Assessment of oxalate absorption from almonds and black beans with and without the use of an extrinsic label

Purpose: Oxalate bioavailability is an important determinant of whether the consumption of a particular food is a high risk in individuals predisposed to kidney stones. We estimated and compared oxalate absorption from a high oxalate containing legume (black beans) and a high oxalate containing nut (almonds). We also compared an isotope method using extrinsically labeled oxalate and an oxalate load method to assess oxalate absorption. Materials and Methods: Six male and 5 female subjects participated in the 4 oxalate load tests, namely almonds, almonds with 20 mg ¹³C₂-oxalic acid, black beans and black beans with 20 mg ¹³C₂-oxalic acid. Each treatment provided a total of 120 mg oxalate, after which timed urine samples were collected for the analysis of oxalate, calcium and creatinine. Results: Average oxalate absorption from the 2 almond treatments (5.9%) using the oxalate load method was significantly higher than that from the 2 black bean treatments (1.8%) during the 24-hour post-oxalate load collection period. In contrast, ¹³C₂-oxalic acid absorption from the almond (7.9%) and black bean (8.6%) treatments did not significantly differ. Conclusions: The higher oxalate absorption from almonds than from black beans suggests that the relative amount of soluble and insoluble oxalate in food has an important role in the determination of oxalate absorption. Since extrinsically provided ¹³C₂-oxalate and oxalate naturally occurring in the high oxalate test foods appeared to be differentially absorbed, the data do not support the use of extrinsically labeled oxalate to assess food oxalate absorption.