Consequences of different strategies of free amino acid supplementation to dietary proteins for physiological utillization

M. Gas

Research output: Thesisinternal PhD, WU


The efficiency of using free amino acids (AAs) as dietary constituent is sometimes lower than that of AAs derived from intact protein. The aim of the project was to evaluate dietary management conditions, which can determine the efficiency of utilization of crystalline AAs in animal diets or in clinical nutrition. The studies in this thesis were focused mainly on differences in short-term catabolism between protein bound and free AAs during the post prandial phase of a meal. The stable isotope technique was used in model studies with laboratory Wistar (WU) rats. In different experiments, so-called [ 13 CO 2 ]-breath test studies were used to compare the metabolic fate of free and protein-bound [1- 13 C]-labeled AAs in a meal in various nutritional situations. The influence of free AA supplementation strategies on weight gain development was also studied. Moreover, protein and fat content in the liver and carcass were analysed.The results of the present study confirm literature that showed a higher post prandial catabolic losses of the tracer when dietary protein is replaced with crystalline AAs. In some, but not all situations a lower weight gain was observed for growing animals. Our results showed that short-term catabolic losses of endogenous leucine were modulated by exogenous AA supply. It increased or decreased depending on the adequacy of the dietary supply.The common practice for free AA supplementation is to mix it with the deficient protein. In our study we showed that during the post prandial period the best utilization of methionine deficient protein did not occur when methionine supplement was given simultaneously with methionine deficient meal. The best utilisation occurred when there was a delay between the supply of deficient meal and free methionine. Therefore, introducing time interval of free amino acid supplementation to poor quality protein should be considered as a factor with potential to improve physiological utilization of dietary amino acids.A 1h time interval for free methionine supplementation influenced the weight gain, but differently depending on the protein level. Animals fed very deficient protein diet (5%) showed higher weight gain when supplementation of the deficient free methionine was provided with a 1h delay (1h interval). With protein deficient diets and less than 7.5 % protein we found fatty livers in our experiment. We concluded that post prandial AA oxidation influences the post absorptive AA catabolism. This does not always mean effect on growth. For growth in rats lysine deficiency is most limiting but it seems that even a 34% methionine deficiency below the NRC recommendation did not limit growth. We found that methionine deficiency influenced fat metabolism and from the literature we concluded that the mechanism probably works via choline and carnitin
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Verstegen, Martin, Promotor
  • Schreurs, Victor, Co-promotor
  • Bujko, J., Co-promotor, External person
Award date20 Jan 2006
Place of Publications.n.
Print ISBNs9789085043355
Publication statusPublished - 20 Jan 2006


  • rats
  • animal models
  • diets
  • feeds
  • free amino acids
  • feed supplements
  • dietary protein
  • catabolism
  • oxidation
  • nutrition physiology


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