Digestion and absorption of casein at different dietary levels in the chick: effect on fatty acid and bile acid absorption.

The site of digestion and absorption of protein was determined in chicks fed diets containing 10, 30 and 45% casein as the protein source, 3% added oil with 91Y Cl3 added as a non-absorbed reference substance. Digestion of protein to low molecular weight (MW) peptides and amino acids was rapid in all diets with major absorption occurring between duodenum and lower jejunum. Increasing the dietary casein resulted in increases in low MW peptide levels in the duodenum. This increase was disproportionate with intake when 45% casein was fed, suggesting that absorptive capacity of the duodenum was exceeded. Overall nitrogen absorption was similar in all treatments reflecting the increased participation of the ileum in nitrogen absorption when high dietary casein levels were fed. Fatty acid and bile acid absorption was depressed when 45% casein was fed, mainly due to inhibition of absorption in the ileum, presumably by binding to undigested protein.