Soluble and insoluble oxalate content of nuts

This study was conducted to determine the oxalate contents in common nuts either locally grown or imported into New Zealand. Samples of imported nuts were purchased from supermarkets in Christchurch while locally grown nuts were obtained directly from the growers. In this experiment gastric soluble and intestinal soluble oxalates were extracted from the nuts using an in vitro assay, which involved incubations of the food samples for 2 h at 37 °C in gastric and intestinal juice. The extracted oxalates were then determined by HPLC chromatography. Roasted pistachio nuts and chestnuts contained very low levels (<85 mg/100 g fresh weight (FW)) of gastric soluble oxalate. Peanuts, Spanish peanuts, peanut butter, ginkgo, cashew nuts and pecan nuts all contained relatively low levels of gastric soluble oxalate (147–250 mg gastric soluble oxalate/100 g FW). Almonds, Brazil, pine and candle nuts contained high levels of gastric soluble oxalate (492.0–556.8 mg/100 g FW). The intestinal soluble oxalate is the fraction that will be absorbed in the small intestine. Peanuts, Spanish peanuts, peanut butter, ginkgo and pecan nuts all contained relatively low levels of intestinal soluble oxalate (129–173 mg intestinal soluble oxalate/100 g FW). Almonds, Brazil, cashew and candle nuts contained higher levels of intestinal soluble oxalate (216–305 mg/100 g FW). Pinenuts contained the highest levels of intestinal soluble oxalate (581 mg/100 g FW), while chestnuts and roasted pistachio nuts were low (72 and 77 mg /100 g FW). Overall the mean soluble oxalate contents of nuts was 78% of the gastric soluble oxalate content (41–100%). The results obtained in this study confirm that the intestinal soluble oxalate contents of nuts range widely and people who have a tendency to form kidney stones would be wise to moderate their consumption of certain nuts.