylamine N-oxide (TMAO), a gut microbiota-derived metabolite of dietary choline and L-carnitine, has emerged as a leading molecular mediator of the gut-heart-kidney axis in heart failure (HF). This state-of-the-art narrative review synthesizes evidence from 14 observational studies (13 prospective cohorts and one cross-sectional analysis), encompassing a heterogeneous range of HF settings, including established chronic HF, acute decompensated HF, incident HF in community cohorts, and subclinical myocardial injury. Across these populations, elevated circulating TMAO has been associated with adverse outcomes, including mortality, rehospitalization, and major adverse cardiovascular events, although the strength and consistency of associations vary by HF phenotype, renal function status, and population ancestry. The prognostic independence of TMAO is attenuated after adjusting for renal function in several cohorts, reflecting both obligatory renal clearance and a potentially bidirectional relationship with kidney injury. TMAO was not reduced during neurohormonal GDMT uptitration in the BIOSTAT-CHF study, suggesting that gut microbiota dysbiosis may represent a pathophysiological axis not reached by current HF treatment. The field has evolved through three thematic stages: TMAO as a single prognostic biomarker, TMAO within cardiorenal pathophysiology, and multimetabolite and multipathway risk profiling. Beyond the TMAO pathway, emerging evidence for phenylacetylglutamine (PAGln), short-chain fatty acids (SCFAs), and protein-bound uremic toxins as parallel gut-derived cardiovascular mediators supports a multidimensional metabolite profiling approach. Several cohorts suggest that selected multimetabolite panels may provide additional prognostic information beyond TMAO alone, although their composition, calibration, and external validity remain uncertain. Population-specific variation in TMAO levels and prognostic thresholds complicates universal clinical application. Without human interventional data demonstrating that lowering TMAO improves cardiovascular outcomes, TMAO remains a prognostic risk marker rather than a validated clinical target. Clinical translation requires resolving the renal confounding problem, validating multimetabolite panels across diverse populations, and conducting intervention trials targeting the gut-heart-kidney axis.