Investigating the Digestibility, Bioavailability and Utilization of Protein Blends in Older Adults Using a Dual Stable Isotope Tracer Technique

Abstract

Objectives: The impact of combining animal and plant protein sources on digestibility is unclear, despite their increasing clinical use. Using a non-invasive dual stable isotope tracer approach, we assessed the digestibility, bioavailability and utilization of distinct protein blends in older adults, and associated plasma amino acid profiles and muscle protein synthesis (MPS) rates.

Methods: Thirty-two older men (69 ± 3 y) consumed one of four protein blends (A (51:49, casein/soy); B and C (35:25:20:20, whey/casein/soy/pea); D (80:20, casein/whey)) alongside primed constant infusions of [1,2-¹³C₂] leucine for 8 h. Arterialized blood and vastus lateralis muscle biopsies were collected during a trickle feed protocol with all blends providing 20 g total protein, universally labeled ¹³C-spirulina, and ²H-cell free amino acid mix to determine digestibility. This trial was registered at Clinicaltrials.gov (ID-NCT07038655).

Results: No differences (¹³C:²H ratios) were found in digestibility between the protein blends (p > 0.05). Mean (±SEM) fed state MPS at 2.5 h was 0.078 ± 0.009%/h, 0.075 ± 0.012%/h, 0.085 ± 0.007%/h and 0.065 ± 0.011%/h for drinks A, B, C and D, respectively, with a main time effect observed (p < 0.01), but no significant differences between drinks. Plasma essential amino acids (EAAs) increased significantly from baseline for all blends by 40 min (p < 0.05), with no differences between blends at any time point.

Conclusions: These findings suggest that protein quantity (and/or leucine content), rather than composition, appears to be the most important factor driving MPS. Future work should focus on clinical populations where protein requirements and digestibility characteristics may differ.

Keywords: ageing; dual stable isotope; protein digestibility; protein synthesis; skeletal muscle.

Figure 1

Change in plasma EAA concentrations compared to baseline over time and area under the curve (AUC) in older males. Values are mean ± SEM. Significant main treatment (p < 0.01) and time (p < 0.0001) effects observed. (A–D) The changes in plasma EAA concentrations compared to baseline in drinks A, B, C and D, respectively. * Significant difference from baseline at the time point (p < 0.05).

Figure 2

Change in plasma leucine concentrations compared to baseline over time and area under the curve (AUC) in older males. Values are mean ± SEM. Significant main treatment (p < 0.01) and time (p < 0.0001) effects observed. † Significant difference between drink D and drink C at time point (p < 0.05). # Significant difference between drink D and drink A at time point (p < 0.05). (A–D) The changes in plasma leucine concentrations compared to baseline in drinks A, B, C and D, respectively. * Significant difference from baseline at the time point (p < 0.05).

Figure 3

Change in plasma ¹³C/²H enrichment ratios (solid lines) and ¹³C/²H enrichment ratios measured in the respective drinks (dotted lines). Values are mean ± SEM. No significant differences between any of the drinks at any time points after reaching the plateau (all p > 0.05).

Figure 4

Myofibrillar protein synthesis (FSR, fractional synthesis rates, %·h⁻¹) measured at baseline and 150 and 300 min post first feed in older males. Values are mean ± SEM. Significant main time effect (p < 0.01).