Serum amino acids as indicators of protein status in gestating ewes.

Changes in the serum amino acids of ewes obtained with regulated protein intakes may be of value for indicating the status of protein metabolism during gestation. Thirty crossbred ewes in midgestation were assigned in equal numbers to three treatments of 12%, 9% and 7% crude protein, isocaloric diets fed al libitum. Serum amino acids were determined by reverse phase, high performance liquid chromatography. Several consistent amino acid changes were related to protein intake over the last half of gestation. Glutamine plus alanine were increased and the branched chain amino acids (isoleucine, valine and leucine) were reduced. These responses suggest that tissues, such as muscle, undergo catabolism during late gestation as a result of low protein intake to release glutamine and alanine for gluconeogenesis. Serum Leu was reduced by a decreased level of dietary protein indicating inadequate protein status. Lowering protein intake also induced a consistent increase in serine through the last half of gestation. Increasing serine values may be the result of induced synthesis by liver in response to the low protein intake. This possibility suggests that the protein status of gestating ewes can be evaluated by following serum amino acid changes and the release of amino acids from specific tissues such as liver and muscle can be monitored through changes in the circulating amino acid values.