Radek and Savage (2008) quantified the proportion of calcium in moringa in the form of oxalate, and it turned out to be a 38%. This percentage would seem dismayingly high, but two important observations emerge from their study.
First, what is the risk of kidney stones from moringa consumption? Radek and Savage showed that moringa leaves contain only non-soluble oxalates. This means that even though there are large amounts of oxalates in moringa leaves, it is found in a form that is simply excreted by the body. Oxalates that are excreted do not circulate in the body, and therefore can't contribute to kidney stone formation.
Second, what does 38% of calcium being bound up in the form of oxalate mean for moringa as a dietary calcium source? Radek and Savage showed that moringa has total calcium values that are extremely high (>20mg/g of dry leaf). So, even with more than a third of the calcium being unavailable, moringa offers very respectable levels of calcium that are potentially available to the body. Powdered milk has around 13 mg/g of calcium (USAID, 2006). So, moringa leaf powder would seem to compare favorably not only with powdered milk in terms of protein content but also in terms of calcium. Even if *all* of the protein or even all of the non-oxalate calcium in moringa turns out not to be readily digestible (more research is needed), the much lower cost of moringa means that it is a protein and calcium source that is more inexpensive and with a much lower environmental impact than milk.
So, the good news is that, in addition to high levels of essential amino acids, calcium, vitamins, and antioxidantes, Radek and Savage's study gives no reason to think that the consumption of moringa has serious potential to contribute to kidney stones, despite the very abundant oxalates. We'll see if more research bears out their findings, but at the moment the situation would seem to be ideal: lots of calcium, and the calcium that is present in the form of oxalates is in the most inoffensive form.
Olson, M. E., and S. Carlquist. 2001. Stem and root anatomical correlations with life form diversity, ecology, and systematics in Moringa (Moringaceae). Botanical Journal of the Linnean Society 135(4): 315-348. pdf
Radek, M., and G. P. Savage. 2008. Oxalates in some Indian green leafy vegetables. International Journal of Food Sciences and Nutrition 59: 246-260.
USAID (U. S. Agency for International Development). 2006. Fact sheet: nonfat dry milk.