A vegetarian meal reduces protein digestibility without altering postprandial protein utilization in old rats

The composition of the meal, particularly its plants-based components, may reduce protein digestibility and amino acid (AA) availability. This issue is particularly relevant given the increasing adoption of plant-based diets, especially in populations with higher protein requirements, including older adults.

To assess the acute effects of meal context on pea protein and AA digestibility and metabolism across lifespan, comparing vegetarian and omnivorous meals.

48 male Wistar rats aged 1, 10, and 18 months (n=16/age) consumed ¹⁵N-labeled peas within an omnivorous or vegetarian meal and were euthanized 6 h later. Digestive contents were collected for protein and AA digestibility. Plasma and urinary ¹⁵N enrichment was measured to evaluate deamination. Liver and muscle protein synthesis was determined using ¹³C-valine flooding dose method. Gene expression related to digestion and protein metabolism was quantified in liver, muscle, and intestinal mucosa.

Fecal pea protein and caecal AA digestibility were slightly reduced with the vegetarian meal (93.5% vs. 94.7%, P = 0.01 and 90.5% vs. 93.7%, P = 0.0009, respectively), particularly in old rats (92.5% vs. 94.2%, P = 0.03 and 89.7% vs. 93.6%, P = 0.02, respectively). However, this reduction did not affect nitrogen utilization or protein synthesis. Young rats showed greater peripheral nitrogen utilization, with higher muscle protein synthesis (+70%, P < 0.0001) and lower deamination (-70%, P < 0.0001), consistent with growth demands. Adult and old rats exhibited higher liver protein synthesis (+40%, P = 0.0004) and higher deamination (+70%, P < 0.0001), indicating a shift toward maintenance metabolism, independent of meal type.

The vegetarian meal slightly reduced protein and AA digestibility, likely due to its higher antinutritional factor content, with greater sensitivity in older rats, but did not impair postprandial protein metabolism. Long-term studies should determine whether these reductions have cumulative effects on muscle maintenance during aging.

This study provides novel evidence that the negative impact of complex vegetarian meals on protein digestibility may be amplified with aging, while postprandial protein utilization remains preserved despite reduced digestibility. It further demonstrates that age-related shifts toward increased splanchnic retention and amino acid oxidation predominate over meal composition effects, highlighting aging as a key determinant of plant protein utilization.