Oxalic Acid

Oxalic acid and oxalates are present in leaves, roots, stems, fruits and seeds of many plants (Prasad et al., 2017). Oxalic acid is a chelating (or binding) agent for cations such as Ca, Mg, Zn, Mn, Fe, etc (“Ullmann’s..” 2005). Plants will, therefore, synthesize oxalates to regulate the balance between inorganic cations (such as Ca+2) and anions in plant cells (Calistan, 2000). Formation of insoluble calcium oxalate is an essential function because both calcium and oxalate can be toxic at high levels (Webb et al., 1995). Insoluble calcium oxalate formation is a mechanism of storing Ca for future needs of the plant (Helper et al., 1985; Franceschi, 1989). Therefore, it becomes clear why it is impossible to find greens high in calcium without also containing high levels of oxalic acid – they go hand-in-hand.

Many authors have risen concern over consuming high levels of oxalate or oxalic acid (Frye, 1991; Highfield, 1990; Frank, 1992), due to the calcium binding nature of oxalates. Calcium oxalate does not disassociate in the intestine, making the calcium unavailable for absorption at that intestinal location (which ties into the topic also discussed, “Calcium Supplementation“). Others, such as Frye (1991), state that feeding high levels of oxalate may lead to oxalate nephroliths or uroliths (kidney stones). Although this may be true with human medicine (Robertson, 1980), Innis (1994) states that his literature search failed to produce a single documented case of oxalate lithiasis in reptiles. We also could not find any documented case. Considering that almost all leafy greens are considered high in oxalate by human nutritionists (Bernard et al., 1991), perhaps tortoises have evolved to better handle higher levels of oxalates (Innis, 1994).

The conclusion is that because oxalates are difficult to avoid when feeding what is recommended – leafy greens – and because we have no proof they are detrimental to tortoises (Innis, 1994), do not freight over this and allow this irrational fear to dictate what is fed. When we start to avoid certain greens because this and that, we fail to provide a varied diet which is very much a key to success.


Bernard, JB. et al. 1991. The response of vitamin D-deficient green iguanas to artificial ultraviolet light. Proceedings from the American Association o f Zoological Veterinarians, 147-150.

Calistan, M., (2000) The metabolism of oxalic acid. Turk. J. Zool., 24, 103–106.

Franceschi, V. R. (1989) Calcium oxalate formation is a rapid reversal process in Lemma minor. Protoplasma. 148, 130–137.

Frank, N. 1992. Green iguanas: their care and captive husbandry. Reptile and Amphibian Magazine, January-February:30- 35.

Frye, F. 1991. Biomedical and Surgical A spects o f C aptive R eptile H usbandry. Krieger Publishing Co. Malabar, Florida.

Helper, P. K. et al. (1985) Calcium and plant development. Annu. Rev. Plant Physiol. 36, 397–439.

Highfield, A. 1990. Keeping and Breeding Tortoises in Captivity. England, R and A Publishing Limited.

Prasad, R. et al. (2017) Oxalic acid/oxalates in plants: from self-defense to phytoremediation. Current Science, Vol. 112, No. 8.

Robertson, WG. 1980. The cause of idiopathic calcium stone disease: hypercalciuria or hyperoxaluria?. Nephron,26:105-110.

Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, pp. 17624/28029.

Webb, M. A., et al. (1995) The intravacuolar organic matrix associated with calcium oxalate crystals in the leaves of Vitis. Plant J. 7, 633–648.