Conversion Of Carotenoids To Vitamin A

The carotenoids of fruits, vegetables, and animal products are generally fat soluble and are associated with the lipid fractions. In some cases they may be in the crystalline state, as lycopene is in the tomato. During the digestion process, esterases, lipases, and proteases act on the carotenoids, which are solubilized by the bile salts. Within the mucosal cell /^-carotene and the other provitamin A, carotenoids are split by carotenoid dioxygenase(s) resulting in an oxidative cleavage. In theory a molecule of /^-carotene should yield two molecules of retinal. Retinol is formed by reduction of retinal. It is esterified and transported to the liver for storage. There is some difference of opinion as to whether there is a central cleavage enzyme or whether there is also a random-splitting enzyme that acts on various places on the chain. The carotenes may also be absorbed, carried by low-density lipoproteins and deposited in the skin, fat, and various organs. The extent of accumulation, rate of accumulation, time to reach saturation, and turnover time were different for each tissue. Liver adrenal and ovary have high concentrations of /?-carotene.

Animals differ in the efficiency of the splitting reaction. The rat is generally regarded as very efficient. The mink and the fox are relatively inefficient in this reaction. Man is average with a ratio of about 6:1. The cat does not contain the enzymes necessary to utilize provitamin A carotenes.

Generally, for a compound to be active it must have an unsaturated central chain and an unsubstituted /?-ring. On the basis of structure out of the greater than 600 carotenoids listed, the provitamin A compounds would be between 50 and 60. Vitamin A conversion factors include:

The Mediterranean Diet Meltdown

The Mediterranean Diet Meltdown

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