TABLE 1.
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). |