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Physiology of vitamin B12 absorption

Vitamin B12, also known as cobalamin, is an essential nutrient that plays a crucial role in various physiological processes, including DNA synthesis, red blood cell production, and neurological function. Absorption of vitamin B12 occurs primarily in the small intestine and involves several intricate steps.

The physiology of vitamin B12 absorption can be summarized as follows:

  1. Dietary Intake: Vitamin B12 is obtained through the consumption of animal-derived foods such as meat, fish, eggs, and dairy products. Plant-based sources do not naturally contain vitamin B12, but fortified foods and supplements are available for individuals following vegetarian or vegan diets.
  2. Release from Food: In the stomach, hydrochloric acid and pepsinogen are secreted to initiate digestion. These gastric secretions help release vitamin B12 from the food matrix, allowing it to bind with proteins present in the food.
  3. Binding to Salivary R-Binders: Once in the small intestine, vitamin B12 encounters salivary R-binders (also called haptocorrins), which are proteins released from the salivary glands. R-binders bind to vitamin B12, protecting it from degradation by stomach acid and enzymes. This binding also facilitates the transport of vitamin B12 through the digestive tract.
  4. Pancreatic Enzyme Release: In the duodenum, the first part of the small intestine, pancreatic enzymes are released, including pancreatic proteases. These enzymes break down the R-binders, freeing vitamin B12 from their complex.
  5. Binding to Intrinsic Factor (IF): Intrinsic factor, a glycoprotein secreted by the gastric parietal cells in the stomach, plays a crucial role in vitamin B12 absorption. Once vitamin B12 is released from the R-binders, it binds tightly to intrinsic factor, forming the vitamin B12-intrinsic factor complex.
  6. IF-B12 Complex Absorption: In the lower part of the small intestine, specifically in the ileum, the vitamin B12-intrinsic factor complex binds to specific receptors on the surface of epithelial cells lining the intestinal wall. These receptors facilitate the uptake of the complex into the cells through receptor-mediated endocytosis.
  7. Release from IF: Once inside the epithelial cells, the vitamin B12-intrinsic factor complex is transported to lysosomes, where it undergoes degradation. This process results in the release of free vitamin B12 from intrinsic factor.
  8. Binding to Transcobalamin II (TCII): Within the epithelial cells, free vitamin B12 binds to a transport protein called transcobalamin II (TCII). The vitamin B12-TCII complex protects the vitamin during its transport to various tissues and organs through the bloodstream.
  9. Delivery to Tissues: In the bloodstream, the vitamin B12-TCII complex travels to various tissues, including the liver, bone marrow, and nervous system, where vitamin B12 is actively taken up by cells to fulfill its physiological functions.

It's important to note that the absorption of vitamin B12 relies on the proper functioning of several components, including gastric acid secretion, intrinsic factor production, and the presence of a healthy ileum for receptor-mediated uptake. Any disruption in these processes can lead to vitamin B12 deficiency, which can result in various health complications, including megaloblastic anemia and neurological disorders.