Dietary Fibre Modulates Gut Microbiota in Late Pregnancy Without Altering SCFA Levels, and Propionate Treatement Has No Effect on Placental Explant Function

Abstract

Background/Objectives: Dietary fibre promotes health, partly by mediating gut microbiota and short-chain fatty acid (SCFA) production. Pregnancy alters the relationship between dietary composition and the gut microbiota, and it is unclear if fibre intake during late pregnancy alters the abundance of SCFA bacteria and circulating SFCA concentrations. The aim of this study was to determine the impact of dietary fibre on faecal microbiome composition and circulating concentrations of SCFA acetate, butyrate, and propionate in late pregnancy. We also aimed to assess the impact of propionate treatment on placental function using cultured placental explants. Methods: 16S rRNA gene amplicon sequencing was performed on faecal DNA collected at 28 weeks of gestation from participants enrolled in the SPRING cohort study consuming a low or adequate fibre diet. Circualting SCFA were assessed. Placental explants were treated with sodium propionate. Results: Fibre intake did not impact microbial diversity or richness but did impact the abundance of specific bacterial genera. Pregnant participants with low-fibre diets had a greater abundance of Bacteroides and Sutterella, and dietary fibre intake (mg/day) negatively correlated with genera, including Sutterella, Bilophila, and Bacteroides. SCFA concentrations did not differ between groups but circulating concentrations of acetate, propionate, and butyrate did correlate with the abundance of key bacterial genera. Propionate treatment of placental explants did not alter mRNA expression of fatty acid receptors, antioxidants, or markers of apoptosis, nor did it impact pAMPK levels. Conclusions: This study demonstrates that the impact of dietary fibre on SCFA concentrations in pregnant women is modest, although this relationship may be difficult to discern given that other dietary factors differed between groups. Furthermore, this study demonstrates that propionate does not impact key pathways in placental tissue, suggesting that previous associations between this SCFA and placental dysfunction may be due to other maternal factors.

Keywords: AMPK; SPRING; microbiome.

Figure 1

Gut microbiota composition at 28 weeks gestation in pregnant participants with adequate (green) and low (purple) fibre diets, as defined by dietary fibre intake (mg/kJ). There was no effect of dietary fibre on α-diversity as estimated by the Shannon Index (A) and the Chao1 Index (B) nor was there any effect on β-diversity as estimated by a supervised canonical co-ordinate analysis (C) or the unsupervised principal co-ordinate analysis (PCoA) using Bray–Curtis metrics (D). The Analysis of similarity using the Bray-Curtis dissimilarity matrix (ANOSIM) depicts intra- or inter-group differences in the gut microbiota composition between groups (E). The linear discriminant analysis (LDA) of the effect size (LEfSe) plot demonstrates that Sutterella and unclassified Clostridiales are the genera most likely to explain differences between adequate- and low-fibre diet groups (F). Participants with a low-fibre diet had an increased abundance of Bacteroides and Sutterella (G). Box plots depict a median with the twenty-fifth and seventy-fifth quartiles and the dotted error bars showing the centiles of 2.5 and 97.5, with the dots outside of this depicting outliers. * denotes significance p < 0.05. For the LEfSe, significance was accepted when LDA > 3.

Figure 2

Correlation analysis between the top 50 most abundant genera and maternal dietary intake at 28 weeks of gestation in all participants (n = 52). Statistical analysis was performed using Spearman’s correlation, with Spearman’s rank correlation coefficients and p-values shown. The darker the red colour, the stronger the positive correlation whereas, the darker blue colour intakes a stronger negative correlation. Significance was accepted when p < 0.05, * for p < 0.05 and blank for non-significant p-values.

Figure 3

There were no differences in serum acetate (A), propionate (B) or butyrate (C) concentrations between participants from the adequate- and low-fibre groups. Serum acetate, propionate, and butyrate concentrations were found to correlate with the abundance of at least one bacteria genus each, with propionate levels correlating with the abundance of five separate bacterial genera (D). Serum concentrations of SCFAs are presented as a median and interquartile range, with individual samples represented by circles in panels (A–C). * denotes significance p < 0.05. Data were analysed using a two-tailed Mann–Whitney U test following Shapiro–Wilk normality testing or Spearman correlation analysis.

Figure 4

The influence of propionate treatments (7 µM and 20 µM) for 4 and 24 h on mRNA expression of the short-chain fatty acid (SFCA) receptors, free fatty acid receptor 2 (FFAR2), FFAR3 (A), the antioxidant genes catalase (CAT), superoxide dismutase 1 (SOD1), and SOD2 (B), and key regulators of apoptosis, caspase 3 (CASP3), and CASP8 (C) in placental explant tissue. Mitochondrial content at 4 and 24 h, as assessed by the ratio of markers of mitochondrial DNA divided by markers of nuclear DNA (D). Phosphorylated AMPKα (E), Total AMPKα (F) and Phosphorylated AMPKα per Total AMPKα (G) after 4 h were also assessed with representative blots shown above (H). Data are presented as mean ± SEM with a comparison between the vehicle (white), 7 µM propionate (light grey), and 20 µM propionate (dark grey). Individual dots represent the mean of technical replicates for four separate placentas that each received the three treatments. Statistical analysis included a repeated measures one-way ANOVA for parametric data or the Friedman test for non-parametric data. Significance was accepted when p < 0.05; blank spaces signify non-significance.