Vitamin A Requirements in the Rat. The Relation of Vitamin A Intake to Growth and to Concentration of Vitamin A in the Blood Plasma, Liver and Retina

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Abstract
When vitamin A was fed daily at different levels to groups of 3 to 4 week old rats for a period of 6 weeks, the rate of growth was found to increase until it reached an optimum at an intake of 25 I. U. daily; it remained at this optimum for intakes up to 1000 units daily. The average plasma vitamin A concentration also increased with increasing levels of vitamin A intake, reaching an optimum at an intake of 50 units daily, and remained at this optimum for intakes up to 1000 units daily. There was no liver storage at intakes of 10 units or less daily, slight storage at 25 units, and increasingly larger storage at higher intakes. Retinal concentrations of vitamin A were usually low at zero units intake and reached an optimal value at an intake of 2 units daily. When rats were placed on a vitamin A-free diet, the plasma vitamin A concentration remained high in the animals with large liver stores, but fell rapidly in those with low reserves. The retinal concentration of vitamin A, however, remained high despite the low vitamin A plasma concentration and the absence of vitamin A in the liver. The minimum vitamin A requirement in the rat was 2 units daily or 20 units per kilogram. Optimal growth occurred at 25 units daily intake, optimal blood concentrations at 50 units daily, and good liver reserves at 100 units daily. Minimum and optimum vitamin A requirements are of the same order in the rat and infant. We wish to express our appreciation to Mr. Bertrand Flusser, to Miss Mildred Weinstock and to Dr. Charles Haig for their cooperation in this study.
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