Elevated serum trimethylamine N-oxide (TMAO) and trimethyllysine in patients with amyotrophic lateral sclerosis (ALS): An exploratory case–control study

Trimethylamine N-oxide (TMAO) is a gut microbiota-derived metabolite implicated in protein homeostasis, inflammation, and chronic disease, but its relevance in amyotrophic lateral sclerosis (ALS) remains poorly characterized. In this exploratory pilot study, we quantified circulating TMAO and related trimethylammonium-containing compounds in a Sardinian ALS cohort using targeted LC-MS/MS. Serum samples were collected under fasting conditions from 12 ALS patients and 8 age- and sex-matched healthy controls. Median serum TMAO levels were markedly higher in ALS patients than in controls (27.9 vs. 4.0 µmol/L, P < 0.05), with substantial inter-individual variability in the ALS group (range 2.4-125.0 µmol/L). Trimethyllysine (TML) concentrations were also significantly elevated in ALS (0.43 vs. 0.34 µmol/L, P < 0.05), whereas choline, carnitine, betaine, ergothioneine, and γ-butyrobetaine levels did not differ between groups. Most ALS patients were receiving acetyl-L-carnitine (ALCAR) supplementation, suggesting that ALCAR intake may contribute to the observed metabolite profiles. Overall, these findings indicate alterations in trimethylammonium-containing compound metabolism in ALS and underscore the need for larger, well-controlled studies to determine whether such changes reflect disease-related mechanisms, treatment effects, or their interaction.