Trimethylamine N-oxide and Homocysteine in Kidney Function and Disease: Distinct Metabolic Origins, Shared Renal Determinants and Pathophysiological Pathways.

Trimethylamine N-oxide (TMAO) and homocysteine are key biomarkers linked to cardiovascular and chronic kidney disease (CKD). Although they originate from distinct metabolic pathways, their circulating levels are jointly shaped by renal clearance, systemic metabolic status, and nutrient availability, complicating their biological interpretation. TMAO derives from gut microbiota-dependent metabolism of dietary precursors followed by hepatic oxidation of trimethylamine, whereas homocysteine is a central intermediate of one-carbon metabolism governed by remethylation and transsulfuration. Choline and betaine provide a nutritional interface between these pathways because choline can contribute to microbial TMA production and betaine acts as a methyl donor for homocysteine remethylation via betaine-homocysteine methyltransferase (BHMT). However, this interface does not establish direct metabolic interdependence between circulating TMAO and homocysteine. Declining renal function promotes the accumulation of both TMAO and homocysteine, although their renal handling and the mechanisms underlying their increase in CKD are not identical. Experimental and clinical evidence also links both metabolites to partially overlapping downstream processes, including oxidative stress, inflammation, endothelial dysfunction, and fibrotic remodeling. These shared pathophysiological responses should be distinguished from convergence of their biosynthetic pathways. This narrative review examines how renal function, metabolic regulation, and nutritional factors influence TMAO and homocysteine biology, emphasizing renal handling, analytical variability, shared downstream mechanisms, and the limitations of isolated measurements. A shift toward context-aware, multiparametric assessment may enhance their interpretation in cardiorenal research, although neither biomarker currently replaces established measures of kidney function.