Oxalic acid availability in human foods

Abstract

Oxalic acid in plant foods has received much research over the past few years, due to the impact of oxalic acid on human health. These impacts include oxalate decreasing the bioavailability of various cations, including calcium, magnesium and iron. Oxalate also binds with calcium in human urine to form insoluble calcium oxalate, which can aggregate and form kidney stones. Very little is known about the solubility and availability of oxalate once ingested. This may be due to the difficulty of measuring oxalate in the stomach and small intestine of humans. The major objective of this study was to investigate the level of oxalate in a human food, and to determine the effect of different cooking methods on oxalate concentration. This data was then compared to the levels of oxalate measured during digestion of that food. This study was carried out on four different cultivars Japanese Taro (Colocasia esculenta var. Schott) and involved the development and use of an in vitro digestion method to measure gastric available oxalate and intestinal available oxalate. The oxalic acid content of Aloe Vera and Aloe extracts was also studied.

The Aloe Vera plant skin contained high levels of total oxalic acid, 147.9 ± 24.6 mg/100 g FW, and 95.4% of it was insoluble oxalate, 141.1 ± 22.2 mg/100 g FW. However, no oxalate was detected in the inner gel of the plant. Oxalic acid was quantified in the commercial gel extract and the Aloe Vera capsules, their total oxalic acid content was 95.8 ± 2.7 and 306.0 ± 12.7 mg/100 g FW, respectively. The percentage of insoluble oxalate in the commercial gel extract was similar to that in the plant skin, while the percentage of insoluble oxalate in the capsules was only 22.3% of the total oxalate. The detection of oxalate in the extract products indicated that the entire leaf of the Aloe Vera plant was used to produce these products, due to the absence of oxalate in the plant inner gel.

The level of total, soluble and insoluble oxalate was variable between the four cultivars of Japanese taro, Akame, Ishikawa-wase, Yamato-wase and Unnamed. Akame was the highest in total oxalate (171.4 ± 11.4 mg/100 g FW) and was significantly higher than the other three cultivars (p<0.001). There was no significant difference in total oxalate between Ishikawa-wase (94.1 ± 4.4 mg/100 g FW), Yamato-wase (73.2 ± 2.6 mg/100 g FW) and Unnamed (95.5 ± 8.8 mg/100 g FW). Cooking Japanese taro by boiling reduced the level of soluble oxalate to below a measurable level. This was due to the removal of soluble oxalate in the cooking water.

There was no significant difference between the total oxalate and gastric available oxalate in raw or cooked Japanese taro (p=0.999), however intestinal available oxalate was significantly higher than soluble oxalate (p<0.001). This indicated that a level of oxalate higher than soluble oxalate, in Japanese taro, was available for absorption in the in vitro intestinal conditions. The in vitro method for measuring potentially available oxalate is an advancement from the chemical method used to measure total, soluble and insoluble oxalate in food. The new data provides us with a measurement of the oxalate in food that may be potentially available for human absorption.