Amino Acid Metabolism and Protein Requirements in Active, Trained Adult Males Using the Indicator Amino Acid Oxidation (IAAO) Technique

Abstract

To better understand the nutritional requirements of active populations, the present study utilized the minimally invasive Indicator Amino Acid Oxidation (IAAO) technique to evaluate the impact of variable intensity exercise on protein requirements in active, trained young adult males. It was hypothesized that protein requirements would be: 1) greater than the current requirements established on the basis of NBAL, and; 2) greater than those requirements previously determined by the IAAO technique in non-active populations. Six active, trained males (23±1 years; 177.5±6.2 cm; 82.3±5.7 kg; 13.5±4.3% body fat; 52.3±5.4 ml O2/kg/min; means±95%CI) partook in 1-10 sequential metabolic trials each consisting of a variable intensity exercise protocol (modified Loughborough Intermittent Shuttle Test) followed by 8 hourly meals providing a variable amount of protein (0.2-2.6g/kg/d), 6g/kg of carbohydrate, and sufficient energy. Protein was provided as crystalline amino acids modeled on the basis of egg protein with the exception of tyrosine (40mg/kg/d) and phenylalanine (30.5mg/kg/d with 5.46 mg/kg over 4h as L-[13C]phenylalanine). After determination of breath 13CO2 by continuous-flow isotope ratio mass spectrometry and CO2 production by indirect calorimetry, the estimated average requirement (EAR) was determined from the breakpoint of the 13C02 excretion after application of bi-phase linear regression. Analysis for the correlation between protein intake and 13CO2 excretion revealed the EAR to be 1.35 g/kg/d (r2= 0.64). Accounting for a safe intake encompassing the upper 95%CI, the protein requirement of 1.64 g/kg/d determined herein exceeds the current dietary reference intake for non-active individuals as determined by NBAL (EAR = 0.66 g/kg/d; RDA = 0.80 g/kg/d) (FAO, WHO 2007) and the minimally invasive IAAO technique (EAR = 0.93 g/kg/d; RDA = 1.2 g/kg/d) (Humayun et al., 2007). In addition the minimum intake determined herein is at the upper range of general recommendations for athletes (i.e. 1.2-1.7g/kg/d) (American Dietetic Association et al., 2009). These results suggest that intensive exercise increases daily protein requirements in healthy, active young men and underscore the need to further evaluate the adequacy of protein recommendations in other active populations (e.g. women, children).