The end-product method of measuring whole-body protein turnover: a review of published results and a comparison with those obtained by leucine infusion

Abstract

The present review summarizes the results of all published papers on whole-body protein turnover in man measured by [ The pros and cons of the two methods are compared and it is suggested that there is a place for further work by the less invasive end-product method, particularly for population studies of the genetic, environmental and functional determinants of whole-body rates of protein synthesis. Protein turnover describes the continual synthesis and breakdown of protein in the body. It is a fundamental biological process in all living organisms, and over the last 50 years workers have attempted to quantify rates of turnover. At the present time the 'gold standard' method of measuring protein turnover in the whole body in man is the 'precursor' method, in which labelled leucine is given by intravenous (IV) infusion for two or more hours until a steady state of enrichment has been achieved in plasma leucine. The purposes of the present review are: to assess what has been achieved over the years by an alternative and historically earlier method, the end-product approach with [ 15 N]glycine; to examine as far as possible the validity of the method by comparing its results with those of the precursor method; to look at the pros and cons of the two methods. The end-product approach would be particularly useful for population studies, which could help to unravel the determinants of individual variation. It is also more easily applied in difficult situations outside a hospital or metabolic ward, as in the studies of Stein et al. (1996) on weightless subjects during a space flight or those of Stroud et al. (1996) during a crossing of Antarctica. We believe that a wider use of the end-product method would help to extend our knowledge of the extremely important biological function, protein turnover.

Postexercise protein ingestion increases whole body net protein balance in healthy children

“…In contrast, 24-h rates of whole body protein turnover in the present study were equivalent to those measured in healthy free-living children utilizing similar methods (7); this is likely due to the incorporation of a fed-state period in the previous (7) and present studies, which would be necessary to elevate rates of protein turnover. In the present study, for example, rates of protein synthesis and protein breakdown as well as WBPB were greatest during the early 9-h exercise and recovery period, which is in agreement with the ability of exercise and protein feeding to increase whole body and muscle protein turnover in adults (11,14). The overnight fasted period, on the other hand, is characterized by lower rates of turnover and a net catabolism of lean tissue, which likely contributed to the attenuation in protein turnover and the subsequent WBPB when measured over 24 h in the present study.…”

“…Therefore, given the relatively small differences in protein intake between conditions, it is possible that if our intervention period persisted for a greater duration of time that rates of whole body protein turnover may eventually diverge between conditions. Alternatively, the sensitivity of oral [ 15 N]glycine may have precluded our ability to detect differences in protein turnover with the relatively small acute differences in protein intake between conditions that other methodology, such as intravenous infusions, may have permitted; however, discrepancies between measured rates of whole body protein kinetics by oral [ 15 N]glycine and [ 13 C]leucine infusion are more pronounced with measurements made in the fasted rather than fed state (14). Nevertheless, in general, whole body protein turnover (i.e., both synthesis and breakdown) is higher in children than adults and can be less sensitive to even large fluctuations in protein intake (19), which opens the possibility that even subtle changes in whole body protein synthesis and/or protein breakdown with exercise or nutrition could translate into physiologically more meaningful differences in WBPB because of these rapid rates of tissue remodeling.…”

“…Our decision to utilize [ 15 N]glycine methodology to measure whole body protein metabolism, which has a long history of use in humans of all ages (14), was related to the ease with which protein kinetics can be measured over relatively long time frames (e.g., up to 24 h) (19), the relatively low withinsubject variability (16), and, because of the oral ingestion and urinary endpoint analysis, its feasible and ethical application in healthy children. In addition, although urinary nitrogen excretion can account for ϳ80 -85% of all nitrogen loss in active adults (39,40), we also utilized the measured exercise-induced change in body mass with average sweat nitrogen content (1) to estimate sweat nitrogen excretion, which can contribute ϳ10% of nitrogen loss in active adults (39) and, if not accounted for, would subsequently increase the apparent net protein balance (1); this approach, in contrast to previous studies in active children that only considered urinary nitrogen excretion (7,8,33), would result in a conservative estimate of WBPB in our study in the absence of the technically and logistically challenging determination of all potential routes of nitrogen loss (e.g., fecal, integumentary, hair, etc.).…”

A critical review of recommendations to increase dietary protein requirements in the habitually active

“…A label of the essential amino acid leucine has also been used to study whole-body protein metabolism during endurance exercise and a comparison between the glycine and leucine tracer techniques has been performed (54) . A stable isotope infusion of the branched-chain amino acid [1-13 C]leucine indicated that there was no difference in amino acid oxidation between rested, fasting endurance trained athletes when compared with age-, sex-and body weight-matched sedentary controls (55,56) .…”

Stable isotopes in the diagnosis and treatment of inherited hyperammonemia

“…The measurement of urea production via isotopic methods is often used as an estimate of net protein catabolism in those with an intact urea cycle [3,4,[13][14][15]. However, the application of metabolic tracers also has an important role in studying the urea cycle flux in patients with defects in the urea cycle.…”