Studies of Protein Retention and Turnover Using Nitrogen-15 as a Tag

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Abstract
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 N15 from 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.