Protein Oxidation as a Relevant Factor Determining Protein Status in Humans

Abstract

INTRODUCTIONDefining optimal protein intake in various metabolic conditions like the newborn, aging and during exercise is highly relevant. Recently, it has been suggested that high protein intake in newborn could lead to excessive weight gain in later life (Kirchberg et al. 2014). Limited protein oxidation could be a relevant parameter directly related to this.OBJECTIVESTo monitor protein oxidation we developed a non‐invasive 13C‐protein‐derived breath test using naturally enriched milk protein (fractions). We validated and applied this technique, in a pilot study, under various metabolic conditions in healthy adult males.METHOD/DESIGNAfter baseline sampling, 30 g of naturally labeled 13C‐milk protein were consumed. Breath samples were taken every 10 min during 5,5 h and 13CO2 was measured by Isotope Ratio Mass Spectrometer. The following variables were used to calculate the amount of substrate oxidized: administered dose, 13C enrichment of substrate, molecular weight of substrate, number of carbon atoms in a substrate molecule, estimated CO2‐production of the subject based on body surface area.RESULTSPostprandial kinetics of oxidation of whey (rapidly digestible protein) and casein (slowly digestible protein) derived from our breath test were comparable to literature data regarding the kinetics of appearance of amino acids in blood (Boirie et al. 1997). Using this test we could demonstrate that 24 ± 3% (mean ± sd) of the milk protein dose was oxidized over 5,5 h which served as our control (100%). A decrease of 31% ± 18% in milk protein oxidation was observed after a 3 day protein restricted diet (~10 g/day) compared to a normal diet and 30 minutes of cycling before protein consumption reduced protein oxidation by 39% ± 37%. A protein restricted diet plus exercise reduced oxidation by 52% ± 19%.CONCLUSIONSProtein oxidation, which can be monitored in breath, is a significant factor in protein metabolism. With our technique we are able to characterize changes in overall protein oxidation under various metabolic conditions such as diet and exercise, which could be relevant for defining optimal protein intake under these conditions and can be extended to measurements during aging and in newborn.Support or Funding InformationThe project is financially supported by Hanze Nutrition BV