Absorptive Behavior of Oligo-L-Methionine and Dietary Proteins in a Casein or Soybean Protein Diet: Porto-Venous Differences in Amino Acid Concentrations in Unrestrained Rats

Abstract

In a low casein methionine-deficient diet, supplementation with oligo-L-methionine (OM) has a similar improvement on rat growth to supplementation of free L-methionine. Rat growth is little improved, however, when OM is added to a low soybean protein isolate (SPI) diet. To evaluate the mechanisms of the different effects of OM supplementation, we observed intestinal absorption of OM added to either an 8 g/100 g casein diet or an 8 g/100 g SPI diet. Using portal and venous cannulated rats under unrestrained conditions, we evaluated OM absorption by subtracting methionine porto-venous differences after feeding the diets without OM from the values after feeding the diets with 3% added OM. OM absorption was faster after feeding the casein-based diet than after the SPI-based diet during the initial stages of the experiment only, but throughout the experiment there were greater increments of methionine concentrations in both portal and venous blood after the casein-based diet than after the SPI-based diet. We also discovered that amino acids from the dietary casein were absorbed faster than those from SPI, using the porto-venous difference of isoleucine as an indication. The larger increment of methionine concentrations in the blood after feeding of OM with a casein diet and the rapid absorption of amino acids from casein may result in much growth improvement in rats.