Effect of Dietary Carbohydrate on Assessment of the Leucine Need for Maintenance of Adult Swine

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Abstract
Nongravid crossbred gilts weighing 135 kg were used in two nitrogen balance assays designed to evaluate the leucine need for maintenance. Gilts fed a crystalline amino acid, “leucine-free” diet containing 70% cornstarch retained more dietary nitrogen than those receiving an other wise identical diet containing 70% sucrose. The cornstarch was analyzed and found to contain 0.30% crude protein of which 9.63% was leucine. This accounted for almost 70% of the contaminating leucine found in the purified “leucine-free” diet. In another assay three supplemental levels of L-leucine were evaluated using sucrose as the source of dietary carbohydrate. A daily nitrogen retention of 1 g (maintenance requirement) was achieved at a leucine intake of 780 mg/day or 19.8 mg/day per kg0.75. The data indicated that cornstarch contains a significant quantity of available leucine which can, if ignored, lead to considerable error in assessment of a maintenance leucine requirement. Morever, even in the purified crystalline amino acid “leucine-free” diet containing sucrose as the carbohydrate source, a small but significant quantity of contaminating leucine existed. Leucine contamination of our purified diet, principally from cornstarch but also from other unknown sources, in this and previous work probably explains why negative nitrogen balance could not be attained in the adult porcine fed what had been considered to be a leucine-free diet.