There have been mixed results regarding the relationship among short chain fatty acids (SCFAs), microbiota, and obesity in human studies. We selected studies that provided data on SCFA levels or fecal microbiota abundance in obese and nonobese individuals and then combined the published estimates using a random-effects meta-analysis. Obese individuals had significantly higher fecal concentrations of acetate (SMD (standardized mean differences) = 0.87, 95% CI (confidence interva) = 0.24–1.50, I2 (I–squared) = 88.5), propionate (SMD = 0.86, 95% CI = 0.35–1.36, I2 = 82.3%), and butyrate (SMD = 0.78, 95% CI = 0.29–1.27, I2 = 81.7%) than nonobese controls. The subgroup analyses showed no evidence of heterogeneity among obese individuals with a BMI >30 kg/m2 (I2 = 0.0%). At the phylum level, the abundance of fecal microbiota was reduced in obese compared to nonobese individuals, but the difference was not statistically significant (Bacteroidetes phylum, SMD = −0.36, 95% CI = −0.73–0.01; Firmicutes phylum, SMD = −0.10, 95% CI = −0.31–0.10). The currently available human case-control studies show that obesity is associated with high levels of SCFA but not gut microbiota richness at the phylum level. Additional well-designed studies with a considerable sample size are needed to clarify whether this association is causal, but it is also necessary to identify additional contributors to SCFA production, absorption, and excretion in humans.

There have been inconsistent findings on the association of obesity and non-constipation irritable bowel syndrome (IBS). Small intestinal bacterial overgrowth (SIBO) with hydrogen (H2) gas forming-microflora causes non-constipation IBS. But, the effect of H2 producing SIBO on obesity in non-constipation IBS patients has not been studied yet. The aim of this study was to investigate the association between obesity and SIBO in non-constipation IBS patients. We reviewed the charts of patients who showed IBS symptoms along with the documented results of their lactulose hydrogen breath test (LHBT) for SIBO. Multivariate models were used to assess the association between obesity and SIBO. Four-hundred fifty-eight patients were retrospectively included in the study. Of the 485 IBS patients, 158 (30.7%) subjects had positive results for LHBT. Subjects without SIBO showed significantly higher levels of body mass index (24.8 vs. 23.3; P < 0.001) and waist circumference (86.5 vs. 82.7; P < 0.001) as compared to subjects with SIBO. In multivariate analysis, the odds ratios of SIBO were 0.396 (P = 0.018) for obesity and 0.482 (P = 0.021) for abdominal obesity. This is the first human study to demonstrate that obesity is inversely related to SIBO with H2 gas production in non-constipation IBS patients.

Micro-inflammation in the gut, assessed by fecal calprotectin (FC), is considered a component of the pathogenesis of functional diarrhea (FD). Since probiotics may suppress micro-inflammation in the intestine by competing with harmful bacteria, we hypothesized that they would reduce the ratio of loose stool symptoms and gut inflammation in patients with FD. We conducted a double-blind, placebo-controlled trial to assess the clinical and laboratory effects of Lactobacillus plantarum CJLP243 in FD patients with elevated FC levels for two months. Twenty-four patients diagnosed with FD with elevated FC levels were randomly assigned to either a probiotic group or a placebo group. After 2 months, 10 patients in the probiotic group and 12 patients in the placebo group completed the study, and FD symptoms, FC values, and intestinal flora were re-evaluated in these subjects. The percentage of subjects who had adequate FD relief (decrease in loose stool frequency) in the probiotic group was significantly increased after two months compared with the baseline. In addition, the probiotic group showed a statistically significant decrease in log-transformed FC values compared with the pre-treatment group, whereas the placebo group showed no difference before and after the intervention. Furthermore, the levels of Leuconostoc genus organisms in the gut microbiota composition in the probiotic group increased significantly after the end of the study compared with the baseline values. In this preliminary exploratory research, we found that two months of Lactiplantibacillus plantarum CJLP243 treatment resulted in FD symptom improvement, reduced FC values, and increased Leuconostoc levels, suggesting that the intake of Lactiplantibacillus plantarum was helpful in those patients. These findings need to be validated via further clinical studies.

Synbiotics, including probiotics and prebiotics, are useful for patients with functional bowel disorders. However, which synbiotics are beneficial for patients with which diseases, especially those with functional diarrhea (FDr) with high fecal calprotectin levels, is currently unknown. FDr is an extension of irritable bowel syndrome with diarrhea (IBS-D). Although fewer studies have been conducted on FDr compared to IBS-D, its importance is increasing as its prevalence increases. The aim of this study was to evaluate the effects of a synbiotic containing a mixture of Lactobacillus and Bifidobacterium and its substrate, fructooligosaccharide, on bowel symptoms, fecal calprotectin levels, fecal microbiota, and safety in FDr patients with high fecal calprotectin levels. Forty patients were randomly assigned to either a synbiotic group or a placebo group. A total of 20 subjects in the synbiotic group and 19 subjects in the placebo group completed the study (8 weeks). Changes in FDr symptoms, fecal calprotectin levels, and gut microbiota were assessed during the intervention period. At 4 and 8 weeks, the number of bowel movements tended to increase in the synbiotic group, with a significant increase in the number of formed stools rather than loose stools (p < 0.05). Bowel movement satisfaction was significantly increased in the synbiotic group, but not in the placebo group. Intestinal flora analysis revealed that Lactobacillales at the order level was increased only in the synbiotic group at the end of the intervention. In contrast, at week 8 of the intervention, log-transformed fecal calprotectin levels were significantly decreased in the synbiotic group, although the change was not significantly different from that of the placebo group. These findings suggest that the intake of a multi-strain-containing synbiotic for 8 weeks could improve gut symptoms and the intestinal microenvironment of FDr patients with high fecal calprotectin levels.

Until now, it is not easy for medical personnel in charge of primary care to adopt hydrogen/methane breath test or next-generation sequencing using feces, which are methods for clinically evaluating gut dysbiosis. Therefore, although not many studies have been conducted, this review discusses serum indicators that can easily suspect gut dysbiosis in primary care based on theoretically explainable mechanisms, and these indicators include serum total bilirubin, liver function level, vitamin B-12, fibrinogen, and ferritin. Total bilirubin rises as a defense mechanism against oxidative stress, liver function rises in relation to leaky gut, and vitamin B-12 increases as a metabolite produced by intestinal bacterial imbalance. Fibrinogen and ferritin are thought to rise due to microinflammation caused by an increase in endotoxin caused by gut dysbiosis. So far, few studies have been conducted on these relationships, so researchers with a functional medicine perspective should be more interested in treatment and research to reveal the relationship in the future.