Trimethylamine Oxide

Trimethylamine oxide (TMAO) is an organic compound produced by liver enzymes and also as a byproduct of intestinal microbial metabolism.  Foods such as meat, seafood, dairy, and eggs¹ contain amines such as choline, betaine, and carnitine, which are metabolized by gut microbes to trimethylamine (TMA). This then enters the circulation and eventually makes its way to the liver. There, it is oxidized by hepatic flavin monooxygenases to form trimethylamine-N-oxide (TMAO), which enters circulation and can be measured in plasma. Some TMA, rather than traveling to the liver, can be further metabolized by gut bacteria into TMAO.
TMAO was originally discovered and investigated as an osmolyte—a small molecule that contributes to the regulation of osmotic pressure in cells and tissues. It can protect native protein conformation against the denaturing effects of urea² and stabilize protein structures against the high osmotic and hydrostatic pressure found in the deepest ocean depths. Thus, it is found at high levels in deep water marine fish.

Trimethylamine N-Oxide and the Gut Microbiome

While several factors, including age, diet, disease status, and gut microbiome, can impact plasma TMAO concentrations, research has shown that intestinal microbiota metabolism may have a stronger impact on plasma levels of TMAO than recent meals³. This underscores the important role of gut microbial metabolism in influencing human health and disease. Specifically, plasma concentrations of TMAO were directly correlated with an increased Firmicutes: Bacteroidetes ratio, whose balance has been reported to be associated with health⁴. Additionally, butyrate-producing bacteria in the gut have been observed at higher levels in individuals with lower plasma levels of TMAO³. A connection between TMAO produced by the gut microbiome and the risk for several chronic diseases has been extensively reported in the literature⁵.

Trimethylamine N-Oxide and Cardiovascular Disease

Since a correlation between TMAO levels and cardiovascular disease (CVD) was first reported in 2011, numerous studies have corroborated these results, and TMAO has been proposed as a CVD biomarker⁶. In one study, elevated plasma TMAO levels were associated with an increased mortality risk from heart failure while lower levels were associated with a better prognosis⁷. Furthermore, a review of 14 metanalyses reported an association between increased circulating TMAO concentration and CVD mortality, hypertension, and major adverse cardiovascular events⁸, which often include nonfatal stroke, nonfatal myocardial infarction, and cardiovascular death.

The relationship between diet and TMAO levels with cardiovascular disease risk is well reported in the literature. Western diets high in animal source foods such as red meat are rich in TMA and thereby impact both gut microbiota composition and metabolism. Patients with hypertension harbor more gut bacteria-derived enzymes related to TMA production than do healthy controls⁸. Furthermore, microbial TMAO production has been shown to influence platelet hyper-responsiveness and blood clot formation⁹. Additionally, studies have reported a connection between diet, TMAO levels, and coronary atherosclerosis burden. Although the exact molecular mechanisms behind this relationship remain to be elucidated, multiple studies have suggested that dietary TMAO promotes atherosclerosis—which can lead to coronary artery disease if left unchecked—via a combination of poor metabolic control (specifically, glucose intolerance and insulin resistance), overexpression of inflammatory markers¹, and altered bile acid metabolism¹⁰.

Trimethylamine N-Oxide and Type 2 Diabetes

Given the connection between dietary TMAO and glucose intolerance/insulin resistance⁸ it’s no surprise that multiple studies have reported a positive association between TMAO concentrations and an increased risk for type 2 diabetes mellitus¹¹. Animal studies have begun to shed light on the mechanism behind this association, suggesting that TMAO may exacerbate glucose intolerance and hyperglycemia by blocking the hepatic insulin signaling pathway and causing adipose tissue inflammation¹². Conversely, in mouse models, silencing of flavin-containing monooxygenase-3 (FMO3), the liver enzyme that generates TMAO, decreases both TMAO and glucose levels in plasma¹³. The unexpected effect on glucose was hypothesized to occur via FMO3 acting in a process that was TMAO-independent.

Trimethylamine N-Oxide and Kidney Health and Disease

Elevated TMAO is also associated with impaired kidney function. Although the renal clearance rate of this metabolite is typically high, renal insufficiency in chronic kidney disease (CKD) reduces TMAO clearance in CKD patients, elevating plasma levels of both TMAO and choline¹⁰ and exacerbating poor renal function⁸. In a study of hemodialysis patients, those with end-stage renal disease had higher baseline levels of TMA and TMAO, with metabolomics analyses pointing to TMAO as a potential biomarker for CKD¹⁴.

These effects are most likely mediated via TMAO-induced inflammatory processes^15,16. Using a rat model of CKD, researchers observed that TMAO promotes vascular inflammation, vascular calcification, and myocardial fibrosis via activation of the NLRP3 inflammasome and NF-κB signaling pathway¹⁴.

Trimethylamine N-Oxide and Cancer

High consumption of animal products is a risk factor for colorectal cancer (CRC), and metabolomics studies have revealed altered TMAO and choline levels in CRC patients^17,18. Specifically, TMAO concentrations were higher in tumor tissues¹⁷ while plasma choline levels were decreased¹⁸.

While the precise mechanism behind this connection isn’t fully elucidated, there is some evidence that TMAO enhances VEGF-A secretion to promote CRC cell proliferation in vitro. This is supported by in vivo studies in mice, where increased systemic TMAO induced by long-term choline feeding led to the formation of new blood vessels and increased tumor growth¹⁹. Furthermore, other groups have shown that TMAO can contribute to CRC development by activating the oncogenic WNT pathway and promoting an immunosuppressive microenvironment²⁰. Studies have also suggested a microbiome link, reporting higher levels of microbial choline trimethylamine-lyase genes in CRC patients²⁰.

TMAO in Research

As of August 2023, there are over 500 citations for “TMAO” in research publications (*excluding books and documents) on PubMed. The extensive number of publications linking TMAO to various disorders related to the Western diet suggests that any researcher interested in the link between this metabolite and conditions such as type 2 diabetes, cardiovascular disease, and cancer should consider including quantitative analyses of TMAO in their study.

References

1. El Hage R, Al-Arawe N, and Hinterseher I. The Role of the Gut Microbiome and Trimethylamine Oxide in Atherosclerosis and Age-Related Disease. Int J Mol Sci 2023;24(3):2399.
2. Querio G, Antoniotti S, Gallo MP, et al. Modulation of Endothelial Function by TMAO, a Gut Microbiota-Derived Metabolite. Int J Mol Sci 2023;24(6):5806.
3. James KL, Gertz ER, and Bennett BJ. Diet, Fecal Microbiome, and Trimethylamine N-Oxide in a Cohort of Metabolically Healthy United States Adults. Nutrients 2022;14:1376.
4. Stojanov S, Berlec A, and Štrukelj B. The Influence of Probiotics on the Firmicutes/Bacteroidetes Ratio in the Treatment of Obesity and Inflammatory Bowel disease. Microorganisms. 2020;8(11):1715.
5. Constantino-Jonapa LA, Espinoza-Palacios Y, Aguirre-García MM, et al. Contribution of Trimethylamine N-Oxide (TMAO) to Chronic Inflammatory and Degenerative Diseases. Biomedicines 2023;11(2):431.
6. Zheng Y and He JQ. Pathogenic Mechanisms of Trimethylamine N-Oxide-Induced Atherosclerosis and Cardiomyopathy. Curr. Vasc. Pharmacol. 2022;20(1):29–36.
7. Lv S, Wang Y, Zhang W, et al. Trimethylamine oxide: a potential target for heart failure therapy. Heart 2022;108(12):917–922.
8. Li D, Lu Y, Li X et al. Gut microbiota–derived metabolite trimethylamine-N-oxide and multiple health outcomes: an umbrella review and updated meta-analysis. Am J Clin Nutry 2022;116(1):230–243.
9. Zhu W, Gregory JC, Hazen SL, et al. Gut Microbial Metabolite TMAO Enhances Platelet Hyperreactivity and Thrombosis Risk. Cell 2016;165(1)111-124.
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11. Zhuang R, Ge X, Zhou X, et al. Gut microbe-generated metabolite trimethylamine N-oxide and the risk of diabetes: A systematic review and dose-response meta-analysis. Obes Rev 2019;20(6)883-894.
12. Gao X, Liu X, Wang Y, et al. Dietary trimethylamine N-oxide exacerbates impaired glucose tolerance in mice fed a high fat diet. J Biosci Bioeng 2014;118(4):476–81.
13. Shih DM, Wang Z, Charugundla S, et al. Flavin containing monooxygenase 3 exerts broad effects on glucose and lipid metabolism and atherosclerosis. J Lipid Res 2015;56(1):22–37.
14. Zixin Y, Lulu C, Xiangchang Z et al. TMAO as a potential biomarker and therapeutic target for chronic kidney disease: A review. Front Pharmacol 2022;13:929262.
15. Fang Q, Zheng B, Ouyang D, et al. Trimethylamine N-Oxide Exacerbates Renal Inflammation and Fibrosis in Rats with Diabetic Kidney Disease. Front Physiol 2021;12,682482.
16. Wang Z, Bergeron N, Hazen, SL, et al. Impact of chronic dietary red meat, white meat, or non-meat protein on trimethylamine N-oxide metabolism and renal excretion in healthy men and women. Eur Heart J 2019;40(7),583-594.
17. Wang H, Wang L, Zhao YL, et al. (1)H NMR-based metabolic profiling of human rectal cancer tissue. Mol Cancer 2013;12(1)121.
18. Brown DG, Rao S, Ryan EP, et al. Metabolomics and metabolic pathway networks from human colorectal cancers, adjacent mucosa, and stool. Cancer Metab 2016;4(11).
19. Yang S, Dai H, Pan S, et al. Trimethylamine N-Oxide Promotes Cell Proliferation and Angiogenesis in Colorectal Cancer. J Immunol Res, 2022;7043856.
20. Liu Y, Lau HCH, and Yu J. Microbial metabolites in colorectal tumorigenesis and cancer therapy. Gut Microbes 2023;15(1):22203968.