QUIZ 5th Dec




The fat-soluble vitamins—A, D, E, and K—follow the pathway for fat absorption described in the previous section. They are solubilized in micelles; thus, any interference with the secretion of bile or the action of bile salts in the intestine decreases the absorption of the fat-soluble vitamins (malabsorption). Malabsorption syndromes can lead to deficiency of fat-soluble vitamins. For example, nontropical sprue—a loss of intestinal surface area due to a sensitivity to the wheat protein gluten—

can lead to vitamin D malabsorption, which ultimately results in a decrease in calcium absorption in the GI tract (Chapter 11).

With one exception,

Watersoluble vitamins are absorbed by diffusion or mediated transport. The exception, vitamin B12

 (cyanocobalamin), is a very large, charged molecule. To be absorbed, vitamin B12

 must first bind to a protein, known as intrinsic factor, secreted by the acid-secreting cells in the

stomach. Intrinsic factor with bound vitamin B12

 then binds to specific sites on the epithelial cells in the lower portion of the ileum, where vitamin B12

 is absorbed by endocytosis. As described in Chapter 12, vitamin B12

 is required for erythrocyte formation, and deficiencies result in pernicious anemia.

This form of anemia may occur when the stomach either has been removed (for example, to treat ulcers or gastric cancer) or fails to secrete intrinsic factor (often due to autoimmune destruction of parietal cells). Because the absorption of vitamin B12

 occurs in the lower part of the ileum, removal or dysfunction of this segment due to disease can also result in pernicious anemia. Although normal individuals can absorb oral vitamin B12

, it is not very effective in patients with pernicious anemia because of the absence of intrinsic factor. Therefore, the treatment of pernicious anemia usually requires injections of vitamin B12

Refrence is

Vander human physiology