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Updated: Jul 24, 2026
Metabolic Pathway Confirmation and Discovery Through 13C-labeling of Proteinogenic Amino Acids
Published on: January 26, 2012
A novel pulse tracer method to estimate the relationship between amino acid meal composition and its intracellular
Nicolaas E P Deutz1, Savanah B Knezek1, Mariëlle P K J Engelen1
1Center for Translational Research in Aging & Longevity, Texas A&M University, USA.
Background:
Continuous intravenous stable isotope infusion approaches underestimate the anabolic response to a protein meal. Pulse tracer approaches can estimate the intracellular appearance of many amino acids simultaneously. We now combined the pulse tracer approach with intake of a mixture of essential (EAA) or a combination of EAA and non-essential amino acids (TAA) to better estimate the anabolic response.
Methods:
In a randomized, single-blind, placebo-controlled crossover study, 11 older adults (60-80 years) consumed, on separate study days at least three days apart, three oral drinks via sip feeding containing: (a) 20 g EAA, (b) 45 g TAA, or (c) water (baseline). The pulse stable isotope method, containing labeled EAA, BCKA, and many non-essential amino acids, was used to assess their whole-body and compartmental kinetics. Blood samples were collected both pre- and post-pulse IV administration and analyzed using LC-MS/MS and GC-MS.
Results:
Plasma amino acid concentrations increased in relation to the composition of the meals, yet showed no clear correlation with the extracellular pool (Q1) or the intracellular pool (Q2) size changes. For phenylalanine and threonine, the intracellular disposal aligned with the intake. For all other essential amino acids, the % increase in the intracellular disposal was more in agreement with % of the recommended daily allowance, independent of whether consumed in the EAA or TAA meal. When insufficient amounts of amino acids were consumed, these amino acids were sourced from other (unidentified) pathways. The intracellular response of non-essential amino acids in relation to the amount consumed is complex.
Conclusions:
We show that intracellular amino acid metabolism can be measured using the pulse amino acid tracer approach during intake of free amino acid mixtures. We observed that both the EAA and TAA mixtures stimulate intracellular protein synthesis, and that the elevated disposal of essential amino acids appears to align with the body's amino acid composition. Furthermore, the intracellular appearance of phenylalanine and threonine, in relation to the intake, can be used to estimate the bioavailability of dietary proteins. Our approach can also be applied to different mixed meals with different types of proteins to estimate the anabolic response and effect on metabolism.
Clinical Trial Registry:
Trial registration ClinicalTrials.gov: NCT05400733.
