Protein glycation compromises the bioavailability of milk protein-derived lysine in vivo in healthy adult males: a double-blind, randomized cross-over trial

Background

Industrial processing and storage of milk products can strongly increase protein glycation level. Previously, we have reported that ingestion of highly glycated milk protein attenuates the postprandial rise in plasma lysine concentrations compared to the ingestion of an equivalent amount of milk protein with a low glycation level. Whether the attenuated increase in plasma lysine availability is attributed to compromised protein digestion and subsequent lysine absorption remains to be established.

Objectives

The present study combined stable-isotope methodology with the ingestion of specifically produced, intrinsically labeled protein to assess protein digestion and amino acid absorption following ingestion of milk protein with a high versus low glycation level in vivo in humans.

Methods

Fifteen recreationally active, healthy young males participated in this double-blinded, randomized cross-over study. Subjects ingested 40 g intrinsically L-[1-¹³C]-lysine-labeled milk protein with either a low (3%) or high (50%) glycation level. Continuous intravenous infusion of L-[4,4,5,5-²H₄]-lysine was combined with frequent blood sample collection during a 6-h postprandial period to evaluate dietary protein-derived lysine release into the circulation.

Results

Postprandial plasma lysine concentrations were lower following the ingestion of milk protein with a high versus low glycation level (time × treatment effect: P = 0.002; ƞ² = 0.214), resulting in a 23 mmol/L x 360 min (95% confidence interval [CI]: 13, 32) lower incremental area under the curve (0 ± 12 vs 23 ± 11 mmol/L x 360 min, respectively, P < 0.001). The postprandial release of milk protein-derived lysine into the circulation was attenuated following ingestion of the protein with the high versus low glycation level (time × treatment effect: P < 0.001; ƞ² = 0.640) and was 31% (95% CI: 26, 36) lower over the full 6-h postprandial period (18 ± 4 vs 49 ± 10% of the ingested lysine, respectively, P < 0.001).

Conclusions

A high level of milk protein glycation strongly reduces postprandial plasma lysine availability in vivo in humans. Industrial processing and storage of (milk) protein products can strongly modulate protein bioavailability and, as such, lower the nutritional value of a protein source.

This trial was registered at www.clinicaltrials.gov as NCT05479916.