Comparison of isotopic tracer techniques for measurement of whole body proteolysis in response to diet intervention and exercise training

Date of Completion

January 2005


Chemistry, Biochemistry|Health Sciences, Nutrition




Research employing simultaneous use of tracers has documented different findings dependent on the specific tracer, even when other study parameters remain constant. This line of inquiry is limited, especially with regard to comparisons between two of the tracers most commonly employed in assessment of protein breakdown: [2H5]-phenylalanine and [1- 13C]-leucine. This investigation consists of two studies which provide a direct comparison of these tracers in response to diet intervention and exercise training. In the first investigation, [2H5]-phenylalanine and [1-13C]-leucine were used to assess and compare rates of whole body proteolysis during recovery from an acute bout of exercise in male runners consuming eucaloric diets providing 3.6(HP), 1.8(MP), and 0.8(LP) g/kg/day of protein for 4 weeks. There was a significant increase (p<0.05) in whole body proteolysis as assessed by [2H5]-phenylalanine and [1-13C]-leucine rates of appearance following an endurance exercise bout in these as athletes protein intakes increased. This showed similar changes in whole body proteolysis in response to habitual consumption of increased dietary protein following recovery from an acute bout of endurance exercise irrespective of the tracer. ^ In the second investigation, [2H5]-phenylalanine and [1-13C]-leucine kinetics, as well as the single and reciprocal pool models of leucine kinetics were used to assess and compare rates of whole body protein breakdown following 4 weeks of an aerobic exercise training in healthy, weight stable individuals consuming 0.8g/kg/day of protein. Aerobic exercise training increased whole body protein breakdown according to estimates based on [1-13C]-leucine rate of appearance, while exercise training had no effect on proteolysis when [2H5]-phenylalanine kinetics were applied. Single and reciprocal pool estimates both documented a decrease in whole body proteolysis posttraining, while only the reciprocal pool detected a decrease in leucine oxidation. This study highlights the impact choice of isotopic tracer and subsequent modeling has on research findings derived from studies seeking to characterize the effects of chronic endurance exercise on whole body protein breakdown. ^ Future research is warranted to investigate similarities and discrepancies that occur when simultaneously employing multiple isotopic tracers and/or models. Studies designed to test for differences between isotopic tracers and/or models in response to interventions would provide key insight into factors that contribute to discrepancies between tracers and/or models. Research designed to investigate the mechanisms that cause estimates of whole body protein breakdown to differ between tracers and/or models would help researchers design studies aimed to specifically capture treatment effects independent of validity of assumptions specific to and required for isotope modeling studies. ^