Effects of calcium supplementation on the composition and activity of in vitro simulated gut microbiome during inulin fermentation†

The gut microbiome influences the availability of micronutrients in the gastrointestinal tract. However, our insights into how colonic fermentation of prebiotic fibers and lactose is modulated by the presence of micronutrients and local pH environment are limited. Here, we investigated the influence of different calcium salts (calcium phosphate (CaPi), calcium citrate (CaCi), and calcium carbonate (CaCa)) on gut microbiome composition and metabolism using inulin and lactose as carbohydrate sources under low, medium, and high in vitro colonic pH gradients. Our results showed that in vitro colonic pH gradient had a significant effect on gut microbiome diversity (observed ASVs and Shannon diversity index, p < 0.05). After 24 hours of fermentation, the calcium sources had a significant effect on beta diversity at all colonic pH gradients (adjusted p < 0.05). Although changes in GM composition were more pronounced after 24 hours, after 6 hours of fermentation, the CaPi group exhibited a higher abundance of Leuconostoc than other groups. After 24 hours of fermentation, the CaPi group exhibited a higher Blautia abundance at high colonic pH gradient and lower Bacteroides abundance at all colonic pH gradient levels. The CaCi and CaCa groups exhibited a pH-dependent decrease in the abundance of Bacteroides. In addition, Bifidobacterium abundance remained over 1% regardless of colonic pH gradient, calcium source, or fermentation time. In addition, short-chain fatty acid (SCFA) production was dependent on the calcium source. For instance, compared to the control group, the CaCi group exhibited higher acetate production at low and high colonic pH gradients, while the CaPi and CaCa groups showed enhanced lactate production at medium and low pH gradients. These findings can increase our understanding of the impacts of calcium-rich diets on the human gut microbiome and its metabolic activity.