Protein fermentation biomarkers in plasma and urine differ between bovine plasma protein and whey protein isolate in a randomized fully controlled dietary intervention

Background: Microbial fermentation of undigested proteins in the large intestine can produce potentially harmful metabolites. While protein intake increases large intestinal protein inflow, the effects of dietary protein composition and digestibility are largely unknown. Objective: To investigate the effects of protein sources differing in amino acid (AA) composition and digestibility on protein fermentation biomarkers (primary outcomes), digesta transit, and gut microbiome composition (secondary outcomes), by contrasting protein and precursor AA delivery into the large intestine using two purified protein sources. Methods: Fifteen healthy adults participated in a randomized, controlled, crossover dietary intervention consuming bovine plasma protein (BPP, poorly digestible) or whey protein isolate (WPI, highly digestible) at 30g/d, divided over 3 meals, for 7-days. Intervention periods were separated by a 7-day washout. Plasma, urine, and fecal samples were analyzed for protein fermentation biomarkers, followed by linear mixed-effects-model analysis. Results: In vitro degree of hydrolysis did not differ between BPP and WPI (32.6±1.38% vs. 26.3±0.05%, P=0.10). Postprandial plasma total AA concentrations were also similar (BPP: 8997±2673 μM, WPI: 9787±3380 μM, P=0.57, n=13). Compared with WPI, BPP consumption induced higher plasma concentrations of p-cresyl sulfate (41.0±27.4 vs 28.5±19.6, P<0.01), phenyl sulfate (2.1±0.6 vs 1.8±0.4, P=0.03), phenylacetyl-L-glutamine (2.3±1.5 vs 1.7±1.2, P<0.01), and indoxyl sulfate (4.8±1.8 vs 3.6±1.4, P=0.03). Urinary p-cresyl sulfate (111.4±63.6 vs 90.9±56.7, P=0.03), phenyl sulfate (13.6±6.1 vs 9.7±2.6, P=0.03), and phenylacetyl-L-glutamine concentrations (55.1±32.2 vs 41.6±25.6, P=0.01) were also higher during BPP consumption. Protein source did not affect fecal ammonia, BCFA, or microbial diversity. BPP consumption tended to prolong colonic transit time (11:54 hh:mm, P=0.07). Conclusions: Consumption of BPP resulted in higher plasma and urinary concentrations of aromatic AA-derived microbial metabolites than WPI. These differences were most likely driven by AA composition rather than digestibility, as in vitro digestibility and postprandial AA responses differed less than expected between the protein sources. Clinical trial registry number: NCT06161155 (https://clinicaltrials.gov/study/NCT06161155).