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 fret 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.
GOOD DIET OPTIONS WITH HIGH CALCIUM AND PHOSPORUS RATIOS
Below are some recommended food options with high calcium/phosphorus ratios. But keep in mind these should not be fed exclusively. Variety is the Spice of life! A well rounded diet made up of many food choices will produce healthier, happier tortoises and turtles.
1.) Kale (Calcium mg. 612.35, Phos. mg 254.01)
2.) Grape Leaves (Calcium mg. 1644.39 Phos. mg. 412.23)
3.) Collard Greens-Raw (Calcium mg. 131.54 Phos. mg. 45.36) 4.) Chickory ( Calcium mg. 453.59 Phos. mg. 213.19)
5.) Figs (Calcium mg. 158.55 Phos. mg 63.42)
6.) Dandelion (Calcium mg. 643.26 Phos. mg. 18.12)
7.) Prickly Pear Cactus Pads ( Calcium mg. 738.39 Phos. mg. (77.01) 8.) Turnip Greens (Calcium mg. 861.82 Phos. mg 190.51)
To a lesser degree you can feed the following options with not quite as good calcium and phosphorus ratio.
Ripe tomatoes, strawberries, Jicama, spinach, pumpkins (leaves and fruit), zucchini, mango, mustard greens, grapes (fruit, not leaves), sweet corn, day lilies, Swiss chard, cabbage, carrots, cantaloupe and broccoli.