Production of egg proteins, enriched with L-leucine-C-13(1), for the study of protein assimilation in humans using the breath test technique

Protein assimilation and metabolism studies are hindered by the lack of an adequate oral tracer, i.e., labeled proteins. We present a new and easily reproducible methodology for producing large amounts of egg proteins labeled with L-leucine-C-13(1),. Laying hens were fed a 0.2% leucine-deficient food supplemented with 0.2% L-leucine-C-13(1) (99 atom %). At plateau, eggs containing highly enriched proteins were obtained. The C-13 content of egg white relative to the total C content was 1.3371 atom %, corresponding to delta = 206 parts per thousand. The overall tracer recovery in egg proteins was high (40.2%), making this method financially attractive as well. Accurately measurable levels of (CO2)-C-13 in breath were obtained after ingestion of a physiological load of labeled egg white proteins. Thus, egg proteins with sufficient C-13 enrichment and applicable for human protein assimilation and metabolism kinetic studies were produced in an easily reproducible and highly efficient manner.