14/11/2025
Trimethylamine N-oxide (TMAO) is a small molecule produced in humans primarily by the gut-liver axis and has become an important biomarker, and possible mediator, of cardiometabolic and renal disease.
Dietary choline, phosphatidylcholine (lecithin), L-carnitine and betaine, mainly from red meat, eggs and dairy, are metabolised by gut microbiota to trimethylamine (TMA). TMA is absorbed and oxidised by flavin mono-oxygenase 3 (FMO3) in the liver to form TMAO.
Multiple large prospective cohort studies have found higher plasma TMAO strongly predicts cardiovascular events, including myocardial infarction, stroke and mortality, independent of traditional risk factors. While there is some mechanistic support for this, causation remains unproven in humans; TMAO may be a marker, not a driver of disease. Some marine fish naturally contain high TMAO, yet fish consumption is cardioprotective, suggesting context-dependent effects.
Vegetarian/vegan diets produce markedly lower TMAO, in part due to their gut flora modulation. Antibiotics or microbiome shifts can suppress its formation.
In an intriguing outcome, allicin, the major bioactive compound in raw garlic, was shown to significantly mitigate atherosclerosis in ApoE⁻/⁻ mice by remodelling gut microbiota and suppressing TMAO formation. Treatment reduced TMA-producing taxa such as Desulfovibrio while enriching beneficial genera including Lactobacillus and Bifidobacterium, resulting in lower plasma lipids, attenuated vascular inflammation and diminished plaque burden—highlighting a gut-TMAO-atherosclerosis mechanism. In the parallel human study, participants identified as high TMAO producers consumed 55 mL of raw garlic juice daily (around 48 mg allicin) for one week. This led to roughly 35–40 % reductions in urinary TMAO and 20–25 % reductions in plasma TMAO, with concurrent improvements in microbial diversity and an increase in health-associated taxa.
The study supports garlic’s capacity to modulate cardiometabolic risk via novel microbiome-dependent pathways. While the dose of garlic juice was quite high, lower amounts of fresh crushed raw garlic or allicin-releasing herbal products could likely have a significant impact over a longer timeframe. Garlic’s beneficial effect might be particularly marked when consumed in conjunction with a meal containing animal protein to lower the resultant spike in TMAO.
For more information see: https://pubmed.ncbi.nlm.nih.gov/35087050/