Physical and serological characteristics
At 10 weeks of age, the mice in both the MSG and MSG+FOS groups were heavier than the control mice (MSG, 26.5 ± 0.6 g; MSG+FOS, 26.3 ± 0.5 g; control, 23.1 ± 0.5 g; P = 0.001 for both; Figure 1A). The mean body weights did not differ between the MSG and the MSG+FOS mice at 18 weeks of age (34.9 ± 2.4 and 35.6 ± 2.0 g, respectively; P = 0.9). Conversely, at 18 weeks of age, the MSG+FOS mice exhibited a lower mean epididymal fat weight than the MSG mice (MSG, 2.7 ± 0.2 g; MSG+FOS, 2.3 ± 0.1 g; P = 0.04; Figure 1B).
The mean total cholesterol level of the MSG mice (148.6 ± 14.5 mg/dL) was higher than that of the control mice (85.6 ± 2.2 mg/dL; P = 0.001) and MSG+FOS mice (115.6 ± 8.7 mg/dL; P = 0.003; Figure 1C).
Histological characteristics of the liver
Hepatic steatosis, inflammatory cell infiltration, and ballooning hepatocytes were observed in the livers of the MSG mice (Figure 2A), whereas these changes were less marked in the livers of the MSG+FOS mice. According to the NAS, steatosis (MSG, 1.7 ± 0.4; MSG+FOS, 0.3 ± 0.4; P = 0.001), lobular inflammation (MSG, 1.0 ± 0.3; MSG+FOS, 0.2 ± 0.1; P = 0.001) and ballooning degeneration (MSG, 1.7 ± 0.4; MSG+FOS, 0.3 ± 0.4; P = 0.001) were less severe in the MSG+FOS mice compared with the MSG mice (Figure 2B).
The mean serum ALT level of the MSG mice was higher than that of the control mice (MSG, 108.3 ± 29.0U/L; control, 39.2 ± 0.9 U/L; P = 0.001; Figure 2C). Conversely, the mean serum ALT level of the MSG+FOS mice was lower than that of the MSG mice (MSG+FOS, 73.5 ± 7.0 U/L; P = 0.001).
The hepatic mRNA expression levels of lipid biosynthesis enzymes
In the MSG mice, the hepatic mRNA levels of enzymes linked to lipid biosynthesis were elevated. The mean relative mRNA expression levels of fatty acid synthase (control, 1.0 ± 0.17; MSG, 1.96 ± 0.51; P = 0.02) and glycerol-3-phosphate acyltransferase (control, 1.0 ± 0.13; MSG, 2.0 ± 0.33; P = 0.01) were significantly higher in the MSG mice than in the control mice (Figure 3). Additionally, the hepatic mRNA expression levels of these enzymes were lower in the MSG+FOS mice (fatty acid synthase, 0.57 ± 0.06; P = 0.02; glycerol-3-phosphate acyltransferase, 1.01 ± 0.07; P = 0.01) than the MSG mice. There were no differences among groups in molecules involved in lipid uptake (fatty acid transport protein 5), lipolysis (carnitine palmitoyltransferase), or lipid transfer (microsomal triglyceride transfer protein).
Histopathological characteristics of the epididymal fat and the frequency of M1 ATM
Enlarged adipocytes and crown-like structures were observed in the epididymal fat of the MSG mice (Figure 4A), whereas FOS treatment led to smaller adipocytes and prevented the formation of crown-like structures in the MSG+FOS mice. To assess the numbers of macrophages in the epididymal tissue, F4/80+ and CD11bhigh cells were collected from the SVF, and M1 ATM were identified as CD11c+ cells [26]. The proportion of M1 ATM was 39.4% ± 3.0% of the SVF cells in the MSG mice, which was significantly higher than that in the control mice (9.1% ± 0.4%; P = 0.001; Figure 4B). In the MSG+FOS mice, the proportion of M1 ATM (22.8% ± 0.7%; P = 0.001) was significantly lower than that in the MSG mice.
The bacterial composition of feces
The analysis of the bacterial composition of feces revealed that the frequency of Clostridium cluster XI, which was 3.6% in the control mice, was reduced to 1.3% in the MSG mice, whereas the frequency of the bacteria classified as “others” was reduced from 17.5% in the control mice to 8.4% the MSG mice. Conversely, the frequency of the genus Prevotella, which was 1.3% in the control mice, was increased to 8.7% in the MSG mice (Supplementary Figure 1; Additional file 1). The feces of the MSG+FOS mice displayed a better bacterial balance. Specifically, the frequencies of Clostridium cluster XI others, and the genus Prevotella were 7.8%, 12.1%, and 1.8%, respectively
The levels of SCFA in feces and serum
The analysis of the SCFA levels in feces revealed that the mean fecal concentrations of n-butyric acid, propionic acid, and acetic acid were significantly lower in the MSG mice than the control mice (0.04 ± 0.01, 0.09 ± 0.03, and 0.65 ± 0.16 mg/g vs. 0.48 ± 0.11, 0.56 ± 0.09, and 2.46 ± 0.43 mg/g; P = 0.02, 0.02, and 0.007, respectively; Figure 5). Conversely, the mean fecal concentrations of n-butyric acid, propionic acid, and acetic acid in the MSG+FOS mice (0.38 ± 0.14, 0.42 ± 0.16, and 1.48 ± 0.29 mg/g, respectively) were significantly higher than those in the MSG mice (P = 0.02, 0.02, and 0.03, respectively).
The mean serum concentrates of n-butyric acid and acetic acid did not differ among the three groups (P > 0.05 in all cases). However, the mean serum concentration of propionic acid was significantly lower in the MSG mice (3.9 ± 0.5 μmol/L) than in the control mice (5.9 ± 0.4 μmol/L; P = 0.03). In the MSG+FOS mice, the mean serum concentration of propionic acid (8.2 ± 0.5 μmol/L) was significantly higher than that in the MSG mice (P = 0.001).