Bile salts metabolic factors

Metabolism—Foods supply vitamin A in the form of vitamin A, vitamin A esters, and carotenes. Almost no absorption of vitamin A occurs in the stomach. In the small intestine, vitamin A and beta-carotene are emulsified with bile salts and products of fat digestion and absorbed in the intestinal mucosa. Here, much of the conversion of beta-carotene to vitamin A (retinol) takes place. There are wide differences in species and individuals as to how well they utilize the carotenoids. Their absorption is affected by several factors, including the presence in the small intestine of bile, dietary fat, and antioxidants. Bile aids emulsification fat must be absorbed simultaneously and antioxidants, such as alpha-tocopherol and lecithin, decrease the oxidation of carotene. Also, the presence of enough protein of good quality enhances the conversion of carotene to vitamin A—a matter of great importance in developing countries where protein is limited in both quantity and quality. 

The impressive range of secondary products to be found in plants contrasts with the paucity of such compounds in animals (the con-stitutents of the bile salts of vertebrates and the contents of the preening glands of birds are in some ways comparable). One factor here may be that animals excrete waste products of their metabolism to the exterior. This can only be achieved to a very limited extent by plants by the shedding of leaves, floral parts or roots. Hence plants may seal off such products in vacuoles, resin canals, tannin cells, heartwood and so on. 

Ail the compounds in Table XIV arc water-soluble salts and their anions iiave molecular weights greater than 400 they satisfy the molecular weight and polarity factors for extensive biliary elimination. Therefore 10% or more of the dose would be expected to be excreted in the bile. This is found with ali the compounds except tartrazine. Ryan and Wright found that injected tartrazine is rapidly excreted in the urine in a conjugated form. Therefore, metabolism to a conjugate with a high affinity for renal clearance may partly account for the low biliary excretion of tartrazine. 

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