Vitamins, absorption definition

Ultraviolet light causes a chemical change in dihydrocholesterol to produce cholecalciferol, a precursor of vitamin D. The latter conforms better to the definition of a steroid hormone than a vitamin. Indeed, the classification of vitamin D as a vitamin is an historical accident. The precursor is released from the skin and is further modified in the liver and kidney to form dihydroxycholecalciferol, which is the active form of the hormone (see Chapter 15 for the reactions). It increases calcium absorption from the... 

Ion exchange resins (colestipol,colestyramine), which complex with the brie salts and can interfere with the absorption of the oil-soluble vitamins Aluminum antacids can complex with some of the vitamins and, when used chronically, most definitely can cause hypophosphatemia Cystic fibrosis, which can cause fat malabsorption (steatorrhea) attributed to inadequate production of pancreatic lipases... 

The concept of bioavailability was introduced in 1945 by Oser et al. during studies of the relative absorption of vitamins from pharmaceutical products. At that time it was called "physiological availability." Since then, many definitions have been used and proposed by various scientists some defining it as the availability of active dmg at the site of action, others as the likely availability of the active ingredients or drugs to the receptors, etc. All these definitions turned out to be far more euphemistic than accurate. 

Vitamin Dj and the vitamin D s which occur in nature appear to be about equally toxic. Some differences were reported in rats by Morgan (1940, 1943), and in dogs and rats by McChesney and Messer (1941, 1942). It appears to be well established that the preparation A.T. 10 has definitely less effect on the absorption of calcium, and that its dominant action is on the mobilization of bone salts (McChesney and Messer, 1942 Albright and Reifenstein, 1948). 

Figure 2 A schematic outlining the path of /3-carotene (/3-C) as it moves out from the food into the intestinal wall. The definition of terms associated with understanding /3-carotene release, absorption, and conversion to retinoi are iiiustrated bioaccessibility, bioavailability, bionconversion, and bioefficacy. (Reproduced with permission from Tanumihardjo SA (2002) Factors influencing the conversion of carotenoids to retinol Bioavailability to bioconversion to bioefficacy. International Journal of Vitamin and Nutrition Research 72 40-45.)...

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