TABLE 3.

Gut microbiota affect the absorption of vitamin B via modifying the physiological properties of gastrointestinal tract.

Key physiological factors for nutrient absorption	Influences of microbiota on physiological conditions	Influence on vitamin B absorption
Permeability	↓ the abundance of Bifidobacterium, Faecalibacterium, and Lactobacillus → ↑ gut permeability → ↑ IBD (239, 240) L. Plantarum, L. casei, B. infants, and S. salivarius → ↓ gut permeability → ↓ IBD (241–244)	Vitamin B (except vitamin B9) could be absorbed by passive diffusion. Bacterial infection might increase vitamin B amount of absorption.
Gastrointestinal motility	Gut bacteria → SCFAs → ↑ gastrointestinal motility IBD mouse (245–247) L. casei and Bifidobacterium animalis → SCFAs → ↓ intestinal motility in rats (232, 248–250) Gram-negative bacteria, E. coli Nissle and L. reuteri →↓ gastrointestinal motility in mice (10, 251–254)	Enhanced gastrointestinal motility resulting from intestinal microbiota might cause a narrowed absorption window (140) and thus results in reduced bioavailability of vitamin B.
The degree of acidity (pH) in gastrointestinal tract	H. pylori infection → ↑ pH (255) Bifidobacterium, Lactobacillus, Enterococcus, and Streptococcus →↓ pH (45, 46)	The absorption progress of vitamin B1, vitamin B3, vitamin B6, and vitamin B9 are pH-dependent. Lactic acid bacteria might change the rate of vitamin B absorption.
Expression of transporter	Gordonibacter → ↓ the expression level and activities of MDR1, BCRP, MRP2, and MRP7 in vitro and mice (256, 257) E. coli → ↓ the expression of THTR-1 and THTR-2 in a Caco-2 cell model (44) S. enterica serovar Typhimurium → ↑ CFTR expression in the intestinal epithelium (258), ↓ the transcription of SLC5A6 S. typhimurium → ↑ MRP2 expression in human intestinal biopsy material (259, 260), ↓ transport function of P-gp (260)	Overgrowth of E. coli might compromise vitamin B1 absorption due to downregulation of THTR-1 and THTR-2. S. enterica serovar Typhimurium might reduce absorption of vitamin B5 and vitamin B7 via inhibiting the SMVT (100, 150).