SOLUBILITY OF OXALATE AND ITS ASSOCIATION WITH METAL IONS ASSESSED BY A SIMULATED IN VITRO DIGESTION MODEL

Abstract

Bioavailability of oxalate is highly influenced by the presence of metal cations in the gastrointestinal tract as a result of the different solubility of oxalate salts. Bioavailability is further influenced by pH effects on oxalate solubility at various stages of digestion, particularly at gastric pH at which insoluble oxalate has been found to solubilize. The aim of the present study was to assess the effect of gastrointestinal pH and different metal ions on the relative solubility of oxalate after treatment with relevant gastric and intestinal enzymes in simulated gastrointestinal digestion conditions. Whole bran cereals and beans were used as test samples. Oxalate solubility increased along with reproducible recovery such as 97 +/- 4 at gastric pH-2 compared to higher pH. High amount of soluble oxalate (32 +/- 1.2) was found in wheat bran after simulated in vitro digestion under gastric conditions as compared to intestinal conditions i.e. 11 +/- 0.5. The availability of calcium for absorption was found to be very low compared to that of magnesium and potassium i.e., 15 +/- 0.3, 13 +/- 0.5 to 10 +/- 0.42 and 4 +/- 1 in wheat bran sample at gastric and intestinal conditions simultaneously. The formation of a complex between calcium and oxalate reduced solubility and hence made calcium less available for absorption. Presence of metals and oxalate not only reduces oxalate availability, but it also reduces availability of metal cations.