Vitamin A Levels in Health and Disease

The antimony-trichloride method for estimating vitamin A has now been available for nearly 25 years, and has been used to study the metabolism of the vitamin in man and animals by numerous workers all over the world.It is not surprising, therefore, that the wealth of literature is quite beyond compression into a short review, SO that we Cannot hope to outline more than a few of the main developments in the field, with emphasis on points in which we have been personally interested.The levels of vitamin A in the blood plasma and tissues are influenced by numerous factors, including the rate of intake of vitamin A or carotene, the efficiency of absorption from the intestines, the storage of vitamin A in the liver and other tissues, its mobilization from the liver into the blood plasma, and its removal from the plasma by transfer to the tissues, by destruction, and sometimes by urinary excretion.The various stages in the absorption, mobilization and utilization of the vitamin, however, probably overlap considerably.Thus, it must not be concluded that the vitamin invariably passes through the liver before being transferred to other tissues. AbsorptionPreformed vitamin A is generally much more efficiently absorbed than carotene, although the disparity decreases, and possibly eventually disappears, at low levels of dosing.Numerous investigations have been made on the effect of massive doses of