Abstract
Total and soluble oxalate content levels were measured in thirteen selected vegetables purchased from a local market in Surin Province in the northeast of Thailand. Total oxalate contents of the leaves, shoots and roots of the fresh vegetables ranged from 249.5 ± 12.1 to 7597.9 ± 77.6 mg oxalate/100g dry matter (DM) while soluble oxalate content ranged from 205.0 ± 2.3 to 2677.6 ± 19.0 mg oxalate/100g DM. Very high levels of total oxalates were found in three of the selected Thai vegetables, Polygonum odoratum (7597.9 ± 77.6 mg/100g DM), Piper aurantaucum (7026.6 ± 76.9 mg/100g DM) and Limnophila aromatica (6179.0 ± 23.6 mg/100g DM). However, the soluble oxalate content of L. aromatica was low and the highest soluble oxalate contents of fresh vegetables were found in P. odoratum, P. aurantuacum and Neptunia oleracea at 2677.6 ± 19.0, 2152.2 ± 65.3 and 1640.8 ± 3.4 mg/100g DM, respectively. Boiling the vegetables reduced the soluble oxalate content between 30.4 and 65.0%. The insoluble oxalate levels increased in eleven of the cooked vegetables while small decreases were observed in L. aromatica and N. oleracea.
Highlights
Vegetables are an important part of the Thai diet
Total oxalate contents of the leaves, shoots and roots of the fresh vegetables ranged from 249.5 ± 12.1 to 7597.9 ± 77.6 mg oxalate/100g dry matter (DM) while soluble oxalate content ranged from 205.0 ± 2.3 to 2677.6 ± 19.0 mg oxalate/100g DM
The soluble oxalate content of L. aromatica was low and the highest soluble oxalate contents of fresh vegetables were found in P. odoratum, P. aurantuacum and Neptunia oleracea at 2677.6 ± 19.0, 2152.2 ± 65.3 and 1640.8 ± 3.4 mg/100g DM, respectively
Summary
Vegetables are important sources of vitamins, minerals, dietary fibre, and antioxidants. Some plants are well known to contain oxalates and these tend to occur in higher concentrations in the leafy parts of vegetables rather than in the roots or stalks [1,2]. The consumption of high oxalate containing foods promotes oxaluria which leads to an increased risk of kidney stone formation [3] with the predominate type being composed mainly of crystals of calcium oxalate [3]. Reductions in the oxalate concentration in the urine can be achieved by avoiding foods which are known to contain high levels of oxalates [3], and by consuming a higher liquid intake to reduce the oxalate concentration of the urine [2,3]. Soaking and cooking food leads to losses of soluble oxalates into the cooking water [1,4] resulting in less oxalates being absorbed
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