Postprandial oxidative losses of free and protein-bound amino acids in the diet: interactions and adaptation

J.A. Nolles, A.M. Verreijen, R.E. Koopmanschap, M.W.A. Verstegen, V.V.A.M. Schreurs

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20 Citations (Scopus)


Postprandial oxidation of dietary free amino acids or egg white protein was studied using the [13CO2] breath test in rats, as well as in humans. Thirty-eight male rats were assigned to four dietary test groups. Two diets only differed in their protein fraction. Diet I contained 21% egg white protein. For the breath test egg white protein, intrinsically labelled with [1-13C]-leucine, was used as a substrate. Diet II contained the same amino acids as diet I, though not as egg white protein but in free form. Free [1-13C]-leucine was used to label this diet. In addition, two 1:1 mixtures of both diets were used. During the breath test either the free amino acid or the protein fraction was labelled as in diets I or II. The animals were breath-tested following short-term (day 5) and long-term adaptation (day 20) to their experimental diet. For all diets, including the mixed diets, postprandial oxidative losses on day 5 were significantly higher for the free leucine compared with the protein-derived leucine. Differences between free and protein-derived leucine oxidation had, however, largely disappeared on day 20. The human subjects were breath-tested without any adaptation period to the diets. The oxidative losses of free leucine were also higher than those of protein-derived leucine. None of the studies showed any indication for an interaction between the oxidation of protein-derived amino acids and free amino acids. It is concluded that free and protein-derived amino acids in the diet are mainly metabolized independently
Original languageEnglish
Pages (from-to)431-438
JournalJournal of Animal Physiology and Animal Nutrition
Issue number4
Publication statusPublished - 2009


  • growing-pigs
  • nitrogen-utilization
  • free lysine
  • breath test
  • humans
  • metabolism
  • assimilation
  • catabolism
  • retention
  • casein


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