Obesity is a complex pathophysiological disease and one of the causes of metabolic syndrome, which is characterized by chronic low-grade inflammation. In 85 obese adults (average BMI = 40.48) and 42 non-obese control individuals (average BMI = 24.03), the serum IPA content was significantly lower in obese patients and was compared with BMI, serum high-sensitivity C-reactive protein (hsCRP), and high-sensitive interleukin 6 (hsIL-6), and the hsCRP and hsIL-6 levels were negatively correlated (40). This suggests that the indole metabolic pathway of Trp is affected in obese patients, which may be related to obesity-related systemic inflammation. However, in obese patients who underwent Roux-en-Y gastric bypass surgery (RYGB) operation, the level of IPA in the blood increased substantially 3 months post-surgery compared with 1 week post-surgery (40). This suggests that IPA content can be used as a marker for obesity. In future research, it will be necessary to perform correlation analyses between IPA and obesity-related complications to provide new diagnostic methods for invasive diagnosis of diseases and to predict obesity-related complications.

Current studies have found that IPA content is closely related to T2D and can predict the risk of T2D, distinguish different stages of T2D, and decrease with the improvement of T2D. IPA can be used as a biomarker of disease progression. When studying the brain–gut–microbiota characteristics of women with obesity and food addiction, a negative correlation was found between IPA in serum and food addiction (41). Bariatric surgery, such as RYGB, can improve T2D, obesity, NAFLD, and other metabolic diseases (42). In clinical studies, IPA content in the peripheral blood of obese patients with T2D was significantly lower than that in healthy participants. The IPA content in blood samples of these patients 3 months post-RYGB surgery was significantly higher than that in blood samples 1 week post-surgery (11).”XenoScan,” a metabolomics platform established by the University of California, Davis, used LC-MS to characterize a series of intestinal microflora metabolites and found that several metabolites, including IPA, could distinguish early T2D rats from rats 3 months after the onset of diabetes (43).

In a clinical trial, researchers used a non-targeted metabolomics approach to investigate whether serum metabolite profiles can predict the incidence of T2D in patients with impaired glucose tolerance. During the 15-year follow-up, patients with glucose tolerance who developed (n = 96) or did not develop (n = 104) T2D had lower and higher serum IPA levels, respectively. This suggests that higher serum IPA levels lead to a low risk of T2D (12). In a clinical study with a 7-year follow-up, it was verified that higher serum IPA levels were negatively correlated with the occurrence of T2D (OR [CI]: 0.86 [0.73–0.99], p = 0.04), directly correlated with insulin secretion (β = 0.10, p = 0.06), and negatively correlated with hsCRP when blood samples were collected (r = −0.22, p = 0.0001), and during follow-up visits (β = −0.19, p = 0.001). This suggests that IPA might be mediated by low-grade inflammation or enhance insulin sensitivity by protecting β-cell function to reduce the risk of T2D (21)

NAFLD manifests as liver fat accumulation, and the disease progresses to non-alcoholic steatohepatitis (NASH) or even hepatocellular carcinoma (HCC). Globally, the prevalence of NAFLD-related HCC may increase with an obesity epidemic (44).

IPA in the intestinal tract is absorbed by the intestinal epithelial cells and diffuses into the blood, which enters multiple target organs such as the liver after passing through the peripheral and portal circulation (30). This suggests that the liver may be a target organ for IPA biology. In 233 patients who underwent bariatric surgery and detailed liver histological examinations, the circulating IPA in patients with liver fibrosis was lower than that in those without fibrosis. Circulating IPA levels are also associated with the liver richness in genes that regulate hepatic stellate cell activation and liver fibrosis signaling. In vitro experiments have verified that IPA reduces the mRNA expression of fibrosis signaling markers such as COL1A2, aSMA, and ITGA3 in LX-2 cells (13).

Cholesterol is considered the primary lipotoxic molecule among liver lipids in NASH development (45–47). Lipotoxicity promotes the progression of NAFLD to NASH, liver cirrhosis, and even liver cancer (48, 49). Depletion of IPA was noted in both hypercholesterolemia-fed HCC mice and in sterile mouse serum transplanted with hypercholesterolemia-fed HCC mouse feces. In vitro experiments showed that IPA could inhibit the accumulation of triglycerides (TG) in the cholesterol-induced human normal hepatocyte line LO2 and inhibit the proliferation of NASH-HCC cell lines (HKCI-2 and HKCI10). Therefore, the partial reason for cholesterol-induced lipotoxicity is the damage to tryptophan metabolism in microorganisms and the reduced serum IPA content, thereby promoting the development of NASH-HCC (50).

CVD is a serious cause of death due to metabolic diseases (51, 52). In a cohort study from an advanced atherosclerosis (n = 100) and gender- and age-matched control group (n = 20), the level of IPA in plasma metabolites of the advanced atherosclerosis group was significantly reduced (0.41 [0.27–0.90] μM vs. 0.22 [0.16–0.34] μM; p < 0.01). In a study of risk factors for atherosclerosis, IPA (OR, 0.27; 95% CI, 0.019–0.91; p = 0.02) was negatively correlated with advanced atherosclerosis (14). In mice experiments, oral administration of IPA significantly reduced high-fat diet (HFD)-induced body weight gain and reduced serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-c), and TG levels, showing sufficient anti-hyperlipidemic effects (16).

In a 1-year follow-up study of patients with chronic kidney disease (CKD), the estimated glomerular filtration rate (eGFR) rapidly decreased by >20% (n = 10) and the control group (n = 10), and the eGFR decreased by <5%. It was found that IPA was the only metabolite that dropped significantly in eGFR rapidly decreased group of plasma. In cross-sectional clinical studies, it can also be found that the serum IPA content of the normal group was significantly higher than that of the CKD group (49.8 ± 15.9 vs. 34.7 ± 10.8 ng/ml; p < 0.01) (53). Intervention with IPA can also inhibit the gene expression of fibrosis and inflammation in proximal renal tubular cells induced by indophenol sulfate (54). In previous studies, oxidative stress was found to be associated with increased kidney damage (55), and IPA as a potent antioxidant may be an important bioprotective agent for CKD.

In another study, oral IPA supplementation reduced the systemic inflammation level in radiation-exposed mice, restored hematopoietic organs, relieved bone marrow suppression, and improved gastrointestinal function and epithelial integrity after irradiation, thereby exerting therapeutic effects on radiation toxicity (56). Supplementation with mouse probiotic Clostridia resulted in an increase in IPA production in the intestinal lumen and increased mitochondrial transcription factor A (Tfam) expression in osteoblasts by promoting Kdm6b/Jmjd3 histone demethylase, thereby inhibiting the epigenetic methylation of H3K27me3 at the Tfam promoter from preventing pathological bone loss in obese mice induced by a HFD (57). IPA is neuroprotective as a potent hydroxyl radical scavenger (58). IPA inhibits β-amyloid fibril formation, a potent neuroprotective agent, and is a potential drug for the treatment of Alzheimer’s disease (59). IPA also exhibits protective effects against streptozotocin-induced diabetic peripheral neuropathy in rats and high-glucose-induced neurotoxicity in neural 2a cells (60).

Impaired glucose tolerance and insulin tolerance are also pathogenic factors in metabolic syndrome. Rats fed a diet rich in IPA had improved glucose metabolism and significantly reduced HOMA index of fasting blood glucose, insulin, and insulin resistance (15).

Cognitive decline is a complication of T2D, and intermittent fasting (IF) is a dietary intervention used to alleviate the symptoms of T2D. In research of its mechanism, IF was found to improve cognition through the microorganism–metabolite–brain axis. Among the metabolites affected by IF, the complementary metabolite IPA showed similar results with IF. In db/db mice, IPA was found to improve cognitive function and insulin sensitivity, enhance mitochondrial biogenesis, and protect the ultrastructure of synapses (61). This may be related to IPA as an antioxidant, preventing neuronal death induced by amylin and β-amyloids and restoring mitochondrial function (62, 63).

First, the protective effect of serum IPA in T2D may be achieved through its efficacy in regulating the secretion of incretin, particularly glucagon-like peptide (GLP)-1 release by intestinal endocrine L cells (64). GLP-1 inhibits the occurrence of T2D by reducing B-cell apoptosis and increasing cell proliferation and regeneration (65)

Second, as a strong oxidant (62), IPA can protect β cells from damage related to metabolism and oxidative stress, and possibly from the accumulation of amyloid (66). These results suggest that IPA may be a promising candidate for the treatment of insulin-resistant metabolic disorders, including T2D.

IPA intervention can improve NASH model mice induced by a HFD through intestinal microenvironment homeostasis (17). In an in vitro experiment, supplementation with oleic acid (OA; 100 μM) resulted in significant accumulation of TG in a human hepatocarcinoma cell line of HepG2 cells, and IPA treatment significantly reduced OA-induced TG accumulation in a dose-dependent manner (10, 25, and 50 µM). Further research showed that IPA dose-dependently reduced the transcription of essential genes involved in fatty acid (Srebp1-c and Fas) and cholesterol biosynthesis (Srebp2 and Hmgr) in HepG2 cells (16).

In addition to inhibiting lipid synthesis in the liver, IPA intervention could inhibit the expression levels of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6 in the liver of NASH rats induced by a HFD. In an in vitro LPS-induced mouse macrophage model, IPA also inhibited nuclear factor kappa B) NF-κB signaling, p65 phosphorylation, and the expression of NF-κB downstream target genes in a dose-dependent manner (17).

Excess free Fe(3+) and was also found to cause oxidative damage, which deteriorates NAFLD to NASH. In an in vitro experiment, FeCl(3+) (0.2 mM) was used to induce the isolated rat liver microsomes to simulate the oxidative damage model and then incubated with IPA. It was found that co-incubated IPA (concentrations of 10, 3, 2, and 1 mM) can prevent the decrease in cell membrane fluidity caused by Fe(3+). The increase in lipid peroxidation caused by Fe(3+) was only inhibited after incubation with the highest concentration (10 mM) of IPA (67). Moreover, IPA at a concentration of 5 mM was able to inhibit lipid peroxidation damage in hamster testes caused by iron ions (68). This suggests that IPA can act as an effective free radical scavenger to prevent iron-induced oxidative damage to cell membranes. The antioxidant effect of IPA is concentration dependent, which also explains the protective effect of high concentrations of IPA on the periphery of the body.

IPA maintains the stability of the intestinal microenvironment. Its primary mechanism is to correct the disordered intestinal microflora, repair the intestinal barrier, and inhibit the intestinal immune response.

The intestinal microbial structure can affect the host’s absorption of dietary monosaccharides and lipids, promoting the accumulation of TG in the adipose tissue and liver and causing metabolic disease (69, 70). An imbalance of intestinal microbes affects the TLR9- and TLR4-related inflammatory pathways in the liver (71). In multi-ethnic cohort studies, intestinal microbial α diversity was generally low in patients with metabolic diseases (72). In a clinical cohort study of 1,018 middle-aged women from TwinsUK, the relationship between serum IPA levels and gut microbial genes was evaluated, and a positive correlation between microbiota alpha diversity and serum IPA content was found (Shannon diversity: β [Shannon diversity: beta (95% CI] = 0.19 [0.13; 0.25], p = 6.41 × 10⁻¹⁰) (73).

In an 8-week NAFLD rat model induced by a HFD, IPA (20 mg/kg) was administered to rats for the 8-week experiment. The 16s rRNA method was used to detect rat feces, and principal coordinate and non-metric multidimensional scale analyses showed that the intestinal microbes of rats in the IPA administration group were significantly different from those in the model group. This suggests that IPA administration can improve the overall structure of the intestinal microbes in NAFLD rats. An increase and decrease in the abundance of Firmicutes and Bacteroidetes, respectively, were biomarkers of obesity (74, 75). Based on this feature, the authors analyzed the intestinal microbial composition, and an HFD was found to increase the ratio of Firmicutes to Bacteroidetes, which could be reversed by IPA treatment. The abundance of the two potential pathogenic bacteria Bacteroides and Streptococcus ( 76) was increased by an HFD and decreased by IPA treatment. It has also been reported that the abundance of the genus Parasutterella ( 77) associated with chronic intestinal inflammation was reduced by IPA treatment. In addition, the abundance of the two genera Oscillibacter and Odoribacter, which are important for maintaining intestinal homeostasis, were reduced in the HFD-fed group (78) and restored in the IPA group (17). In addition, IPA supplementation can inhibit the growth of Mycobacterium tuberculosis by blocking the synthesis of Trp in M. tuberculosis through the catalytic step of TrpE, thereby exerting an anti-tubercular effect (79).

Increased intestinal permeability and abnormal intestinal tight junctions caused by ecological imbalance are frequently observed in patients with metabolic diseases (80). Intestinal ecological imbalance leads to an increase in LPS and bile acid production, which is related to whole-body low-grade inflammation (81).

Treatment of HFD-fed mice with IPA reduced intestinal permeability (decreased circulating FITC-dextran) and reduced circulating LPS levels. In vitro, researchers used monolayers of T84 cells incubated with the pro-inflammatory cytokines interferon-γ (IFN-γ) and TNF-α. IPA was found to reduce the permeability of monolayers through an FITC-dextran permeability experiment (82).

The ratio of villi to crypts in the ileum of HFD-fed rats was reduced (83), and the villi height was restored by IPA treatment, which also promoted the protein expressions of zonula occludens-1 (ZO-1), occludin, and tight junction proteins in the rat ileum (17). The end of the afferent neurons of the vagus nerve is located in the intestinal mucosa, and the increase in LPS changes the afferent signals of the vagus nerve and reduces the satiety induced by cholecystokinin, thus promoting appetite and leading to obesity (84). Thus, IPA can inhibit appetite by inhibiting LPS levels in the plasma.

The Caco-2/HT29 co-culture model was used to evaluate the effect of IPA on the intestinal barrier and explore its potential mechanism. Studies have shown that IPA increases transepithelial resistance and decreases paracellular permeability. Simultaneously, IPA enhances the mucus barrier by increasing the expression of mucins MUC2 and MUC4 and the goblet cell secretion products TFF3 and RELMβ. In addition, IPA reduces the expression of inflammatory factors in LPS-induced Caco-2/HT29 cells. These findings provide a new perspective for the intestinal microbial metabolite of Trp to improve the intestinal barrier (85). SLC2A5 (GLUT5-encoded) is the leading fructose absorption transporter in the kidneys, small intestine, and proximal tubules, and its overexpression causes metabolic syndrome by increasing fructose intake (86). Expression of the fructose transporter SLC2A5 mRNA was increased in IFN-γ-induced intestinal epithelial T84 cells, and IPA intervention reversed SLC2A5 mRNA expression (11).

Impaired intestinal barrier function and increased leakage of intestinal-derived antigens may lead to visceral lipid deposition and metabolic dysfunction (87). Serum IPA was reported to be decreased by approximately 60% in patients with active inflammatory bowel disease compared with that in healthy controls. During the recovery period of inflammatory bowel disease, the level of IPA in the serum gradually recovered (27, 88).

Administration of IPA showed significant induction of IL-10 receptor protein 1 expression in cultured intestinal epithelial cells T84 (27), based on a close correlation between epithelial IL-10 receptors and the maintenance and recovery of epithelial barrier function (89), which further supported the role of IPA in the maintenance of intestinal immunity.

Recent studies have suggested that IPA is an endogenous ligand for intestinal PXR. IPA induces the transcription of PXR target genes Mdr1, Cyp3a11, and Ugt1a1 mRNA in vivo ( 82). However, IPA alone is a weak human PXR ligand (82). Inoculated Clostridium sporogenes in germ-free mice accompanied with L-Trp-supplemented diets promoted the production of IPA to protect mice from dextran sulfate sodium-induced colitis through the PXR pathway (82). Studies have also confirmed that IPA improves intestinal permeability (FITC-dextran permeability) in a colitis (indomethacin-induced) mouse model with intestinal barrier defects. Intestinal TNF-α mRNA expression and p38-MAPK protein phosphorylation were inhibited, while in PXR-deficient (Nr1i2^−/−) mice, the benefits of IPA were inhibited, suggesting that IPA improved the intestinal barrier via the PXR pathway (82). Furthermore, IPA can modulate vascular function by modulating PXR activity, and IPA exposure reduces the vasodilatory responses of nitric oxide-mediated muscarinic and protease-activated receptor 2-stimulated mouse aortic tissue (90).

In another study, IPA was shown to be an agonist of the aromatic meridian receptor (Ahr) of a commensal bacterial product (91), and Ahr activation was beneficial to the maintenance of intestinal homeostasis and the regulation of immunity (92, 93). Therefore, these studies indicated that promoting IPA formation by bacteria or the direct administration of IPA is beneficial for inflammatory bowel disease.