Pressing the trimethylamine N-oxide narrative

We read with interest the correspondence titled “Gut Microbe-Generated Trimethylamine N-Oxide from Dietary Choline is Prothrombotic in Subjects”, published by Zhu et al. in the journal Circulation (1). This work builds on an emerging body of evidence which has suggested a role for the gut-derived metabolite, trimethylamine N-oxide (TMAO), in the development of cardiovascular disease (CVD). This current study is an important clinical test of hypotheses generated from innovative preclinical and observational evidence, and seeks to build upon the nature of TMAO’s relationship with CVD risk in humans. To assess this relationship, Zhu et al. enrolled 18 subjects (10 omnivores and 8 vegetarians) and provided choline bitartrate as a supplement (450 mg choline/day) for 2 months. Following choline supplementation, fasting plasma TMAO concentrations increased and were associated with increased ADP-induced platelet aggregation, an effect that was partially blunted by aspirin.

While the results of this trial are enticing and add to the evolving TMAO story, history reminds us to be cautious about the preemptive translation of this evidence into public health and clinical recommendations. The field of nutrition has seen multiple instances where promising associations, coupled to preclinical evidence and trials with surrogate outcomes, have not yielded clear evidence of benefits in large randomized controlled trials, and in some cases, have led to unintended harm (2-5). This current trial was relatively small, with no control group or random treatment allocation, and utilized a single, ex vivo surrogate outcome without established thresholds to assess thrombotic risk (6,7). Despite provocative results that appear in line with some previous animal evidence, caution is warranted in their interpretation. Indeed, not all cohort studies have found associations between choline, TMAO and CVD risk (8,9), and many have not accounted for confounding factors, such as impaired kidney function, insulin resistance and an altered gut microbiome (10). Additionally, not all animal models have found an adverse effect of TMAO on cardiovascular health. In ApoE knockout mice engineered to express human cholesteryl ester transfer protein (CETP), a protective effect of TMAO on the atherosclerotic process was observed suggesting that the lipid environment interacts with TMAO to impact CVD pathogenesis (11).

Interpretation of these findings and their relevance to dietary guidance for the public and patients remains challenging, despite widespread media oversimplification of the results of this study (12). The dose of choline supplemented in this study was relatively large (approximating recommended intake levels) and administered in addition to background dietary choline intakes. Furthermore, the form of choline utilized in this study was choline bitartrate, a water-soluble form of choline which serves as a potent substrate for microbial production of TMA at high doses; however, foods are comprised of primarily phosphatidylcholine, a relatively poor substrate for TMA biosynthesis (13-15). Whether the lower TMAO levels achieved by consumption of foods sources of phosphatidylcholine, such as eggs, affects thrombotic risk remains to be investigated in a trial. While not focused on in media coverage, extrapolation of these results to TMA/TMAO containing foods, such as fish, is particularly challenging, given the long-standing recommendation to consume fatty fish rich in long chain −3 fatty acids to reduce the risk of CVDs. In a recent study, a commonly consumed fish (cod) produced substantially higher TMAO levels than eggs and beef (15). The inverse association between fish consumption and CHD risk suggests that either TMAO is not a causal factor in disease development or that other factors in the food matrix (i.e., −3 fatty acids) may modify the potential effect of TMAO (16,17).

As animal model, epidemiological and clinical data continue to emerge regarding the role of TMAO in cardiovascular health, it appears that many factors, such as age, comorbidities, the gut microbiome, and nutrient intakes, modify the relationship between TMAO and disease risk. Whether these risks are causal and for whom they are relevant requires further evidence from randomized controlled trials. Any recommendations to modify choline, carnitine and/or TMA/TMAO intakes must be targeted at specific populations and consider the potential unintended consequences of reducing relevant food sources and other associated nutrients. For example, misrepresentation of the relationship between choline and health may have negative impacts on subsets of the population, such as pregnant women, for whom choline needs are high (18,19). Future trials employing gold standard designs with adequate power, in defined populations, using an array of pre-specified surrogate outcomes assessing thrombotic risk, and advanced statistical modeling to account for relevant confounding factors and relevant interactions will be critical to advancing our understanding TMAO’s role in cardiovascular health.

Acknowledgements

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned and reviewed by the Section Editor Xiaoyan Wang (Department of Cardiology, Zhongshan Hospital, Fudan University, Shanghai Institute of Cardiovascular Disease, Shanghai, China).

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/amj.2017.06.08). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Zhu W, Wang Z, Tang WH, et al. Gut Microbe-Generated Trimethylamine N-Oxide From Dietary Choline Is Prothrombotic in Subjects. Circulation 2017;135:1671-3. [Crossref] [PubMed]
2. Lippman SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA 2009;301:39-51. [Crossref] [PubMed]
3. Albanes D, Heinonen OP, Taylor PR, et al. Alpha-Tocopherol and beta-carotene supplements and lung cancer incidence in the alpha-tocopherol, beta-carotene cancer prevention study: effects of base-line characteristics and study compliance. J Natl Cancer Inst 1996;88:1560-70. [Crossref] [PubMed]
4. Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. N Engl J Med 1994;330:1029-35. [Crossref] [PubMed]
5. Smedberg M, Wernerman J. Is the glutamine story over? Crit Care 2016;20:361. [Crossref] [PubMed]
6. Bonello L, Tantry US, Marcucci R, et al. Consensus and future directions on the definition of high on-treatment platelet reactivity to adenosine diphosphate. J Am Coll Cardiol 2010;56:919-33. [Crossref] [PubMed]
7. Gorog DA, Fuster V. Platelet function tests in clinical cardiology: unfulfilled expectations. J Am Coll Cardiol 2013;61:2115-29. [Crossref] [PubMed]
8. Mueller DM, Allenspach M, Othman A, et al. Plasma levels of trimethylamine-N-oxide are confounded by impaired kidney function and poor metabolic control. Atherosclerosis 2015;243:638-44. [Crossref] [PubMed]
9. Meyer KA, Benton TZ, Bennett BJ, et al. Microbiota-Dependent Metabolite Trimethylamine N-Oxide and Coronary Artery Calcium in the Coronary Artery Risk Development in Young Adults Study (CARDIA). J Am Heart Assoc 2016;5: [Crossref] [PubMed]
10. Cho CE, Caudill MA. Trimethylamine-N-Oxide: Friend, Foe, or Simply Caught in the Cross-Fire? Trends Endocrinol Metab 2017;28:121-30. [Crossref] [PubMed]
11. Collins HL, Drazul-Schrader D, Sulpizio AC, et al. L-Carnitine intake and high trimethylamine N-oxide plasma levels correlate with low aortic lesions in ApoE(-/-) transgenic mice expressing CETP. Atherosclerosis 2016;244:29-37. [Crossref] [PubMed]
12. American Heart Association. Gut Bacteria May Turn Common Nutrient Into Clot-Enhancing Compound. Accessed: 30 May 2017. Available online: http://newsroom.heart.org/news/gut-bacteria-may-turn-common-nutrient-into-clot-enhancing-compound
13. Zeisel SH, Wishnok JS, Blusztajn JK. Formation of methylamines from ingested choline and lecithin. J Pharmacol Exp Ther 1983;225:320-4. [PubMed]
14. Miller CA, Corbin KD, da Costa KA, et al. Effect of egg ingestion on trimethylamine-N-oxide production in humans: a randomized, controlled, dose-response study. Am J Clin Nutr 2014;100:778-86. [Crossref] [PubMed]
15. Cho CE, Taesuwan S, Malysheva OV, et al. Trimethylamine-N-oxide (TMAO) response to animal source foods varies among healthy young men and is influenced by their gut microbiota composition: A randomized controlled trial. Mol Nutr Food Res 2017;61: [PubMed]
16. Zheng J, Huang T, Yu Y, et al. Fish consumption and CHD mortality: an updated meta-analysis of seventeen cohort studies. Public Health Nutr 2012;15:725-37. [Crossref] [PubMed]
17. Phang M, Garg ML, Sinclair AJ. Inhibition of platelet aggregation by omega-3 polyunsaturated fatty acids is gender specific-Redefining platelet response to fish oils. Prostaglandins Leukot Essent Fatty Acids 2009;81:35-40. [Crossref] [PubMed]
18. Yan J, Jiang X, West AA, et al. Maternal choline intake modulates maternal and fetal biomarkers of choline metabolism in humans. Am J Clin Nutr 2012;95:1060-71. [Crossref] [PubMed]
19. Zeisel SH. Nutrition in pregnancy: the argument for including a source of choline. Int J Womens Health 2013;5:193-9. [Crossref] [PubMed]