Protein Type, Protein Dose, and Age Modulate Dietary Protein Digestion and Phenylalanine Absorption Kinetics and Plasma Phenylalanine Availability in Humans.
Level 1 - systematic review of randomized trials
Pooled individual participant data analysis / meta-analysis of 18 randomized controlled trials
PubMed 32069356 · doi:10.1093/jn/nxaa024
What was done
A pooled analysis was conducted using data from 18 randomized controlled trials with 602 participants (mean age 53 ± 23 y, BMI 24.8 ± 3.3 kg/m²). Participants ingested intrinsically L-[1-¹³C]-phenylalanine-labeled whey (n = 137), casein (n = 393), or milk protein (n = 72) with concurrent intravenous L-[ring-²H₅]-phenylalanine infusions to quantify dietary protein digestion, phenylalanine absorption kinetics, and systemic release over a 5-hour postprandial period. The effect of aging was evaluated in a subset of 82 young (aged 22 ± 3 y) and 83 older (aged 71 ± 5 y) participants.
What was found
Across all participants, 50% ± 14% of dietary protein-derived phenylalanine appeared in systemic circulation over 5 hours. Appearance varied by protein source (P < 0.001): 45% ± 11% for casein, 57% ± 10% for whey, and 65% ± 13% for milk protein. Postprandial availability increased with larger protein doses (P < 0.05). Absorption kinetics were attenuated in older compared with young individuals, with 45% ± 10% versus 51% ± 14% of dietary-derived phenylalanine appearing in circulation (P = 0.001).
Why it matters
This study provides large-scale in vivo human tracer data quantifying how dairy protein matrix, ingestion dose, and age independently influence amino acid bioavailability. These findings help explain differences in postprandial muscle protein synthetic responses and inform dietary protein recommendations, particularly for aging populations.
Limits
The dataset was limited to dairy-derived protein sources in liquid form and single-amino-acid tracer kinetics; it did not evaluate plant proteins, complex mixed meals, or direct muscle protein synthesis endpoints. The postprandial monitoring period was capped at 5 hours, potentially underestimating late-phase absorption for slower-digesting proteins like casein, and the age comparison was restricted to a subset (n = 165) rather than the full cohort.
Cited by
- supports Following ingestion of 20 grams of protein, approximately 10 grams of amino acids are released into the systemic circulation over the subsequent 4 hours.