Integrating TMAO into the pathogenesis of obesity and type 2 diabetes: a mini review

TL;DR

This mini review explores how TMAO, a gut-derived metabolite, may contribute to obesity and type 2 diabetes, and highlights differences in findings between children and adults.

Contribution

The paper introduces a novel comparison of TMAO's role in pediatric and adult populations with obesity and diabetes.

Findings

Higher TMAO levels are linked to obesity, type 2 diabetes, and cardiovascular complications.

TMAO may reflect hepatic insulin resistance, connecting it to diabetes and cardiovascular risk.

Findings vary across studies due to differences in controlling for confounders and population characteristics.

Abstract

Increasing circulating levels of trimethylamine N-oxide (TMAO), a metabolite originating from both dietary sources and microbial metabolism in the gut, have drawn significant attention for their possible contribution to cardiometabolic disorders, such as type 2 diabetes and carotid intima–media thickening in individuals with excess weight. Yet, evidence from longitudinal and retrospective investigations remains inconsistent, with studies often reaching divergent conclusions. Higher circulating TMAO levels have been observed in people with overweight, obesity, and type 2 diabetes who subsequently develop cardiovascular complications, as well as in diabetic individuals who experience renal impairment. Still, the strength and consistency of these links vary considerably between cohorts and are shaped by how thoroughly studies control for underlying confounders. Given that insulin modulates FMO3 activity within hepatocytes, circulating TMAO concentrations might reflect underlying hepatic insulin resistance, offering a possible mechanistic link to its reported associations with diabetes and cardiovascular risk. This mini review summarizes current evidence on TMAO in obesity and type 2 diabetes, highlighting the novel comparison between pediatric and adult findings. This approach may help clarify how TMAO-related mechanisms evolve across the lifespan and inform future risk assessment and prevention strategies.

Tables

Table 1. Key biological and methodological confounders influencing circulating and urinary TMAO interpretation (8, 35).

Confounder	Influence on TMAO levels	Relevance for interpretation
Dietary intake	Acute and habitual consumption of fish, seafood, red meat, eggs, and dairy directly affects circulating and urinary TMAO	May reflect recent exposure rather than chronic metabolic status
Gut microbiota composition	Determines the capacity for TMA production from dietary precursors	Contributes to inter-individual variability across age groups
Renal function	Reduced glomerular filtration rate leads to higher systemic TMAO concentrations	Major determinant of circulating TMAO; requires adjustment in analyses
Age	Pediatric vs adult populations show different metabolic and microbiota profiles	Influences interpretation across the life course
Gender	Hormonal and metabolic differences may modulate FMO3 activity and TMAO levels	Potential source of biological heterogeneity
Fasting status	Non-fasting samples may reflect postprandial TMAO fluctuations	Affects comparability between studies
Sample type	Plasma/serum vs urine reflect systemic exposure vs excretion	Determines the clinical and research utility of TMAO
Preanalytical handling	Storage temperature, processing delay, freeze–thaw cycles	Impacts measurement reliability

Full text

Introduction

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Trimethylamine N-oxide (TMAO) has emerged as a key metabolite at the intersection of habitual diet, host metabolism, and gut microbiome activity, gaining increasing attention as a potential contributor to cardiometabolic disease risk (1, 2). Dietary nutrients commonly consumed in Western-type dietary patterns, particularly L-carnitine and choline, undergo microbial conversion in the intestine to trimethylamine (TMA) (3, 4), which is subsequently oxidized in the liver by flavin-containing monooxygenase 3 (FMO3) to form TMAO (5, 6). This metabolic pathway represents a fundamental mechanistic link between diet-driven microbial metabolism and systemic metabolic regulation, with growing relevance for nutrition-based therapeutic strategies (1, 7).

Accumulating evidence has linked elevated circulating TMAO concentrations to major metabolic disorders, including obesity and type 2 diabetes mellitus (8, 9). Proposed mechanisms include disturbances in lipid metabolism, impairment of insulin signaling, and promotion of chronic low-grade inflammation (10, 11). Several adult studies have reported positive associations between TMAO levels and body mass index, visceral adiposity, and markers of arterial stiffness in individuals with excess weight (12, 13). Together, these observations have positioned TMAO as both a potential biomarker of metabolic dysfunction and a candidate mediator involved in the pathophysiology of cardiometabolic disease, although the relative contribution of these roles remains under debate.

Importantly, the current body of evidence is heavily weighted toward adult populations, while data in children and adolescents remain limited and fragmented (14). This imbalance is particularly relevant in the context of obesity, which affects both adults and younger age groups and continues to rise globally as a major public health concern (15). Early metabolic alterations, including insulin resistance, endothelial dysfunction, and subclinical vascular changes, have already been documented in pediatric populations with obesity (12). However, whether TMAO-related pathways are similarly activated in children, or whether they evolve progressively across the life course, remains insufficiently explored.

In this context, the direct comparison of pediatric and adult data represents a critical and underdeveloped area of investigation. Evaluating TMAO concentrations and associated metabolic features across age groups may offer unique insights into the temporal dynamics of diet–microbiota–host interactions, helping to clarify whether TMAO reflects early metabolic vulnerability or predominantly mirrors cumulative exposure to cardiometabolic risk factors over time (16). Such a life-course perspective is essential for understanding the potential role of TMAO in the early origins of metabolic disease and its utility in risk stratification and prevention strategies.

Accordingly, this mini review synthesizes current evidence on the involvement of TMAO in obesity and type 2 diabetes, with a deliberate and novel emphasis on contrasting findings from pediatric and adult studies. By integrating data across age groups, this review aims to highlight both shared and distinct patterns in TMAO-related metabolic alterations and to identify gaps that warrant further longitudinal and mechanistic research.

From a mechanistic standpoint, TMAO production occurs along the diet–microbiota–liver axis. The process is initiated by ingestion of dietary precursors containing trimethylamine moieties, including choline, phosphatidylcholine, betaine, and L-carnitine, derived primarily from eggs, dairy products, red meat, and certain fish and seafood (3, 17, 18). Within the intestinal lumen, anaerobic bacteria convert these compounds into TMA through well-characterized enzymatic pathways (4, 19). Choline metabolism is mediated by the choline TMA-lyase CutC, activated by the radical-SAM enzyme CutD, while L-carnitine conversion involves the Rieske-type carnitine monooxygenase/reductase system CntA/CntB (20, 21). Additional pathways, including γ-butyrobetaine utilization and homologous gene clusters, contribute variably depending on the host’s microbial composition (19, 20). After intestinal absorption, TMA is transported via the portal circulation and oxidized predominantly by hepatic FMO3 to form TMAO (4, 22). FMO3 expression is influenced by genetic factors, hormonal regulation, and nutritional status, contributing to inter-individual variability in circulating TMAO levels observed in both adults and children (23, 24). Notably, fish consumption provides not only precursors but also preformed TMAO, resulting in marked postprandial increases independent of microbial metabolism (25, 26). Renal excretion represents the primary elimination pathway, making kidney function a major determinant of systemic TMAO exposure (27, 28). Collectively, these mechanisms underscore the complex interplay between diet, gut microbiota, hepatic metabolism, and renal clearance in shaping the TMAO phenotype across different stages of life.

Assessment of TMAO: serum versus urinary measurements

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Quantification is performed predominantly by isotope-dilution liquid chromatography–tandem mass spectrometry, employing a deuterated internal standard (29, 30). This approach ensures accuracy and correction for matrix effects; separation is frequently achieved on hydrophilic-interaction liquid-chromatography columns for highly polar analytes (31). In addition, analytical validations have demonstrated low limits of detection, excellent linearity, and adequate analyte stability at −80 °C and after multiple freeze–thaw cycles (inter-cycle coefficient of variation < 10%) (29, 31, 32).

Plasma TMAO is useful for etiological research and for cardiometabolic risk prediction (33). Its concentrations are influenced to some extent by recent dietary intake, most notably elevated after consumption of fish or seafood, by fasting status, and, most importantly, by renal function, a major driver of inter-individual variability (26, 34). Accordingly, blood sampling is recommended in the fasting state after at least eight hours without food, and results should be interpreted with adjustment for the estimated glomerular filtration rate (35, 36). The latter correlates inversely with TMAO (higher concentrations with reduced renal function), necessitating rigorous control for confounding by renal impairment when interpreting results (37). Beyond renal function, circulating TMAO levels are also modulated by habitual dietary patterns, gut microbiota composition, and host-related factors such as age and sex, which should be considered when comparing results across populations or study designs.

Urinary TMAO quantifies excretion and recent exposure, functioning as a sensitive biomarker of short-term dietary intake; concentrations rise consistently after fish consumption (25, 38). Twenty-four-hour urine collections provide more accurate estimates of total body load than spot samples. In practice, creatinine-normalized spot urine is more feasible but more susceptible to variation in hydration status; therefore, for diet–biomarker studies the twenty-four-hour collection remains the reference standard, whereas spot sampling is useful for population surveillance or for repeated intervention monitoring (39–41). Interpretation of urinary TMAO similarly requires consideration of dietary intake, renal handling, and intra-individual variability related to hydration and collection protocol.

Nevertheless, a range of preanalytical and standardization factors, such as the interval to processing, the temperature at which the specimen is stored, and the number of freeze–thaw cycles, can influence the measurement outcome. Under standard laboratory conditions, both plasma and urinary TMAO appear stable. However, meaningful comparison across studies requires harmonized procedures, including an explicit choice of specimen type, appropriate storage conditions, consistent reporting of units, and the universal use of isotope-labeled internal standards. Careful accounting for biological and methodological confounders is therefore essential to ensure valid interpretation of TMAO as a metabolic biomarker.

Accordingly, plasma or serum TMAO is preferable when the objective is to characterize systemic exposure and to support cardiometabolic risk assessment or prevention, whereas urinary TMAO, ideally from a twenty-four-hour collection, or alternatively from spot urine normalized to creatinine offers greater sensitivity to recent dietary intake as well as to total excretion. The analytical gold standard remains isotope-dilution liquid chromatography, tandem mass spectrometry, and broader adoption of standardization guidelines would likely enhance reproducibility at the population level (29). The major biological and methodological confounders influencing circulating and urinary TMAO interpretation are summarized in Table 1.

Mechanistic pathways underlying the cardiorenal-metabolic effects of TMAO

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TMAO has been most extensively investigated in relation to cardiovascular diseases (42). Elevated circulating levels have been associated with atherosclerosis, myocardial infarction, stroke, and heart failure (43–45). Mechanistic evidence suggests that TMAO may contribute to impaired cholesterol metabolism, endothelial dysfunction, foam cell formation, and enhanced platelet reactivity, thereby promoting a pro-atherogenic and pro-thrombotic environment (46, 47). Prospective cohort studies and meta-analyses indicate that elevated TMAO levels predict recurrent cardiovascular events and mortality independently of traditional risk factors, and in populations with heart failure, they have been consistently linked to poorer prognosis (43). Although these associations are robust, causality remains under debate, and ongoing clinical trials targeting microbial TMA generation or hepatic FMO3 activity aim to clarify the therapeutic potential of this pathway (48).

In chronic kidney disease (CKD), the interplay between TMAO and renal function appears to be bidirectional (49). Impaired renal clearance contributes to systemic accumulation of TMAO, while experimental models indicate that elevated concentrations may, in turn, exacerbate renal injury by promoting interstitial fibrosis, oxidative stress, and low-grade inflammation (50–52). Observational data consistently reveal higher circulating TMAO levels in individuals with CKD, and quantitative syntheses report associations with accelerated decline in estimated glomerular filtration rate (eGFR) and increased all-cause mortality (53, 54). This dual nature: TMAO as both a surrogate marker of reduced excretory capacity and a potential pathogenic mediator, poses interpretative challenges (25). Furthermore, variability in dietary choline and carnitine intake, interindividual differences in gut microbiota composition, and methodological heterogeneity across studies limit comparability and causal inference (55). These complexities underscore the necessity of well-controlled prospective trials to determine whether therapeutic modulation of TMAO generation or clearance could mitigate renal functional decline (56, 57).

The relationship between TMAO and metabolic disorders has also gained considerable scientific attention (58). Individuals with type 2 diabetes (T2D) frequently exhibit higher circulating TMAO concentrations compared with healthy controls (59, 60) and several prospective cohorts have reported an increased risk of incident T2D associated with elevated baseline TMAO levels (61, 62). Proposed mechanisms include impaired insulin signaling, altered bile acid metabolism, enhanced hepatic gluconeogenesis, and chronic low-grade inflammation, all consistent with the metabolic dysfunction characteristic of obesity (63). However, findings across studies remain partly inconsistent, as some cohorts have reported null associations, suggesting that dietary patterns, genetic variability in FMO3 activity, and interindividual differences in gut microbiota composition may modulate these relationships. Overall, current evidence supports TMAO as a biomarker of metabolic dysregulation rather than a firmly established causal mediator (64).

Evidence linking TMAO to cancer remains limited and inconsistent. A large population-based study found no association between serum TMAO concentrations and overall cancer incidence, arguing against a generalized carcinogenic role (65). Nonetheless, site-specific associations have been reported, particularly for distal colorectal cancer, where higher levels of TMAO and its precursors have been correlated with increased risk (66). Preclinical data further suggest that TMAO may promote tumor cell proliferation and migration in prostate cancer through mechanisms involving oxidative stress and MAPK signaling pathways; however, these findings are preliminary and require validation in prospective human cohorts (67). Collectively, current evidence indicates that the relationship between TMAO and malignancy is likely context-dependent rather than universal (68).

Emerging research has also implicated TMAO in neurological and inflammatory disorders (69). Elevated circulating TMAO levels have been associated with a greater burden of white matter lesions, cognitive decline, and poorer functional outcomes following ischemic stroke (70). Experimental studies demonstrate that TMAO can trigger microglial activation and neuroinflammatory responses, providing a plausible mechanistic basis for these associations (71). More broadly, TMAO appears to modulate systemic inflammatory pathways, with reported links to hypertension, vascular stiffness, and circulating inflammatory biomarkers (72). These findings extend the potential pathophysiological relevance of TMAO beyond cardiometabolic disease; however, human evidence remains limited and often heterogeneous (3). Further longitudinal studies with standardized assessment protocols are warranted to elucidate the role of TMAO in neurological and inflammatory conditions (68).

Circulating TMAO in obesity

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In pediatric cohorts, circulating and urinary TMAO have begun to be characterized, with several cross-sectional studies reporting higher serum TMAO concentrations in children with obesity compared to normal-weight controls, along with positive correlations with adiposity indices such as BMI, waist circumference, and waist-to-height ratio (73). A multicenter Romanian study further identified TMAO as an independent predictor of these anthropometric parameters and linked elevated levels to markers of subclinical vascular injury, suggesting that microbial methylamines represent measurable obesity-related exposures from early life (12). Urinary TMAO also reflects habitual dietary patterns in children, particularly the intake of fish, meat, and eggs and has been shown to decrease following lifestyle interventions in prepubertal children with obesity (73). However, it should be noted that most available pediatric data are derived from cross-sectional analyses with relatively small sample sizes and limited longitudinal follow-up, which constrains causal inference and the assessment of long-term metabolic trajectories. These findings support the concept that pediatric TMAO levels are influenced by both diet and gut microbiota composition and may be responsive to behavioral modification (12, 74).

Links between TMAO and inflammation, lipid metabolism, vascular stiffness, and hepatic alterations are increasingly reported in younger populations, although sample sizes remain smaller than those in adult studies (16). Elevated TMAO levels in children with obesity have been shown to co-vary with carotid intima–media thickness, pulse wave velocity, and central blood pressure, findings that parallel evidence of early arterial stiffening as a subclinical cardiovascular phenotype of risk in pediatric obesity (12). Some epidemiological datasets suggest that TMAO precursors (e.g., choline and betaine) may correlate more strongly than TMAO itself with adverse inflammatory and cardiometabolic profiles in children, highlighting nuances in biomarker interpretation and pathway specificity (14). Pediatric hepatic steatosis, currently referred to as MAFLD is highly prevalent in obesity; however, direct pediatric evidence linking TMAO to liver disease defined by biopsy or elastography remains scarce (75). Contemporary syntheses and consensus statements on MAFLD emphasize its high prevalence and metabolic determinants without establishing a specific causal role for TMAO (32).

In adults, the evidence base encompasses both cross-sectional and longitudinal investigations (58). Multiple cohort studies and meta-analyses have reported elevated TMAO concentrations in individuals with type 2 diabetes and metabolic syndrome, as well as prospective associations with incident diabetes and adverse cardiovascular outcomes (60, 76). Diet represents a major determinant of circulating TMAO levels: chronic consumption of red meat increases systemic TMAO through enhanced microbial and host production, whereas predominantly plant-based dietary patterns are generally associated with lower levels, underscoring exposure variability across studies (77). Overall, adult data support TMAO as an enriched biomarker of cardiometabolic risk, although causal inference remains limited pending interventional trials directly targeting microbial TMA generation or hepatic FMO3 activity (57).

The emerging evidence positions TMAO as an important metabolic intermediary that connects higher BMI with metabolic impairment and cardiovascular pathology (58). Elevated circulating TMAO concentrations show a robust relationship with central adiposity, supporting its role in the cascade of obesity-associated metabolic alterations. Its interaction with resistin points to a combined contribution to insulin resistance and cardiometabolic complications, underscoring their relevance in evaluating overall metabolic risk (76). Moreover, TMAO has been identified as an independent predictor of obesity, with a significant interaction effect that implies a more intricate, interdependent relationship rather than isolated action (12, 76). This interplay suggests a synergistic contribution to obesity susceptibility and highlights the value of jointly assessing these biomarkers for enhanced cardiometabolic risk prediction.

Circulating TMAO in type 2 diabetes

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Over recent years, numerous studies have examined the relationship between TMAO and type 2 diabetes mellitus, proposing its involvement both as a marker of metabolic dysfunction and as a potential contributor to disease pathophysiology. Associations have been reported between circulating TMAO concentrations and reduced insulin sensitivity, reflected by higher HOMA-IR values, elevated HbA1c levels, increased waist circumference, and greater central adiposity (32). These observations support the relevance of TMAO as an indicator of adverse metabolic profiles in individuals with excess body weight and insulin resistance. However, existing evidence remains heterogeneous, and it is not yet clear whether elevated TMAO represents a causal mediator of glucose dysregulation or predominantly reflects upstream influences such as dietary intake, renal function, and gut microbiota composition. This distinction is critical for clinical interpretation, as it determines whether TMAO should be viewed primarily as a risk marker or as a therapeutic target in the prevention and management of type 2 diabetes (32).

Moreover, one of the most robust observational studies demonstrated that individuals with elevated serum TMAO levels had more than twice the risk of developing diabetes compared to those with lower TMAO concentrations, independent of confounding factors and FMO3 gene polymorphism (78). These findings were supported by the prospective Guangzhou Nutrition and Health Study, in which Li et al. followed middle-aged and older adults for nearly nine years and observed that individuals with higher serum TMAO levels had a significantly increased risk of developing type 2 diabetes, as well as a progressive rise in fasting blood glucose over time, even after adjustment for multiple confounding factors (8). These findings were supported by the prospective Guangzhou Nutrition and Health Study, in which Li et al. followed middle-aged and older adults for nearly nine years and observed that individuals with higher serum TMAO levels had a significantly increased risk of developing type 2 diabetes, as well as a progressive rise in fasting blood glucose over time, even after adjustment for multiple confounding factors (35). As evidence connecting TMAO with the onset of diabetes continues to accumulate, though not always consistently, interest has shifted toward determining whether TMAO can serve as a reliable biomarker for future diabetes risk (11). Longitudinal cohort investigations, which overcome many of the limitations of cross-sectional designs, have provided insight into the temporal link between circulating TMAO at baseline and the later emergence of diabetes. For type 2 diabetes specifically, two independent groups documented that individuals with elevated baseline TMAO exhibited a greater likelihood of developing the disease over time (8, 79). Huang et al. further demonstrated that not only initial TMAO concentrations but also their longitudinal trajectories predicted incident type 2 diabetes (79).

Conversely, even though TMAO remained associated with parameters of insulin resistance, including HOMA index and fasting insulin, other prospective analyses failed to detect such a relationship (35, 80). Adding another layer of complexity, Papandreou et al. observed an inverse association in which higher TMAO concentrations appeared to correlate with a reduced risk of developing diabetes in the future (34). This unexpected result underscores the intricate physiology of TMAO and raises the possibility that context-dependent or compensatory mechanisms may modulate its metabolic effects. Moreover, the pronounced intra- and inter-individual variability in TMAO levels documented among patients with diabetes may further contribute to the heterogeneity reported across studies (81). Friedrich et al. offered a potential explanation for these divergent findings, showing that the increased diabetes risk linked to higher TMAO occurred exclusively in women, with no parallel effect observed in men (82).

Evidence linking TMAO to diabetes dates back to 1998, when Messana et al. first reported that individuals with diabetes exhibited markedly higher urinary TMAO levels, a finding that persisted regardless of metabolic control, glucosuria, or HbA1c values (83). Multiple investigations expanded these observations, documenting significant associations between circulating TMAO concentrations and diabetes across several types of cohorts, including large population-based samples, as well as studies enrolling patients with established cardiovascular conditions (84, 85). Although observational research cannot definitively establish causality, consistently identifying biomarkers that track with disease across diverse populations remains fundamental for defining potential therapeutic targets and improving risk stratification.

Future perspectives

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Although the adult literature has established a consistent link between TMAO, obesity, and type 2 diabetes, pediatric evidence remains fragmented and preliminary, with heterogeneous findings. Most studies conducted in children are cross-sectional, based on small sample sizes, and often lack methodological standardization in biospecimen collection and analysis (fasting status, adjustment for renal function, or recent dietary intake). The absence of longitudinal data limits our ability to determine whether TMAO acts as an early determinant of metabolic disturbances or merely reflects concurrent dietary and microbial exposures. Furthermore, age-specific factors, such as puberty, metabolic plasticity, and gut microbiome maturation, add layers of complexity and highlight the need for age-tailored research.

In this context, well-powered prospective, multicenter studies with repeated measures and comprehensive phenotyping (including vascular, inflammatory, hepatic, and body composition parameters) are essential to clarify the longitudinal dynamics of TMAO throughout childhood and adolescence. Such designs would allow for the evaluation of TMAO as a predictive biomarker for the future development of cardiometabolic pathologies, including insulin resistance, arterial stiffness, and early endothelial dysfunction. Incorporating TMAO into risk prediction models, alongside established metrics such as BMI, lipid profile, HOMA-IR, and inflammatory markers, could improve early risk stratification and guide preventive interventions well before overt disease onset.

Moreover, nutritional and microbiota-targeted interventions could test the causal hypothesis that reducing TMAO concentrations positively influences cardiometabolic health trajectories. In the long term, TMAO may evolve from a passive biomarker of metabolic exposure to an active predictive and therapeutic target, enabling more refined strategies for early prevention and personalized management of metabolic and cardiovascular disease. Integrating pediatric and adult data is therefore crucial to building a comprehensive, life-course perspective on TMAO biology and its potential role in shaping lifelong cardiometabolic risk.

Conclusions

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Integrating evidence from both pediatric and adult populations adds substantial value to understanding the complex role of TMAO in the pathogenesis of obesity and type 2 diabetes mellitus. The pediatric–adult comparison highlights a clear life-course gradient, in which elevated TMAO levels in children and adolescents appear to reflect early, potentially modifiable metabolic alterations closely linked to dietary exposure and gut microbiota composition. In contrast, adult data consistently associate higher TMAO concentrations with visceral adiposity, chronic low-grade inflammation, impaired metabolic regulation, and established cardiometabolic complications. This age-dependent pattern underscores the importance of considering developmental stage when interpreting TMAO-related findings.

From a translational perspective, these observations support the concept that TMAO may function primarily as an early biomarker of metabolic vulnerability in pediatric populations, while in adults it may increasingly reflect cumulative metabolic burden and disease progression. Distinguishing between these roles is central to clinical interpretation and may inform the timing and targets of preventive strategies. Importantly, the life-course framework presented in this review suggests that early modulation of diet–microbiota interactions could influence long-term cardiometabolic trajectories.

Given the escalating global burden and progressively earlier onset of obesity and type 2 diabetes, there is a pressing need for biomarkers that capture both metabolic disturbance and responsiveness to intervention. Positioned at the intersection of diet, gut microbiota, hepatic metabolism, and renal clearance, TMAO represents a promising candidate for risk stratification and personalized prevention strategies. However, its transition from an associative marker to a clinically actionable tool will require well-designed longitudinal and interventional studies, particularly those addressing causality, key confounders, and age-specific effects. Such evidence will be essential to define the future clinical relevance of TMAO in the prevention and management of metabolic disease across the lifespan.