Postprandial fate of amino acids: adaptation to molecular forms

J.A. Nolles

Research output: Thesisinternal PhD, WU


During the postprandial phase dietary proteins are digested to peptides and amino acids and absorbed. Once absorbed the peptides are further hydrolyzed to amino acids and transported to the tissues. These amino acids are largely incorporated into body proteins. Not all amino acids are, however, incorporated into body proteins part of these amino acids are oxidized, and can, thus, no longer be utilized to support protein metabolism in the body. The objective of this thesis was to increase the understanding of those processes that determine the utilization efficiency of dietary proteins. The studies described in this thesis, focused on the appearance rate of dietary amino acids in the free amino acid pool of the body. The rate of appearance of dietary amino acids in this pool has been shown to modulate the postprandial oxidation of amino acids and thereby also their utilization efficiency for physiological purposes. As a consequence postprandial oxidative losses influence the nutritional protein status of the body. This thesis aimed to elucidate whether the body is able to cope with diets in which the amino acid appearance rate is high and what mechanisms are involved in this process.

First an effort was made to establish the metabolic consequences of amino acid sources with a high appearance rate in a rat and a human model. Postprandial oxidation of free or protein derived [1-13C]- leucine was determined in a [13CO2]-breath test, using both a diet consisting of only free amino acids including [1-13C]-leucine and a diet consisting of proteins in which [1-13C]-leucine was incorporated, and 1:1 mixtures of both diets. In those mixed diets either the protein part or the free amino acid part was labeled. The postprandial oxidative losses of dietary leucine after 5 days being fed these diets (short-term adaptation) appeared to be significantly higher for the free amino acid diet compared to the protein diet. These differences between dietary free amino acids and dietary protein persisted in the mixed diets, as measured by the [13CO2] breath-test. It was concluded that amino acids derived either from a free amino acid or a protein diet, were handled independently even when ingested simultaneously during the same meal. Results obtained in rats were comparable to the results obtained in humans.

The differences in oxidation between a free amino acid and a protein diet had largely disappeared after long-term adaptation (after 26 to 30 days on the diet). An adaptive decrease in the oxidation of free amino acids was observed. In the second study it was examined to which extent increasing levels of methionine supplements in a diet (50, 100 or 200% methionine supplement relative to casein) were retained in body protein. This was thus far not clear since a higher appearance rate in the free pool has been reported to have a negative influence on the efficiency of utilization of amino acids from the diet. Moreover, only specific patterns of amino acids are supposed to be deposited in body protein. Higher dietary methionine levels resulted in higher postprandial oxidative losses of methionine. The groups, which were fed the diets with the highest methionine levels, showed the lowest methionine retention as part of intake but the highest retention in absolute terms. After long term adaptation, however, to the free amino acid diets, methionine retention was increased in all groups. It was concluded that postprandial retention of dietary amino acids is, at least in part, driven by the amino acid composition of the diet.

In third study it was examined whether the postprandial fate of different dietary amino acid was regulated by hormonal responses to the diet. It has been observed that the differences in oxidative losses between diets consisting of free amino acids or protein were not mediated by the combined action of insulin, glucagon, corticosterone and GH. Hence, postprandial catabolism of amino acids is probably regulated by other mechanisms. As stated above, the amino acid appearance rate plays a crucial role in determining the postprandial utilization of amino acids. In the fourth study it was, therefore, investigated, whether the amino acid absorption rate can adapt to dietary free amino acids. Rats were kept on a free amino acid diet for 0 (nonadapted), 5, or 26 to 30 days (long-term adaptation). The methionine absorption of long-term adapted rats was lower than that of the non-adapted rats. It was concluded that the absorption of amino acid by the intestine plays a crucial role in minimizing the postprandial oxidative losses.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Verstegen, Martin, Promotor
  • Schreurs, Victor, Co-promotor
Award date9 Jan 2006
Place of PublicationS.l.
Print ISBNs9789085043423
Publication statusPublished - 9 Jan 2006


  • rats
  • animal models
  • amino acids
  • diets
  • animal nutrition
  • oxidation
  • metabolism
  • adaptation
  • intestinal absorption
  • nutrition physiology


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