Elsevier

The Journal of Nutrition

Volume 150, Issue 8, August 2020, Pages 2041-2050
The Journal of Nutrition

Editor's Choice
Protein Type, Protein Dose, and Age Modulate Dietary Protein Digestion and Phenylalanine Absorption Kinetics and Plasma Phenylalanine Availability in Humans

https://doi.org/10.1093/jn/nxaa024Get rights and content
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open access

ABSTRACT

Background:

Dietary protein ingestion stimulates muscle protein synthesis by providing amino acids to the muscle. The magnitude and duration of the postprandial increase in muscle protein synthesis rates are largely determined by dietary protein digestion and amino acid absorption kinetics.

Objectives:

We assessed the impact of protein type, protein dose, and age on dietary protein digestion and amino acid absorption kinetics in vivo in humans.

Methods:

We included data from 18 randomized controlled trials with a total of 602 participants [age: 53 ± 23 y; BMI (kg/m2): 24.8 ± 3.3] who consumed various quantities of intrinsically ʟ-[1-13C]-phenylalaninelabeled whey (n = 137),casein (n = 393), or milk (n = 72) protein and received intravenous infusions of ʟ-[ring-2H5]-phenylalanine, which allowed us to assess protein digestion and phenylalanine absorption kinetics and the postprandial release of dietary protein–derived phenylalanine into the circulation. The effect of aging on these processes was assessed in a subset of 82 young (aged 22 ± 3 y) and 83 older (aged 71 ± 5 y) individuals.

Results:

A total of 50% ± 14% of dietary proteinderived phenylalanine appeared in the circulation over a 5-h postprandial period. Casein ingestion resulted in a smaller (45% ± 11%), whey protein ingestion in an intermediate (57% ± 10%), and milk protein ingestion in a greater (65% ± 13%) fraction of dietary proteinderived phenylalanine appearing in the circulation (P < 0.001). The postprandial availability of dietary proteinderived phenylalanine in the circulation increased with the ingestion of greater protein doses (P < 0.05). Protein digestion and phenylalanine absorption kinetics were attenuated in older when compared with young individuals, with 45% ± 10% vs. 51% ± 14% of dietary protein–;derived phenylalanine appearing in the circulation, respectively (P = 0.001).

Conclusions:

Protein type, protein dose, and age modulate dietary protein digestion and amino acid absorption kinetics and subsequent postprandial plasma amino acid availability in vivo in humans. These trials were registered at clinicaltrials.gov as NCT00557388, NCT00936039, NCT00991523, NCT01317511, NCT01473576, NCT01576848,NCT01578590, NCT01615276, NCT01680146, NCT01820975, NCT01986842, and NCT02596542, and at http://www.trialregister.nl as NTR3638, NTR3885, NTR4060, NTR4429, and NTR4492.

Keywords:

healthy aging
muscle protein synthesis
muscle mass maintenance
splanchnic extraction
gsarcopenia

Abbreviations used:

BM
body mass
Ra
rate of appearance
RCT
randomized controlled trial
T
tertile

Cited by (0)

Supported by Top Institute Food and Nutrition , which is a publicprivate partnership on precompetitive research in food and nutrition.

Author disclosures: LJCvL and LBV have received research grants, consulting fees, speaking honoraria, or a combination of these, from Friesland Campina, Nutricia Research, and PepsiCo. The other authors report no conflicts of interest.

The industrial partners contributed to the project through regular discussions.