Analyzing Short Term Protein Oxidation Provides Evidence about Its Role in Whole Body Protein Metabolism

INTRODUCTION Protein intake has been suggested to play an important role in development of obesity; on one hand in newborn, where excess protein intake might stimulate development of obesity in later life (Kirchberg et al. 2014). On the other hand, protein consumption is promoted for weight management (Martens et al. 2014). At present, no formal recommendations for the upper limit of protein intake have been presented. Information about rate‐limiting steps in protein catabolism is scarce. We therefore studied the role of protein oxidation in this process. OBJECTIVES To monitor protein oxidation we have developed a non‐invasive natural enriched 13 C‐protein breath test using naturally enriched 13 C‐milk protein (fractions). We used this technique to analyze the kinetics of protein oxidation and its interfering factors. METHOD/DESIGN The breath test was preceded by an overnight fast. After consuming the substrate, dissolved in 500 ml water, breath samples were collected up to 330 minutes and analyzed by Isotope Ratio Mass Spectrometry for the 13 CO 2 : 12 CO 2 ratio. RESULTS Whey was oxidized for 28.6% ± 6.8% (mean ± SD) after 240 minutes, compared to 22.9% ± 6.2% for glucose in the same period. Postprandial oxidation kinetics of glucose and whey started to diverge after 70 min after which whey was being oxidized faster than glucose. Different doses of milk protein (10, 30, 50, 60, 70 g) were oxidized for 15.3, 24.2, 22.3, 21.5 and 14.5% respectively. Maximal milk protein oxidation was measured during the 60 g dose at a rate of 0.062 g/min, leading to a maximal oxidation capacity of 1.3 g/kg bodyweight/day. Combining protein (30 g 13 C‐whey) with carbohydrates (30 g non‐labeled glucose) resulted in two‐fold increase in oxidation of 13 C‐whey compared to oxidation of 13 C‐whey alone. A three‐fold increase in oxidation of 13 C‐glucose was measured when non‐labeled whey was added compared to oxidation of 13 C‐glucose alone. CONCLUSIONS Whey is oxidized faster than glucose from 70 min onwards, which seem to suggest that stomach emptying was not the cause of the difference in oxidation. The interaction of whey and glucose suggest that the oxidation of whey is not driven by energy demand. The stimulating effect of glucose might hint to the involvement of insulin. Oxidation of whey protein is quantitatively of high importance and not down regulated by concomitant administration of other substrates. Analyzing biological variations in acute protein oxidation in defining the upper limit of intake, seem to be relevant. Support or Funding Information The 13 C‐milk substrates were provided by Hanze Nutrition BV.