The World Health Organization (WHO) defines metabolic syndrome (MetS) as a pathological state consisting of obesity, insulin resistance, high blood pressure, elevated lipid levels, and waist-to-hip ratio (WHR); MetS occurs if at least three of the above-mentioned parameters are observed (1). It is estimated that 34% of Americans suffer from metabolic syndrome (2). Based on data from the Tehran Lipid and Glucose Study (TLGS), the prevalence of metabolic syndrome in adult adolescents in Iran was 24% in women and 42% in men (3). In addition, the whole incidence of metabolic syndrome in individuals 20 years of age and older was found to be 550.9 per 10,000; for men, it was 794.2 per 10,000, and for women, it was 443.5 per 10,000 (4). Numerous variables, such as chronic inflammation, autonomic disorders, and oxidative stress, are linked to the etiology of MetS (5). Several lines of recent scientific research suggest that alterations in the gut microbiota play a crucial role in the pathogenesis of MetS (6, 7). With the extensive westernization of lifestyle, changing the gut microbiota composition (dysbiosis) and its metabolites such as Trimethylamine N-oxide (TMAO), as well as affecting cellular pathways like the Tryptophan (TRP) pathway to produce kynurenine (KYN), has received more attention than previously due to their possible roles as an early predictor of cardiovascular disease (CVD) and MetS (8–10).

Trimethylamine N-oxide is an organic compound, gut microbiota-derived metabolite, which has recently been found as a new potentially important reason for increased atherosclerosis and MetS (11). Of all the pro-atherosclerotic mechanisms postulated for TMAO the most important are, a rise in vascular inflammation, the platelet hyper-responsiveness, the blockage of reverse cholesterol transport, reducing high-density lipoprotein cholesterol (HDL) in the liver, and affecting bile acids metabolism (12, 13). So TMAO may promote dyslipidemia via regulating cholesterol balance (14). Several systematic reviews and meta-analysis studies found a dose-dependent relationship between circulating levels of TMAO and elevated cardiovascular risk and mortality in humans (15–18). However, there is controversy about the specific role of TMAO in the pathogenesis of MetS, as a collection of cardiometabolic risk factors consisting of abdominal obesity, disrupted lipid profile, high blood sugar, and hypertension which predispose individuals to CVD (11). The kynurenine pathway (KP), which is known as the TRP degradation pathway, can be influenced by the gut microbiota (19). TRP is an exogenous amino acid that is metabolized to KYN by indoleamine 2, 3-dioxygenase (IDO) (20). Inflammatory factors like LPS-derived pathogenesis microorganisms or interleukin (IL-6) can induce IDO activation (21). KP appears to be involved in the development of many chronic inflammatory metabolic disorders, including Mets, and atherosclerosis, all of which are commonly recognized as risk factors for CVD (22–24). TRP degradation via KP has been reported to increase in patients with ischemic heart disease and atherosclerotic lesions (23). Furthermore, it has been shown that elevated IDO activity and KYN are positively correlated with overweight and dyslipidemia (23, 25). However, studies on the relationship between the KYN pathway and dyslipidemia are still ambiguous and contradictory. For example, in an experimental study on IDO-deficient mice, an increase in TG levels was observed along with a decrease in HDL (26), while in another animal study, it was observed that one of the downstream metabolites of the KP (3-hydroxy anthranilic acid) by decreasing hepatic fat accumulation, lowered plasma levels of triglyceride (TG) and cholesterol and improving atherosclerosis in LDLR^–/– mice (27).

Altogether, the literature suggests a connection between the KP pathway and TMAO and some components of MetS. However, most of these data derive from experimental research in animals, and little is known about its relevance in human cohorts. Further research needs to clarify the pathway and cellular-molecular mechanism in this context which may lead one to consider whether plasma KYN and TMAO can be a new risk factor for predicting MetS in predisposed persons. So, the primary aim of the current study is the measurement of gut microbiota metabolites such as KYN and TMAO and the main goal is to investigate the mentioned metabolites association with MetS and its components in adults.

To the best of our knowledge, this survey is the first study that investigated two types of metabolites obtained from gut microbiome (Serum KYN and TMAO) with MetS and its components in the Iranian population. The present study showed that there is a positive correlation between TMAO and MetS. In the findings of our study, it was observed that high levels of TMAO can increase the odds of hypertriglyceridemia and abdominal obesity and can have a reducing effect on low HDL. In the case of KYN, the odds of MetS could be increased after adjusting for confounders, in addition, individuals with high levels of KYN were more likely to have abdominal obesity. As one of the basic risk factors for cardiovascular diseases and all-cause mortality, MetS plays an important role in people’s health (36). Despite its widespread in different countries, including America, as well as its rising graph in Iran, it has attracted a lot of attention in recent years (36–38). The findings of the present study showed that people with MetS had higher DBP, TG, LDL, TC, FBS, WHR, WC, and HC than people without MetS. People with MetS also had lower HDL than the other group without this disorder. In another study, people with MetS had higher BMI, DBP, TG, SBP, body weight, FBS, WC, and lower HDL compared to healthy people(39). Today, studies are reporting on the role of metabolites obtained from the gut microbiome in the pathogenesis of MetS, and TMAO and KYN are among the metabolites investigated in this study (6, 7).

TMAO is a metabolite that is produced by the microbial metabolism of choline from animal food sources and L-arginine from meat. The L-arginine metabolism pathway leads to the production of two other intermediate metabolites, namely crotonobetaine, and γ-butyrobetaine, which ultimately cause TMAO (40). In the present study, TMAO levels in people with MetS were about 51.49 (pg/mL), and in people without this disease, it was 37.50 (pg/mL). In another study, the circulating levels of TMAO in people with MetS were about 10.65 ± 1.62 (μM), and in people without this disease, it was reported as 6.82 ± 3.17 (μM) (11). In our study, we observed that levels higher than 30.39 pg/mL could increase the odds of MetS. In the other study, the acceptable cut-offs for circulating levels of TMAO for predicting the presence of MetS was ≥ 8.74 μM (11). (The reason for the different expression of the measurement units is the use of various kits in the measurement of TMAO and KYN and as the level of TMAO depends on various factors including age, gender, race species of the microbiome, and food intake, maybe the level of TMAO in our study was low). In the case of TMAO, there was a significant positive correlation between high levels of TMAO and MetS, which increased odds after adjusting for confounders, and also, there was a positive correlation between TMAO and abdominal obesity and also with low HDL. The studies conducted on TMAO reveal the destructive effects of this metabolite, such as the formation of macrophage foam cells (41), endothelial dysfunction (42), vascular inflammation and activation of inflammation (43–46), increased platelet reaction and thrombosis (47, 48), and reduced cholesterol reverse transport (49). Meanwhile, diet plays a key role in TMAO production. In a study conducted in 2023 by Meng Wang et al., more meat consumption can increase the risk of atherosclerotic cardiovascular disease through increasing metalloids such as TMAO (40). According to the studies conducted on TMAO, this metabolite can cause insulin resistance by blocking the hepatic insulin signaling pathway and also by causing inflammation in adipose tissue in mice, which are themselves one of the components of MetS (50). Meanwhile, insulin signaling suppresses flavin-containing monooxygenase 3 (FMO3), which is the enzyme that produces TMAO (51). So in people with insulin resistance or obese people, the level of the FMO3 enzyme increases(51, 52). Moreover, FMO3 can induce the production of FoxO1, which is responsible for regulating some gluconeogenic genes and as a result, adjusting glucose production in the liver. The findings indicate that the suppression of FMO3 through the inhibition of FoxO1 can prevent blood fat, hyperglycemia, and atherosclerosis (51). According to the studies, high levels of TMAO can increase lipid metabolism disorders and inflammatory response (43, 47, 50). Based on the evidence, TMAO can be a factor in the occurrence of obesity as well as the basis of obesity-related disorders such as body weight, BMI, and visceral fat index (VAI), and VAI itself can reduce insulin sensitivity (11, 52–54). With the help of new findings, TMAO is considered as a prospective indicator for detecting MetS (11). The evidence of the present study shows that there was a positive and statistically significant correlation between TMAO levels and hypertriglyceridemia. In a study conducted by Lin Ding et al in 2018, mice with the apoE genotype were fed a diet containing TMAO for 8 weeks. The result of the study was the fat in the serum of mice, and the possibility of inhibiting the synthesis of hepatic bile acid is considered the reason for this increase in fat (55).

Kynurenine is a metabolite that is created in the human body in 90% of the way of metabolizing TRP and can be directly or indirectly affected by factors such as diet and gut microbiota (10). Diet can alter KYN levels in the body. In the study conducted by Sandra Tillmann and her colleagues in 2021, it was shown that a methyl-deficient diet can regulate the KYN pathway (56). From other studies in this field, the relationship between the ketogenic diet, ginseng polysaccharides intake, a diet with limited calories, and a high-fat diet has been reported (57–59). In this study, the relationship of KYN levels obtained from serum on MetS and its components was investigated. The findings of the present study show that high levels of KYN increase the odds of MetS. There are not many studies on the relationship between KYN and MetS and its components. The relationship between inflammation and depression is not hidden from anyone and many studies have shown the relationship between these two disorders (60–63), but in the meantime, KYN also plays its role under the influence of inflammation (64). In the study published in 2010 by Gregory F et al., despite considering inflammation as one of the important factors of MetS, it reveals the role of inflammation on KYN (64). So inflammation can change the pathway of TRP synthesis to KYN production by inducing the transcription of one of the rate-limiting enzymes of the TRP /KYN pathway, called indoleamine 2,3-dioxygenase (IDO). Furthermore, KYN can cause the development of MetS through neurotoxic and prooxidative, apoptotic effects as well as positive regulation of phospholipase A2, prostaglandin, 5-lipoxygenase, arachidonic acid, nitric oxide synthase, and leukotriene cascade (64). picolinic acid and quinolinic acid are two intermediary metabolites in the metabolism of KYN, and the increase of KYN causes the increase of these two metabolites. These metabolites can increase lipid peroxidation through the stimulation of nitric oxide synthase (NOS) and the mediation of 3-hydroxy anthranilic and 3-hydroxykynurenine acids, and as a result, through the stimulation of arachidonic acid release, the activation of inflammatory factors such as leukotrienes, through the activation of arachidonate 5-lipoxygenase (5-LOX) and prostaglandins, through the active action of cyclooxygenase (COX) (Melillo et al., (65); Oxenkrug, (66)). Xanthurenic acid is another metabolite in the KYN-nicotinamide adenine dinucleotide pathway that creates an antigenic compound unrecognizable from insulin by reacting with insulin and eventually causes insulin resistance by reducing insulin sensitivity (67–70). According to studies, IDO has been reported to be inversely related to high-density lipoprotein and directly related to C-reactive protein (71, 72). The present study shows that there is a correlation between KYN levels and abdominal obesity. In the study conducted by Judith A. Finkelstein et al., free TRY was decreased in the blood of obese mice (73). Harald Mangge and colleagues showed that there is a relationship between KYN/TRP and MetS, BMI, and abdominal obesity, so that the higher this ratio, the more relationship was observed, and also MetS and this ratio both affect each other (74). In a study conducted on people with HIV in 2020, the ratio of quinolinic acid to kynurenic acid increased due to abdominal obesity, which means that the pro-inflammatory pathway of KYN metabolism is activated and the result of this pathway can increase systemic inflammation and decrease anti-inflammatory molecules (75).

The result of this investigation shows that there was a positive correlation between TMAO and MetS and its components. Also, we observed a positive correlation between KYN and abdominal obesity. Also, our study showed that high levels of TMAO (≥30.39 pg/mL) can increase the odds of MetS, hypertriglyceridemia, abdominal obesity, and low HDL. Moreover, high levels of KYN (≥297.18 nmol/L) can raise the odds of MetS and abdominal obesity. Therefore, the metabolites obtained from the gut microbiome can have effects on MetS and the risk of cardiovascular diseases, and it is very important to pay more attention to these metabolites with a larger sample size in the future.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The studies involving humans were approved by TUMS evaluated and approved the study protocol (IR.TUMS.MEDICINE.REC.1401.064). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

AM: Writing – original draft. MM: Writing – original draft. AK: Writing – original draft. HA: Writing – original draft. FA: Formal analysis, Writing – original draft. NS: Writing – original draft. MH: Writing – original draft. MR: Supervision, Writing – original draft. PK: Writing – review and editing. KM: Supervision, Writing – review and editing.