Blood 15N:13C Enrichment Ratios Are Proportional to the Ingested Quantity of Protein with the Dual-Tracer Approach for Determining Amino Acid Bioavailability in Humans.
J Nutr 2020;
150:2346-2352. [PMID:
32710787 PMCID:
PMC7467852 DOI:
10.1093/jn/nxaa205]
[Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2020] [Revised: 04/06/2020] [Accepted: 06/25/2020] [Indexed: 12/28/2022] Open
Abstract
BACKGROUND
Assessment of amino acid bioavailability is of key importance for the evaluation of protein quality; however, measuring ileal digestibility of dietary proteins in humans is challenging. Therefore, a less-invasive dual stable isotope tracer approach was developed.
OBJECTIVE
We aimed to test the assumption that the 15N:13C enrichment ratio in the blood increases proportionally to the quantity ingested by applying different quantities of 15N test protein.
METHODS
In a crossover design, 10 healthy adults were given a semi-liquid mixed meal containing 25 g (low protein) or 50 g (high protein) of 15N-labeled milk protein concentrate simultaneous with 0.4 g of highly 13C-enriched spirulina. The meal was distributed over multiple small portions, frequently provided every 20 min during a period of 160 min. For several amino acids, the blood 15N- related to 13C-isotopic enrichment ratio was determined at t = 0, 30, 60, 90, 120, 180, 240, 300, and 360 min and differences between the 2 meals were compared using paired analyses.
RESULTS
No differences in 13C AUC for each of the measured amino acids in serum was observed when ingesting a low- or high-protein meal, whereas 15N AUC of amino acids was ∼2 times larger on the high-protein meal (P < 0.001). Doubling the intake of 15N-labeled amino acids increased the 15N:13C ratio by a factor of 2.04 ± 0.445 for lysine and a factor between 1.8 and 2.2 for other analyzed amino acids, with only phenylalanine (2.26), methionine (2.48), and tryptophan (3.02) outside this range.
CONCLUSIONS
The amino acid 15N:13C enrichment ratio in the peripheral circulation increased proportionally to the quantity of 15N-labeled milk protein ingested, especially for lysine, in healthy adults. However, when using 15N-labeled protein, correction for, e.g., α-carbon 15N atom transamination is advised for determination of bioavailability of individual amino acids. This trial was registered at www.clinicaltrials.gov as NCT02966704.
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