The use of 15 N-alanine as a tracer to measure whole body protein turnover (WBPT) in man - abstract

ABSTRACT

A preliminary study was designed to determine the most suitable metabolic condition (fed, fasted) and appropriate time (day, night) to measure whole body protein turnover. A primed intermittent oral dose of the tracer was given; urinary ammonia and urea were used as end-products. With feeding, an isotopic steady state was maintained but upon withdrawal of food it was disrupted. Also, as fasting progressed, the enrichment in urinary ammonia and urea kept increasing. This indicates that alanine did not seem to be a good tracer to measure WBPT under metabolic conditions, where the intake of food was not constant. In the fed state, rates of WBPT were measured after a primed, intermittent oral dose and after a primed, continuous intravenous infusion. Six subjects were used in the oral study and two of the six subjects were chosen for the intravenous study. During the oral administration, mean rate of WBPT as measured by the enrichment in urinary ammonia and urea, was found to be 2.25 ñ .2 gm protein/kg/day and 3.00 ñ .4 gm protein/kg/day respectively. The value obtained from the intravenous study was 1.87 gm protein/kg/day with ammonia and 4.57 gm protein/kg/day with urea. The results show that, within studies estimates of protein flux from ammonia and urea generally were not comparable, oral doses gave higher estimates with ammonia whereas the intravenous dose gave higher estimates with ureas. In contrast with the most commonly used tracer (15 N-glycine), 15N-alanine gave estimates that were lower when the respective technique and end product was considered. However, the rank order of these values for the different method was similar to those with 15 N-glycine (AU)