The hallmark of gout is the concentration of serum uric acid (SUA), whose solubility threshold is well-known to be affected by temperature, pH, and sodium concentration (11). In general, the definition of hyperuricemia is established when the concentration of SUA reachs 6.8 mg/dl (416 mmol/l), which is equivalent to the solubility threshold of urate at pH of 7.4 and temperature of 37°C (11, 12) in human. However, the definition of hyperuricemia varies with age and gender (13). It has been defined as SUA>7.0 mg/dL (420 mmol/l) in men; >6.0 mg/dL (360 mmol/l) in women; and >5.5 mg/dL (330 mmol/l) in children and adolescents (13). Uric acid (UA) is the final catabolic product of human exogenous and endogenous purine nucleotide metabolism (14). In most mammals, UA produced by metabolism can be degraded by uricase into 5-hydroxyisoourate and allantoin to be eliminated from the body (15). However, humans and great apes who lack uricase because of its mutation or inactivation during evolution, have higher circulating urate concentrations (16). Hyperuricemia can result from excessive purine intake, purine metabolism in the body, or mainly by renal and intestinal underexcretion (17), which is an important step in the development of gout. However, only about 36% of patients with hyperuricemia will eventually develop into patients with gout, whereas most remain in a state of asymptomatic hyperuricemia (18, 19). Therefore, hyperuricemia is generally considered asymptomatic in the absence of inflammatory episodes (gout flares) caused by monosodium urate (MSU) crystals.

Asymptomatic hyperuricemia is highly prevalent worldwide, ranging from 2.6 to 36% in different populations, and it has been increasing for decades (4, 20). Epidemiological and experimental data obtained over the past few decades show solid correlations between asymptomatic hyperuricemia and hypertension (HTN), renal disease, cardiovascular events, type 2 diabetes (T2DM), and other metabolic diseases. A meta-analysis of 55,607 patients found a dose-dependent relationship between serum urate and HTN; for every 1 mg/dl increase in SUA, the relative risk of HTN increased 1.13 times (21). The meta-analysis of Li et al. that reviewed 190,718 participants in 13 observational cohort studies inferred that elevated serum uric acid levels are associated with the onset of chronic kidney disease [OR = 2.35 (1.59–3.46)] (22). Krishnan et al. conducted a follow-up study of 5,012 diabetes-free participants aged 18–30 years for 15 years and reported that asymptomatic hyperuricemia is an independent risk factor for T2DM and insulin resistance [HR = 1.87 (1.33–2.62) and 1.36 (1.23–1.51), respectively] (23). Two large-scale meta-analyses that explored the correlation between the incidence of T2DM and serum urate also found similar results, suggesting that for every 1 mg/dl increase in serum urate, the combined relative risk of T2DM increases by 6–11% (24, 25). Another meta-analysis includes six studies of 5,686 patients with acute myocardial infarction showed that patients with hyperuricemia are more likely to have major adverse cardiac events [RR = 3.44 (2.33–5.08)] and in-hospital mortality [RR = 2.10 (1.03–4.26)] (26). Cell culture and animal studies further prove that hyperuricemia may be related to renal failure, hypertension, and metabolic syndrome (15, 27). Therefore, the researchers then conducted Mendelian randomized studies to confirm the causal association; however, the inconsistent results led to the causal link among hyperuricemia and kidney disease, hypertension, diabetes, or other metabolic diseases remains controversial (28–30). Indeed, the time interval from the onset of hyperuricemia to that of cardiovascular and renal comorbidities is long, making it unclear whether hyperuricemia is a risk marker for non-gout diseases or independent causal factors. In addition, although hyperuricemia is earlier than comorbidities, reverse causality cannot be ruled out. Moreover, the population with heterogeneous and environmental factors are not considered in the Mendelian randomization study. Also, there is a lack of understanding of the potential impact of genetic polymorphisms on the urate-related biological mechanisms.

The causality between hyperuricemia and comorbidities such as kidney disease, cardiovascular events, and diabetes is still not entirely resolved. However, given the rising prevalence of asymptomatic hyperuricemia, as well as metabolic and renal diseases, it is vital to explore the pathogenesis of hyperuricemia, especially the risk factors that can be manipulated.

Gout flare is the most significant and unique manifestation of gout, leading to the extreme pain of synovitis in the joints and surrounding areas, soft tissues and other organs bounded by monosodium urate (MSU) crystal deposits (31). Theoretically, once the SUA level reaches the solubility threshold, it can precipitate as needle-like crystals and cause inflammation. In fact, even in hyperuricemia patients whose SUA concertation is higher than 10 mg/dL, only 50% of them had a gout flare over 15 years (32). Therefore, it is suggested that hyperuricemia is necessary but insufficient to induce gout, and additional molecular/gene factors can affect the formation of MSU. Approximately 25% of patients with hyperuricemia have MSU, and it often occurs in the first metatarsophalangeal joint (19).

MSU crystals are a potential trigger for inflammation, but the mechanism of how MSU crystals induce inflammatory response has not been fully elucidated. Activation of PYD domains-containing protein 3 (NLRP3) inflammasomes in macrophages and monocytes by MSU is recognized as central to the initiation of the gout flare (33). NLRP3 inflammasome is a cytoplasmic protein complex involved in the maturation and expression of pro-inflammatory cytokines such as IL-1β, IL-33, and IL-18 (34). In the last decade, trials of inflammation-modifying treatments, such as IL-1β inhibition, have been tested as a treatment for gout with successful outcomes (35). The activation of NLRP3 inflammasome has been described as a dual-signal initiation process. The first signal stimulates NF-κB through TLR4 and TLR2 and synthesizes pro-IL-1β and inflammasome components (36). The second activation signal is more specific and is mediated by MSU crystal, causing the assembly of inflammasomes. The oligomerization of NLRP3 inflammasomes leads to the activation of Caspase-1, which proteolyzes pro-IL-1β into biologically active IL-1β. Then, IL-1β interacts with IL-1β receptors, triggering a downstream signal cascade including pro-inflammatory cytokines and chemokines, causing the recruitment of neutrophils and other cells to the site of crystal deposition (37).

Acute gout attacks, especially in the early stages of the disease, are self-limiting by nature (38). Current research shows that there are multiple mechanisms involved in the spontaneous resolution of acute gout, including protective protein-encapsulated crystals, and changes in the expression balance of pro-inflammatory and anti-inflammatory factors when the cell population in the inflammatory joint changes (39). After years of acute intermittent gout, advanced gout characterized by tophi, chronic gouty synovitis, and structural damage in joints usually occurs. The cytokines, chemokines, proteases and oxidants involved in acute inflammation induced by UA can cause chronic inflammation, leading to chronic synovitis, cartilage loss and bone erosion (40).

In addition to the excruciating arthritic pain and premature death, gout is associated with a high frequency of comorbidities, such as insulin resistance syndrome, hypertension, obesity, nephropathy, and cardiovascular disorders. The prevalence of comorbidities with increasing duration of gout is associated with all components of metabolic syndrome (41). Prospective studies have consistently shown that patients with hypertension have an increased risk of gout (42–44). According to the third NHANES (spelt out its whole names), abdominal obesity and T2D prevalence were found higher in gout patients (respectively 62.9 and 33.1%) than that in non-gout patients (respectively 35.3 and 10.8%) (45). Moreover, in prospective studies, gout also increases the risk of T2D, and diabetes reduces the risk of gout (46, 47), which can be partly explained by the increased excretion of urate in urine of diabetic patients (48). In addition, the prevalence of CKD in stage 3 or above in gout patients is estimated to be 24% and (49) gout prevalence was found to increase from 16.0 to 35.6% for CKD patients (50). Moreover, various heart diseases are independently associated with gout, which lead to increased frequency of cardiovascular deaths (51, 52).

Therefore, comorbidities of gout must be given special attention for the disease because they may contribute to the critical prognosis of gout patients and complicate the management of gout. Furthermore, deep understanding of the underlying molecular mechanism of gout flare is incredibly crucial.

The essence of hyperuricemia is the result of the imbalance between UA production and excretion. Interestingly, fructose is the only common ingested carbohydrate that produces uric acid during metabolism (81). Fructose is a kind of hexose, and its chemical formula C₆H₁₂O₆ is the same as glucose. It differs from glucose in that it has a ketone group at position 2 of the carbon chain, while there is an aldehyde group at position 1 of the glucose carbon chain. In solution, it can exist in the form of α or β pyranoside and furanoside rings. The content of fructose and glucose in ordinary sucrose is generally in a 1:1 ratio. High fructose corn syrup also contains a mixture of fructose and glucose. The outcomes of systemic fructose homeostasis are mainly 2-folds: intestinal absorption and clearance, the latter of which is generally considered to be mainly mediated by the liver (~55–71%) and the kidney (<20%) (82).

Figure 1 has depicted the details of fructose metabolism in the body. Once the body directly ingests pure fructose or HFCS, or fructose is produced from the digestion of sucrose on the brush border membrane, it will be transported from the lumen to the enterocyte via a specific fructose transporter GLUT5, which is located at the apical pole of the enterocyte. Subsequently, fructose is firstly absorbed and metabolized by the small intestine in an insulin-independent manner (83). In the first step, fructose is rapidly phosphorylated under the action of fructokinase [also known as ketohexokinase (KHK)] to form fructose-1-phosphate. This process requires ATP to provide a molecule of phosphate to be converted into ADP. In the second step, fructose-1-phosphate is decomposed by aldolase B into glyceraldehyde and dihydroxyacetone phosphate. In the final step, trikinase catalyzes the phosphorylation of glyceraldehyde by ATP to form glyceraldehyde- 3-phosphate (83). In this process, the rapid phosphorylation of fructose into fructose 1-phosphate reduces the intracellular levels of ATP, GTP and phosphate, leading to the accumulation of AMP (84). The decrease of intracellular GTP and phosphate activates AMP deaminase, and the accumulation of fructose 1-phosphate has did the same effect. AMP generates IMP under the action of AMP deaminase. IMP levels rise, and it is metabolized along with GMP to generate uric acid (85). In turn, uric acid inhibits AMP-activated protein kinase, thereby promoting more AMP metabolism by AMP deaminase. Influencing aldose reductase and increasing the expression and activity of fructokinase at the same time can stimulate fructose production further (86–88). In addition, the accumulation of IMP, GDP and GMP inhibits aldolase B, further enhances fructokinase activity and stimulates ATP conversion (84). As uric acid accumulates, xanthine oxidase is inhibited, but it may be at the cost of continuous activation upstream of the AMP deaminase pathway (89).

After fructose absorption and clearance, the remaining fructose inside the enterocyte diffuses into the portal circulation via a transport GLUT2 located at the basolateral pole of the enterocyte (90) and is further extracted by the liver rapidly and efficiently. Considering that the liver presents a high level of fructose catabolism enzymes (like KHK) expression and the sensitivity to fructose, it has generally been assumed to be the leading site of fructose metabolism (91). However, this notion has recently been challenged. Researchers (92, 93) have discovered that the small intestine shields the liver from fructose exposure. Most of the fructose intake is metabolized through the small intestine of mice. Using isotope tracers and mass spectrometry in mice, it was surprising that most dietary fructose has been converted into glucose and various organic acids in the portal vein, which connects the small intestine to the liver (93). The extent to which unmetabolized fructose enters the liver through the small intestine depends on the absorption and clearance rate of fructose in the small intestine. When ingesting low-dose fructose (< 0.5 g kg⁻¹), about 90% of fructose phosphorylation occurs in the jejunum, duodenum, or ileum. In contrast, intake of high doses of fructose (≥1 g kg-1) will saturate the absorption and catabolism of fructose in the small intestine, causing fructose to overflow into the liver (>30%) (92, 93). Namely, intestinal fructose metabolism seems to be a saturated process, allowing only high doses of dietary fructose to enter the liver. Most of the entered fructose is rapidly phosphorylated in the liver, and under the action of three key enzymes, glyceraldehyde 3-phosphate, an intermediate product of the glycolysis pathway, is generated (92). After that, the metabolic pathways of fructose and glucose in the liver are qualitatively similar and finally metabolized to produce pyruvate and acetyl-CoA, leading to lipogenesis (82). Another consequence of the fructolysis is the rapid depletion of intracellular ATP and Pi and the production of corresponding uric acid.

Another important organ for fructose metabolism is the kidney. The proximal straight tubule is the main site of fructose metabolism under physiological conditions. There, the glucose transporter 5 expressed at the top of the cell membrane mediates the absorption of fructose in the urine, which is then metabolized by fructose kinase in the cytoplasm. However, fructose metabolism is not limited to the proximal straight tubules; the proximal curved tubules have also been recorded. In fact, the proximal tubules express fructokinase and aldolase B, the latter being easily induced (94). On the contrary, continuous intake of a large amount of fructose may cause the physiological mechanism of the kidney to metabolize fructose to exceed the threshold. As a result, a large amount of ATP consumption and inflammatory reactions occur, and ultimately lead to a large amount of uric acid production and renal tubular damage (95). In addition to exogenous fructose, fructose can be synthesized from glucose through the polyol pathway. Glucose is converted to fructose in two steps in the polyol pathway. Firstly, aldose reductase regulates the conversion of glucose to sorbitol. Subsequently, sorbitol dehydrogenase catalyzes the oxidation of sorbitol to fructose. The endogenous fructose obtained via the polyol pathway in the kidney may be a potential mechanism for causing kidney damage (96). In mice with acute ischemic kidney injury, an increase in the concentration of aldose reductase, sorbitol, and endogenous fructose was found in the renal cortex, indicating that the polyol pathway was significantly activated. Moreover, polyol pathway restriction can limit acute ischemic kidney injury and promote recovery after kidney injury (97).

Based on the metabolism of fructose, it is can be concluded that fructose intake could induce UA synthesis through increasing of ATP decomposition as a precursor of UA, and increased insulin level following fructose intake has also been shown to raise of urate reuptake and reduce the excretion of UA in kidney. More specifically, fructose is rapidly phosphorylated upon intake, which is thought to lead to ATP depletion and the subsequent AMP accumulation (98). And the lack of free phosphate leads to AMP convert to IMP, thus resulting in the production of UA and the amelioration of UA levels in the serum (98). High fructose levels and associated decrease in ATP in the body will increase the compensatory effect of purine nucleotide synthesis, which subsequently results in further overproduction of UA in the presence of additional fructose (99). In addition, high fructose levels induce hyperinsulinemia and insulin resistance, which may lead to further elevated circulating UA levels via reducing UA excretion (96).

Given that ~30% of UA was excreted by the gastrointestinal (GI) tract, it is not surprising that gout or hyperuricemia-related studies have focused on the analysis of gut microbiota in recent years. The GI tract provides one of the largest interfaces (250–400 m²) among the host, environmental factors, and antigens in the human body (100). Over 10¹⁴ microorganisms have been inhabiting the GI tract, which offers many benefits to the host through a series of physiological functions. However, there is potential for these mechanisms to be destroyed as a result of an altered microbial composition, known as dysbiosis or dysregulation, which are associated with a variety of diseases (101–103). As for hyperuricemia, the gut microbiota is known to be involved in the metabolism of purines and uric acid. For example, the key enzyme in the oxidative metabolism of purines, xanthine dehydrogenase, was proved to be secreted by the Escherichia coli group of human intestinal bacteria (104–106). Proteus bacteria can secrete xanthine dehydrogenase to convert purines into UA (104). Lactobacillus can reduce the absorption of purines in the intestine, prevent the increase levels of serum UA and further induce hyperuricemia (107). Common members of the human gut microbiota, Lactobacillus and Pseudomonas, are found to be active in the synthesis of key enzymes (such as uricase) in the catabolism of UA (108). Moreover, the study also found that various indigenous microorganisms in the human gut secrete uric acid transporters, thereby affecting the excretion of UA (109). Therefore, intestinal microbiota can potentially affect the production and excretion of UA, thereby participating in the pathogenesis of hyperuricemia or gout.

Indeed, recent sequencing studies have shown special changes in microbial gene richness and diversity in gout subjects when compared to healthy controls (110). The abundance of Bacteroides caccae and B. xylanisolvens was significantly enriched in gout subjects, while those of Faecalibacterium prausnitzii and Bifidobacterium pseudocatenulatum was reduced (110). Further metagenomes study also found that the abundances of Prevotella, Fusobacterium, and Bacteroides were increased in gout patients, whereasthat of Enterobacteriaceae and butyrate-producing species were decreased (111). Also, imbalanced intestinal flora was observed in the rat model of nephropathy induced by hyperuricemia using the 16S rRNA technology (112). Flavobacterium, Myroides, Corynebacterium, Alcaligenaceae, Oligella and other conditional pathogens increased greatly in the rat model group, whereas Blautia and Roseburia, and the SCFAs producing bacteria diminished significantly (112).

Diet is one of the well-known significant factors that impact the composition of gut microbiota. A high-fiber diet appears to be involved in the increased abundance of Rominococcus bromii (113) and depletion of Frimicutes /Bacteroidetes (F/B) ratio (114), leading to the high production of SCFAs and butyrate in the host (113, 114). Diet supplemented with high-fat in mice indicated an association with the enrichment in F/B ratio, Ruminococcus and Rickenellaceae, as well as thedepletion in Lactobacillus, Bifidobacterium, and Prevotella (115, 116). In addition, artificial sweeteners were demonstrated to have deleterious effects on the composition of the gut microbiota, resulting in the rise in Fecal Bifidobacteria, Lactobacilli, propionate and reduction in Bifidobacteria, Lactobacilli, and Bacteroides (117, 118). Diet supplemented with a high-fructose syrup (HFS) compared to a fruit-rich diet can induce significant differences in the composition of the microbiota, resulting in a lower abundance of Firmicutes and Ruminococcus and a higher Bacteroidetes abundance (119).

Collectively, these research results made us comes up with a new question, that is, how does diet affect the interaction between the human body and its gut microbiota? Unfortunately, there is a lack of evidence to investigate the impact of fructose-rich diets on gut microbiota and the subsequent effects of high-fructose diet-induced effects on gout/hyperuricemia. An animal study in Rats with intragastrical administration of a high-dose fructose to increase uric acid resulted in an increased abundance of Lachnospira, Parasutterella, Marvinbryantia, and Blantia (120). However, a cross-over intervention study of 26 healthy adults who consumed either orange juice (OJ) or caffeine-free cola in -between 3 meals per day within 2 weeks, have found that beverage did not induce microbial changes and the uric acid level was decreased in the OJ group (121). The result makes sense since the complex nutrients which may have a positive effect on gut microbiota and stimulate UA excretion do exist in the OJ. Therefore, further studies with large-sample and other experimental conditions (e.g., different sources and forms of fructose, various doses and durations) might be needed.

How does fructose-mediated microbiota population shift affect the onset of gout/ hyperuricemia? To elucidate this doubt, the prerequisite is to understand the host-gut microbiota metabolic interactions (100). The composition and activity of the gut microbiota are subject to complex interactions that depend on the host's genome, nutrition, and lifestyle. On the other hand, the metabolism and homeostasis of the resident microbiota are closely related to the health of the host.

Recent studies found that high-dose fructose intake with the capability of inducing the overproduction of plasm inflammatory cytokines, such as IL-6, TNF-α, MIP-2, and IL-1β, decreased the level of anti-inflammatory marker IL-10 in rats (120, 122). Furthermore, excessive fructose intake alters the composition of the intestinal microbiota and impairs intestinal barrier function via reduction expression of TJ proteins, thereby triggering the inflammatory process (123, 124). As a complex, multi-gene inherited auto-inflammatory disorder, innate inflammatory pathways deservedly played a crucial role in the pathogenesis of gout (125). IL-1β is considered to be the core of gout inflammation. By stimulating pro-inflammatory cytokines and chemokines and up-regulating adhesion molecules on endothelial cells, inflammatory cells (like neutrophils and monocytes) are recruited to the MSU crystal deposition site, ensuing a positive feedback loop of inflammation, leading to ongoing inflammation (126). MSU crystals can stimulate the recruited monocytes to produce TNF-α (127). TNF-α facilitates the activation of caspase-1 by ATP via its receptors TNFR1 and TNFR2, leading to the activation of the NLRP3 inflammasome, which leads to the severity of gout. In addition, TNF-α can also promote the synthesis of pro-IL-1β mRNA, thereby promoting the production of IL-1β (128, 129).

Interestingly, these inflammatory reactions, which play a key role in the pathogenesis of gout, are also important factors for shaping and maintaining the homeostasis of intestinal flora. For instance, NLRP3 knockout mice exhibit both quantity and composition alterations in the microbiota, including a significant decrease in the F/B ratio (130) and only detected members of the genera Mycobacterium and Collinsella (131). In addition, the trafficking of immune cells following the increased release of inflammatory factors has been widely confirmed to synergistically promote the disruption of the intestinal mucosal barrier, thereby affecting the homeostasis of the intestinal flora (132). In turn, inflammatory reactions are influenced by gut microbiota and its products. The absence of microbiota is correlated with decreased production of SCFAs, especially acetate, which is necessary for inflammasome assembly and IL-1β production, and it is dependent on the activation of GPR43 (133), whose signal transduction can regulate the production of ROS (134) following activation of the NLRP3 inflammasome (135). Another interesting study found that gut dysbacteriosis can increase intestinal permeability and promote the translocation of bacteria or bacterial products, such as lipopolysaccharide [LPS (136)]. High levels of serum LPS can induce chronic inflammation and increase the risk of hyperuricemia (137). The high levels of LPS sustained in gout are positively related to the abundance of Gram-negative bacteria in the intestine, especially Proteobacteria (138). In addition, LPS is also a metabolite of the intestinal microbiota. Abnormal levels of LPS in blood circulation are usually accompanied by an increase in XO activity, which is an important enzyme in purine oxidation metabolism Furthermore, a recent study has stated that there were consistent associations between gut bacteria and immune cell dynamics, especially in the taxa of Faecalibacterium, Ruminococcus 2, and Akkermansia (139).

Additionally, the homeostasis of intestinal microbes will lead to changes in the metabolome, which are mainly manifested as the up-regulation of glucose, acetate, succinate and some amino acids, and the down-regulation of α- ketoisocaproate, phenylalanine, valine and citrulline (140). These metabolites are shown to be related to UA excretion, purine metabolism and inflammation. For example, acetate, succinate and glucose, materials that are involved in energy metabolism, can provide energy for intestinal epithelial cells, promote UA excretion and thereby relieve hyperuricemia (141). Glycine and aspartate are associated with the biosynthesis of purine nucleosides, and their abnormal expression leads to disorders of purine metabolism in patients with gout, thereby promoting the risk of disease (142). Besides, researches have shown that indigenous microbes in the human gut can secret key transporters to mediate UA absorption (109, 143–145). These transporters include ABCG2, SLC2A9, SLC16A9, SLC17A4, SLC17A1, SLC17A3, SLC22A11, SLC22A12, and SLC16A9, which in turn affect the absorption and metabolism of fructose. When the absorption of fructose in the intestine changes, the concentration of fructose in the lumen will also change, thereby affecting the ecology of intestinal microbes (90).

Moreover, the concept of oxygen metabolism and oxygen barrier in shaping the composition of the gut microbiota has recently been proposed (146). High fructose intake will increase the permeability of intestinal epithelial cells (147), resulting in increased oxygen levels in the lumen and following the destroy of living environment of epithelial anaerobes, which can further be facultative anaerobes or potential Aerobic bacteria provide the advantage of ecological selection, making them more competitive when expanding (148). For example, pathogenic bacteria such as Salmonella undergo aerobic proliferation under the destruction of anaerobic bacteria (147). These pieces of evidence support that oxygen levels can act as a shaper of the host's regulation of the gut microbiota. Overall, while understanding of the relationship among the microbiome, fructose, and gout or hyperuricemia has increased, more data are necessary to draw more concise and causal conclusions.