Hamid M. Said, Ebba Nexo
ABSTRACT
Nine compounds are classified as water‐soluble vitamins, eight B vitamins and one vitamin C. The vitamins are mandatory for
the function of numerous enzymes and lack of one or more of the vitamins may lead to severe medical conditions. All the vitamins
are supplied by food in microgram to milligram quantities and in addition some of the vitamins are synthesized by the intestinal
microbiota. In the gastrointestinal tract, the vitamins are liberated from binding proteins and for some of the vitamins modified
prior to absorption. Due to their solubility in water, they all require specific carriers to be absorbed. Our current knowledge
concerning each of the vitamins differs in depth and focus and is influenced by the prevalence of conditions and diseases
related to lack of the individual vitamin. Because of that we have chosen to cover slightly different aspects for the individual
vitamins. For each of the vitamins, we summarize the physiological role, the steps involved in the absorption, and the factors
influencing the absorption. In addition, for some of the vitamins, the molecular base for absorption is described in details,
while for others new aspects of relevance for human deficiency are included. © 2018 American Physiological Society. Compr Physiol 8:1291‐1311, 2018.
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Figure 1. Absorption of water‐soluble vitamins in the small intestine. The figure depicts key proteins involved in the uptake of vitamins supplied with food or recycled with bile and absorbed in the small intestine. The question mark indicates that the molecular identity of the system involved has not been identified yet.
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Figure 2. Absorption of water‐soluble vitamins in the large intestine. The figure depicts key proteins involved in the uptake of vitamins synthesized by the gut microbiota and absorbed in the large intestine. The question mark indicates that the molecular identity of the system involved has not been identified yet.
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Teaching Material
H. M. Said, E. Nexo. Gastrointestinal Handling of Water-Soluble Vitamins. Compr Physiol 8: 2018, 1291-1311.
Didactic Synopsis
Major Teaching Points:
- Water-soluble vitamins cover eight B vitamins and vitamin C.
- The vitamins are supplied with food in quantities of microgram to milligram, and some are synthesized also by gut microbiota.
- The intestinal uptake requests concerted action by several cellular transporters.
- The vitamins function as coenzymes or antioxidants for numerous intracellular metabolic reactions.
- Lack of one or the other of the vitamins may lead to severe disease.
Didactic Legends
The figures—in a freely downloadable PowerPoint format—can be found on the Images tab along with the formal legends published in the article. The following legends to the same figures are written to be useful for teaching.
Figure 1 Absorption of water-soluble vitamins in the small intestine. The figure depicts key proteins involved in the uptake of vitamins supplied with food or recycled with bile and absorbed in the small intestine. The question mark indicates that the molecular identity of the system involved has not been identified yet.
Teaching points:
- All water-soluble vitamins supplied with the food or by hepatic recirculation are absorbed by the enterocytes in the small intestine.
- The left part of the figure indicates membrane-bound proteins so far described to be involved in the intestinal uptake of the vitamins.
- Vitamin B12 will be absorbed only if bound to the soluble gastric intrinsic factor, on the figure indicated together with its receptor.
- Once in the enterocyte the vitamins are exported by specific proteins indicated at the right part of the figure.
- The capacity for the carrier-mediated vitamin uptake is limited. Administration of high-dose vitamin can increase the uptake because around 1% is absorbed by passive diffusion.
Figure 2 Absorption of water-soluble vitamins in the large intestine. The figure depicts key proteins involved in the uptake of vitamins synthesized by the gut microbiota and absorbed in the large intestine. The question mark indicates that the molecular identity of the system involved has not been identified yet.
Teaching points:
- Many vitamins are synthesized by the gut microbiota and absorbed in the large intestine.
- The figure indicates proteins so far described to be involved in the uptake and export of water-soluble vitamins.
How to Cite
Hamid M. Said, Ebba Nexo. Gastrointestinal Handling of Water‐Soluble Vitamins. Compr Physiol 2018, 8: 1291-1311. doi: 10.1002/cphy.c170054
