The Riboflavin Economy of the Rat

Abstract

The concentration of riboflavin in the liver and in the rest of the carcass has been measured for weanling rats maintained for three to 5 weeks with zero to 160 µg of riboflavin per day. With no dietary riboflavin tissue concentrations fall to a little less than half their initial value by the end of three weeks and then remain stationary. Maximum riboflavin concentrations are obtained with intakes of about 40 µg per day. Maximum growth occurred with tissue riboflavin concentrations that were about 75% of the maximum levels attainable. Male rats grew more rapidly than females even with suboptimum tissue concentrations; hence with a given suboptimum intake of riboflavin, male tissues were lower in riboflavin than were female tissues. It was found that when allowance was made for riboflavin deposited in the tissues or released from the tissues, as the case might be, that approximately the same expenditure of riboflavin was required by the growing rat as the non-growing rat to maintain a given tissue concentration of riboflavin. If, however, the gross intakes alone are compared, a rapidly growing rat may require up to three times as much riboflavin per gram as a non-growing rat to maintain a specific tissue level. Destruction of riboflavin by the rat varies from as little as 0.04 µg per gram per day in severe deficiency to more than 6 µg per gram per day when a large excess of riboflavin is administered. Loss of riboflavin from the rat by excretion is negligible compared to destruction except with excessively large levels of intake. In addition to any riboflavin deposited in the tissues, approximately 0.2 µg of riboflavin per gram per day is needed to balance destruction with tissue levels which permit maximum growth. No increase in riboflavin destruction by riboflavin-deficient rats was observed when metabolism was increased by means of thyroxin or by a cold environment. Nor did changing the metabolic mixture to emphasize carbohydrate, protein or fat influence riboflavin destruction. Hence it is concluded that there is little or no direct destruction of riboflavin associated with its use.