International Journal of Molecular Medical Science, 2025, Vol.15, No.2, 98-106 http://medscipublisher.com/index.php/ijmms 102 5.3 The impact of fiber metabolites on intestinal health The fermentation of dietary fiber by intestinal flora can produce short-chain fatty acids (SCFAs), which are very important for intestinal health. SCFAs such as acetic acid, propionic acid and butyric acid can help maintain the intestinal barrier and reduce inflammation (Zhai et al., 2018; Vinelli et al., 2022). The amount of these metabolites depends on the type and structure of the fibers. Some fibers produce more SCFAs (Vinelli et al., 2022). SCFAs, as an energy source for colonic cells, can also enhance barrier function and regulate immunity, and affect overall health, such as improving lipid metabolism and reducing the risk of metabolic disorders (Cronin et al., 2021). These benefits of SCFAs further highlight the role of dietary fiber in maintaining a healthy intestinal environment and preventing related diseases (Zhai et al., 2018; Vinelli et al., 2022). 6 Research Methods and Evaluation Criteria 6.1 Comparison between animal experiments and clinical trials: design and results Animal studies and clinical trials are both important means to study how dietary fiber affects the gut microbiota. Animal experiments, such as those conducted on mice and rabbits, typically involve altering the fiber ratio under controlled conditions to observe changes in the microbiota and metabolites. For instance, mouse studies have shown that different ratios of insoluble fiber to soluble fiber can significantly affect the contents of certain intestinal bacteria and short-chain fatty acids (SCFAs) (Figure 2) (Zhai et al., 2018; Zhang et al., 2023). Similarly, in the rabbit experiments, changing the ratio of non-fibrous carbohydrates to neutral detergent fibers in the diet would also affect the structure of the intestinal flora and immune function (Li et al., 2023). Figure 2 Effects of nine ratios of insoluble to soluble dietary fibers on SCFAs of healthy mice (Adopted from Zhang et al., 2023) Image caption: Significant differences between groups were evaluated using one-way ANOVA; p<0.05 was considered significant, (A) Acetic acid, (B) propionic acid, and (C) butyric acid. *p<0.05, **p<0.01, and ***p<0.001 were compared with I1S9DF group; p<0.05, p<0.01, andp<0.001 were compared with I9S1DF group (n=7) (Adopted from Zhang et al., 2023) In contrast, clinical trials take human subjects as the subjects and evaluate microbiota changes and health effects through dietary intervention. Such experiments often use randomized controlled designs to detect the effects of dietary fiber on microbiota diversity and SCFAs generation. For instance, systematic reviews and meta-analyses revealed that interventions with fructans and galactooligosaccharides could increase the numbers of Bifidobacteria and Lactobacillus in healthy adults (So et al., 2018; Vinelli et al., 2022). These results provide a basis for dietary fiber to improve human health. 6.2 Intestinal flora analysis methods: 16S rRNA gene sequencing, metabolite analysis, etc In animal and clinical research, molecular techniques are commonly used to analyze the intestinal flora. The most common one is 16S rRNA gene sequencing, which can identify and quantify the types of bacteria in the intestine and evaluate the effect of fiber on the diversity and abundance of the microbiota (Liu et al., 2020; Zhang et al., 2023). Li et al. (2020) pointed out that the fiber in sweet potato residue can significantly increase the ratio of Bacteroidetes to Firmicutes, which is a sign of a healthy microbiota. Metabolite analysis is important for understanding the functional changes of the microbiota, especially the measurement of SCFA. Studies have found that dietary fiber intervention can increase the production of short-chain fatty acids such as acetic acid, propionic acid and butyric acid. These metabolites are associated with a
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