Development of a folacin bioassay in rats.

The objective of this study was to determine if the relationship between dietary folic acid and liver folacin concentration could serve as a quantitative standard response curve for a folacin bioassay. This paper examines the time required for liver folacin depletion, the effect of the supplementation period on the range of the linear response and the precision of the standard curve. Male weanling Fisher strain rats were offered a low folacin diet for 31 days. At the end of the depletion period rats were assigned to diets containing 0. 0.25, 0.5, 1.0, 2.0 or 4.0 mg added folic acid/kg of diet. Total liver folacin was measured on three animals from each group after 2, 4, 7, 14, and 28 days on the repletion diets. The liver folacin response curves at 7, 14, and 28 days had a linear range of 4, 2, and 1 mg of folic acid per kg of diet with standard errors equal to 11, 6, and 7% of the slopes respectively. The 28 day repletion period gives the most sensitive standard range. These responses establish that liver folacin concentration is suitable for use as the response variable in the folacin bioassay. With appropriate controls the model should be useful for bioassay of folacin in natural materials.