This study showed that the people with visceral obesity had lower level of Bifidobacterium, and there was a negative correlation between Bifidobacterium and visceral fat area. Serum uric acid was an independent influencing factor for Bifidobacterial. Serum uric acid was negatively correlated with Bifidobacterium and serum uric acid was positively correlated with visceral fat area.
Most studies have showed that there was gut microbiota dysbiosis in obese individuals. Special gut microbiome leaded to fat deposits. Transplant gut microbiota from mice with diet induced obesity to lean germ-free mice, The germ-free mice developed more fat deposits17. The vast majority of gut microbiota belong to four main families (phyla): Firmicutes, Bacteroidetes, Proteobacteria and Actinobacteria18. At the genus and species level, obesity individuals had higher count of Fusobacterium、Enterococcus, Prevotella, and lower counts of Faecalibacterium, Bifidobacterial than lean people9ཞ13. In this study, the counts of Faecalibacterium prausnitzii were also decreased in visceral obesity, but the difference was not significant. The counts of Bifidobacterium significantly decreased in visceral obesity, that was the same as previous studies based on BMI criteria for obesity12,13. This result emphasized the role of Bifidobacterium as probiotics in obesity.
The gut microbiota is an important micro-ecosystem. Gut microbes contribute to the pathogenesis of obesity by fermenting indigestible dietary polysaccharides, producing short-chain fatty acids, and regulating energy homeostasis19. Supplementation of Bifidobacterium breve to high-fat diet-induced obese mice, significantly dose-dependently suppressed the accumulation of body weight and epididymal fat, and improved the serum levels of total cholesterol, fasting glucose and insulin20. Epididymal fat in this study was visceral fat. This study did not detect serum uric acid level. In another study, Supplementation of probiotic yogurt with Bifidobacterium lactis Bb12 decreased the level of serum uric acid21. In our study, we found that serum uric acid was an independent influencing factor for Bifidobacterium, Combining the factors of TG、LDL-C、HDL-C,FBG and age. Serum uric acid was negatively correlated with Bifidobacterium. Meanwhile, serum uric acid was positively correlated with visceral fat area. So serum uric acid is probably to be the mediator of the link between decreased Bifidobacterium and increased visceral adipose tissue.
Serum uric acid is a product of purine nucleotide metabolism, mainly derived from exogenous diet and endogenous nucleic acids. A certain level of serum uric acid is considered to be a beneficial antioxidant, but excess uric acid is associated with various diseases, such as hypertension22, diabetes23, cardiovascular disease24. Multiple epidemiological studies have shown a positive correlation between visceral fat and serum uric acid levels25,26. High levels of serum uric acid can increase insulin secretion, thereby promote fat synthesis27. Uric acid can also directly promote fat synthesis in hepatocytes via ER stress-induced activation of SREBP-1c28. This suggests that controlling serum uric acid levels may reduce the accumulation of visceral fat.
except for exogenous diet, the gut microbiota also plays an important role in serum uric acid levels. About one-third of uric acid is excreted through the gut29. The gut microbiota has gradually become a new target to study the pathogenesis of hyperuricemia. Transplant fecal microbiota of diet induced hyperuricemia rats into recipient rats, Serum uric acid levels were significantly increased in recipient rats30. The abundance of gut Faecalibacterium prausnitzii, Clostridium butyrate-producing bacterium, Bifidobacterium decreased in the gout people31. As a member of probiotics, Lactobacillus can reduce serum uric acid levels by synthesizing uric acid degrading enzymes32.The mechanism of Bifidobacterium lowering serum uric acid remains unclear. Several studies have found that Bifidobacterium can reduce endotoxin levels, reduce intestinal mucosal permeability, and have a protective effect on the intestinal mucosal barrier33,34.Normal intestinal mucosal barrier helps to prevent the translocation of intestinal bacteria or bacterial lipopolysaccharide(LPS) into the blood. Elevated LPS levels in the blood increased the risk of hyperuricemia35.
Currently the main and widely used medications for lowering serum uric acid are xanthine oxidase inhibitors such as allopurinol. Some people cannot tolerate these medications because of its side effects. Bifidobacterium is a kind of probiotic that has been widely used. Its clinical safety has been proven, almost no side effects. In addition to direct supplementation with Bifidobacterium, supplementation with specific prebiotics, such as chicory, could also help to reduce serum uric acid levels36. Our study found that serum uric acid was probably a mediator to the link between decreased Bifidobacterium and increased visceral adipose tissue. By lowering serum uric acid levels, we can lower the accumulation of visceral adipose tissue, further reduce the risk of metabolic diseases caused by visceral obesity.
Our study also has some insufficiency. First, we only detected ten gut bacteria. There is an interaction between the vast gut microbiota. It's easy to miss other meaningful gut microbiota. We need to further use 16S rRNA gene amplicon sequencing to detect gut microbiota in people with visceral obesity. Assess the species composition of the gut microbiota and its relative abundance information in visceral obesity. Then use q-PCR to analyze the role of specific gut bacteria. Second, We just found a correlation between Bifidobacterium, serum uric acid, and visceral adiposity. The specific cause and effect relationship need more research to confirm.