Considering vegetables to grow, I started studying a group of chemicals all too common in leafy greens: oxalic acid, and the oxalates it forms when it chelates with minerals.
Spinach contains moderately high oxalate in soluble form, which easily frees the acid again so it can combine with ... well, let's talk about calcium. Calcium oxalate is not soluble: it locks up the calcium, preventing the human body from using it. Under certain circumstances, this forms kidney stones. In the mouth, since saliva is rich in calcium, calcium oxalate crystals deposit on the teeth, giving that mouth-puckering, fuzzy feeling.
Since I've had kidney problems, I searched for information on oxalates in plants. Unfortunately, the situation is complex, and much advice simplifies it too much. For example, oxalic concentration alone does not determine risk. Enough calcium in the plant will sequester the oxalic acid.
Not many lists of oxalate content compare the calcium except at the Food Science website of Lincoln University in New Zealand (see http://www.foodscience.ac.nz/research_topics/oxalate/Oxalate_containing_food.html). There I learned that the amaranth I've fallen in love with, although super high in oxalic acid, also has plenty of calcium, and so is safer than spinach or the beet greens I've been planning to grow.
Another commonly-cited fallacy is that any cooking will render oxalates safe. If you cook the plant with a calcium source you can probably chelate preventatively or better, leach out the soluble oxalate by cooking in water. That means boiling greens, not just steaming them! And it means throwing out the cooking water, even though I hate losing the flavor and vitamins. But now I've started to do it with the leaves of Swiss Chard. Luckily, stems contain much less oxalate; this is true generally (think of rhubarb, where we eat the stems but the leaves' levels are poisonous). Roots have even less.
Some greens I was considering show unfavorably high oxalate/calcium ratios: New Zealand Spinach (Tetragonia expansa) and Orach (Atriplex hortensis). Also lambs' quarters, a weed I love to eat, as well as purslane (Portulaca oleracea), a weed I have tried to get myself to eat for its high Omega-3 content, though I don't like the flavor. I had just discovered cultivars of purslane developed for salads, so was thinking they might have more interesting flavor, and I should try them.
I'm glad to forget about all of these. I had too many choices already.
© Copyright 2006 Catherine Holmes Clark
|Published in the six Nashoba Publications papers on Friday, 17 February 2006
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|For more information (yes, I know a lot of this is contradictory!)
- Oxalic acid
- Recurrent Kidney Stones: a Naturopathic Approach , by Silena Heron and Eric Yarnell, from Alternative & Complementary Therapies, February 1998:60-67
- Information from Lincoln University:
- The Metabolism of Oxalic Acid, by Mahmut Caliskan, Department of Biology, Mustafa Kemal University, Turkey, 1998 (PDF)
- The Truth about Oxalate, by Michael Liebman, Ph.D., Professor of Human Nutrition, University of Wyoming. Dietary oxalate is one of his research specialties
- Iron is also chelated by oxalic acid.
- Improving Iron Status through Diet, by Lindsay H. Allen, Ph.D. (Dept. of Nutrition, Program in International Nutrition, UCal Davis, and Namanjeet Ahluwalia, Ph.D., Nutrition Dept, Penn State U, 1997. (PDF)
- Effect of domestic processing and cooking methods on total, hcl extractable iron and in vitro availability of iron in spinach and amaranth leaves, by Yadav SK, Sehgal S., in Nutr Health. 2002;16(2):113-20. Is iron oxalate soluble? "blanching and cooking resulted in significant improvement of iron availability, and a significant reduction in oxalic acid content, while only blanching significantly reduced phytic acid and polyphenol contents. Thus cooking and blanching are good ways to improve HCl-extractability and in vitro availability of iron."
- Tetragonia tetragonioides (New Zealand spinach, sea spinach, Warrigal greens)
- Atriplex hortensis (Orache, Mountain Spinach)
- Chenopodium album (Lambs' Quarters)
- The two varieties of purslane I was considering: