GASTRO-JEJUNAL DIGESTION OF SOYA-BEAN-MILK PROTEIN IN HUMANS

In order to determine how soya-bean proteins are digested acid metabol ized in the human intestine before colonic bacterial fermentation and to estimate their true digestibility, the gastro-jejunal behaviour of soya-bean proteins in water and in two other forms (a concentrated soy a-bean-protein solution (isolate) and a drink composed of crude soya-b ean proteins (soymilk)) was studied in humans. Experiments were carrie d out in eight healthy volunteers using a double-lumen steady-state in testinal perfusion method with polyethyleneglycol (PEG) as a non-absor bable volume marker. Gastric emptying and N and electrolyte contents o f the jejunal digesta were analysed. Gastric half-emptying time (min) of the liquid phase after water ingestion (12.59 (SE 0.12)) was shorte r (P < 0.05) than those for soymilk (37.74 (SE 11.57)) and isolate (36 .52 (SE 11.23)). Electrolytic balances showed that for all meals, Na+, Cl- and K+ were secreted when Ca2+ was efficiently absorbed from the jejunal lumen. Gastro-jejunal N absorption for isolate and soymilk wer e 63 and 49 % respectively, and were not significantly different from one another; after water ingestion, endogenous N was estimated to be 2 1 mmol. An estimate of the exogenous: endogenous values for the efflue nts was obtained from the amino acid compositions of soymilk and efflu ents after water or soymilk ingestion, indicating that 70 % of the tot al N was exogenous and 30 % endogenous. Under these conditions the end ogenous fraction represented 31 mmol after soymilk ingestion and the g astro-jejunal N balance indicated that 54 % of the soymilk was absorbe d. This finding indicates that the true gastrojejunal digestibility of soya-bean proteins is similar to that of milk proteins.