Search

Nitrogen-15 from single tagged feeding has been used to measure nitrogen in urine from a given protein source. Comparative studies of retention of absorbed protein in young and older subjects have been presented. The average retention in the young subjects was found to be 57.6%, but only 49.1% in the older subjects. The half-life of N15from yeast in the body was found to average 75 days for the entire group studied: for the young controls 61 days and for the older subjects 86 days. The rate of protein turnover in the young controls was found to be approximately one and one half times that in the older subjects. Increased physiologic age would seem a probable cause for this reduced utilization of protein. The average nitrogen requirement from the pool for protein synthesis in the body was found to be 0.23 gm per kilogram per day. The nitrogen requirement for maintenance for a 70-kg man was found to be 16 gm per day.

Methods for the study of protein absorption in man have been discussed. A yeast protein tag with an N15content of 58.3 atom % excess was prepared. Evidence for the general distribution of the tag throughout the protein fraction of the yeast has been reviewed. Comparative absorption studies in young and old subjects have been presented. A mean yeast protein absorption of 90.3% among 6 subjects has been determined. Achlorhydria, hypochlorhydria, and age do not appear to produce depressing effects on the capacity to absorb a protein tracer during continued high-level protein intake. The role of gastric acidity in protein utilization will be further discussed in a subsequent communication. The single-dose tracer method of determining protein absorption has been shown to obviate the need for evaluation of endogenous nitrogen.