Protein digestion and absorption in young chicks and turkeys.

The site of digestion and absorption of protein was examined in chicks fed soybean meal and casein and in turkeys fed soybean meal, using 91YCl3 as a non-absorbed reference substance. In soy-fed chicks, peptides with molecular weight (MW) between 8,000 to 15,000 accumulated in the proximal small intestine, and low MW peptides accumulated distally. A similar pattern was observed in soy-fed turkeys, although with events occurring proximally to those in the chick. In casein-fed chicks, a gradual reduction in the peptide MW was observed with increasing distance from the pylorus. The rates of formation and absorption of peptides with MW below 500 were similar throughout the small intestine in soy-fed chicks; whereas, the rate of their formation exceeded absorption rate in casein-fed chicks. Net secretion of nitrogen into the duodenum was observed in soybean-fed chicks, no net secretion in casein-fed chicks and 50% absorption in soybean-fed turkeys with net secretion in the gizzard. From the sequence of digestive and absorptive events, apparently solubilization and breakdown of intermediate peptides are rate-limiting in protein absorption from soybean meal diets. But when casein is fed, the rate of uptake of low MW peptides appears to be limiting.