TABLE 1.

Summary of vitamin B absorption processes.

Vitamin	Daily intake	Hydrolysis from food	Absorbing location	Transporter
Vitamin B1	1.1–1.2 mg (28)	Intestinal alkaline phosphatase (33).	Small intestine and large intestine (27).	A combination of unsaturated passive diffusion and saturated active transport (31, 32, 230). By intestinal epithelium through THTR-1 and THTR-2 (SLC19A2 and SLC19A3) (33).
Vitamin B2	1.1–1.3 mg (59)	The diet needs to be hydrolyzed to riboflavin through protein denaturation and hydrolysis by alkaline phosphatases and FMN/FAD pyrophosphatases (59, 231).	Small intestine (58, 231).	Specific carrier-mediated processes (57).
Vitamin B3	28 mg for male and 18 mg for female in the diet (79)	Synthesized from tryptophan by pyridine carboxylase (83).	Stomach and upper intestine (84–86).	Proton cotransporters SMCT1 (SLC5A8), GPR109A (HCAR2) (25).
Vitamin B5	4–7 mg for adults and 5–9 mg for pregnant women (28)	Dietary vitamin B5 is firstly hydrolyzed to pantetheine by alkaline phosphatase in the intestinal lumen, and then is converted into the absorbable forms of pantothenic acid by pantetheinase (99).	Intestinal lumen (99).	At low luminal concentrations, free pantothenic acid is actively transported into the absorptive cells via the SMVT (100–103).
Vitamin B6	1.3–1.7 mg (123)	The vitamin B6 in the diet is hydrolyzed by pyridoxal phosphatase (83, 232, 233).	Jejunum, would also occur in ileum or cecum (234).
Vitamin B7	150–300 μg for adults, 35 μg for infants (149)	Ingested protein-bound forms of biotin are first broken down by gastrointestinal proteases and peptidases to biocytin (biotinyl-L-lysine) and biotin-oligopeptides (9).	Small intestine (149).	SMVT (100, 136)
Vitamin B9	400 μg for adult, 600 μg for pregnant women (28)	Glutamate carboxypeptidase II, dihydrofolate reductase (167).	Brush border of the proximal part of the jejunum (167).	PCFT (168, 169)
Vitamin B12	5–30 μg (187)	Vitamin B12 is released from the protein carriers with the help of gastric acid and pepsin (197). Pancreatic proteases.	Duodenum (197, 199).	Transmembrane protein amnionless and megalin/LRP2 (197, 199).