Soybean trypsin inhibitor(s) reduce absorption of exogenous and increase loss of endogenous protein in miniature pigs

It was the purpose of this study to define whether trypsin inhibitors impair protein digestibility via enhanced loss of exogenous or endogenous protein by quantifying those losses using the homoarginine technique, recently developed in this laboratory. Pigs fitted with permanent ileal T-cannulas were fed test meals containing homoarginine-labeled protein. The meals contained casein and increasing doses of trypsin inhibitors (Experiment 1) or alternatively either heat-treated or raw ground soybeans (Experiment 2). Following a casein meal (425 mmol nitrogen, no trypsin inhibitors), ileal protein was predominantly of endogenous rather than of exogenous origin (105 vs. 9 mmol nitrogen). Addition of isolated trypsin inhibitors (3000 mg) enhanced appearance of both endogenous and exogenous protein at the ileum (by 73 and 9 mmol nitrogen, respectively). Feeding raw instead of heat-treated soybeans in one single test meal caused a significant increase of endogenous protein from 217 +/- 42 to 263 +/- 47 mmol (mean +/- SEM) and of exogenous protein from 16 +/- 3 to 48 +/- 14 mmol. If fed continuously for 1 wk, a raw soybean diet caused endogenous protein loss to rise significantly from 221 +/- 26 to 432 +/- 85 mmol. We conclude that ingestion of food containing trypsin inhibitor affects nitrogen balance more by losses of amino acids of endogenous secreta than by losses of dietary amino acids.