Effect of soy protein and casein intake on intestinal absorption and lymphatic transport of cholesterol and oleic acid.

Rats were fed for 4 wk on defined diets containing either casein or soy as the protein source, or diets in which the lysine/arginine ratios were modified by addition of arginine to casein, and lysine to the soy diet. During this period, weight gains and food intakes were comparable in the four dietary groups. Animals were subjected to cannulation of the left thoracic lymphatic duct, and after an overnight fast, were given a single intragastric dose of a lipid emulsion containing oleic acid and cholesterol. The overall 24-h recoveries of cholesterol and fatty acid in lymph were similar in the four groups, as were the distribution of lipids among the major lipid fractions and lipoprotein classes of thoracic duct lymph. However, analysis of timed lymph collections indicated that absorption of lipids was more rapid in casein-fed rats than in those fed soy protein. Furthermore, addition of arginine to the casein diet resulted in a slowed rate of lipid absorption, and addition of lysine to the soy diet markedly increased the rate of lipid absorption.