Oxalic Acid and Foods

Before we even begin, let us emphasize that we are not physicians or trained medical persons; if you drink bleach and get sick, don't say "but they said oxalic acid is no big deal."

What Oxalic Acid Is

Oxalic acid is, of course, a chemical substance. At high concentrations, it is a dangerous poison, but such immediately toxic levels are not found in foodstuffs but rather in manufactures, such as some bleaches, some anti-rust products, and some metal cleaners (among other things). It is also a naturally occurring component of plants, and is found in relatively high levels in dark-green leafy foods (relatively high, though, is just that).

In the human body, ingested oxalic acid is not (so far as is known today) a useful nutrient; so, like all such unneeded components of diet, it is processed by the body to a convenient form and that byproduct is then excreted--in this case, in the urine. In the course of being processed by the body, oxalic acid combines with other substances to form various salts, called oxalates; usually, those salts are in solution (like salt in salt water or sugar in coffee), but if their concentration is high enough some may precipitate out in crystalline form. Such tiny crystals of these salts can be irritating to human tissue, especially to the stomach, the kidneys, and the bladder. It is commonly believed that oxalates contribute to the formation of kidney and bladder stones; one common nutrient with which oxalic acid combines is calcium, making the salt calcium oxalate, and calcium oxalate is found in kidney stones.

Assessing Oxalic-Acid Risks

Despite the plethora of articles on the web, there is little hard data--many references are either from inherently unreliable sources, or seem to be parroting material they scarcely understand. Here is what we have gleaned.

Toxicity

Sheer toxicity--actual poisoning--from ingested oxalic acid is wildly unlikely. The only foodstuff that contains oxalic acid at concentrations high enough to be an actual toxicity risk is the leaves (not the stalks, which is what one normally eats) of the rhubarb plant. For just about every other foodstuff, the risk--if any--is not immediate toxicity but a contribution to the development of oxalate crystals.

Stones and Gout

It is now generally believed that the normal human body can dispose of oxalic acid at even relatively high dietary quantities without trouble. Trouble comes only to those unfortunate enough to have one or another genetic condition that impairs, to a greater or lesser degree, their bodies' ability to process oxalic acid. (And sometimes stones and gout are not related to oxalic acid at all.) For those folk, oxalic acid is not the cause of their problems, but it is the raw material for it, and they do indeed need to regulate their intake of it, just as diabetics need to monitor their sugar intake despite sugar normally being a harmless substance.

(Gout is a painful condition caused by crystals, such as oxalates, being deposited in the extremities, typically the feet; it is thus clearly related to kidney- and bladder-stone conditions.)

Calcium Deprivation

Some have argued that by readily combining with calcium, oxalic acid in the diet reduces one's effective intake of dietary calcium. That is true, but the size of the effect is, for anyone getting decent nourishment, not meaningful. Even the conservative RDA for calcium is a gram or so (1000 mg) a day, and many believe that 1.5 to 2 g a day is better. (As one source put it: "While research studies confirm the ability of phytic acid and oxalic acid in foods to lower availability of calcium, the decrease in available calcium is relatively small.")

Nor need one be afraid to boost one's calcium intake. The belief that high calcium intake aggravates the formation of kidney or bladder stones has now been pretty well discarded, with studies showing that even intakes well above 2 g/day do not participate in stone formation in persons who do not otherwise have a stone problem. (Incidentally, getting decent amounts of potassium in one's diet will minimize the effects of calcium participation in stone formation for those who do have a problem; also, magnesium tends to combine with calcium in digestion, so magnesium can lower available calcium levels--which may be good or bad, depending on one's overall calcium status.)

Further Reading

Here are a few links for those interested in following out some primary sources:

• Kidney Stones: Causes
• Clinical Chemistry: Reading the Stones
• Oxalosis and Hyperoxaluria Foundation
• Rhubarb Poisoning (a general oxalic-acid-discussion)
• An Experimental Study of Oxaluria

Appraising Foods' Oxalic-Acid Content

Unfortunately, a simplistic tabling of "oxalic-acid content" is not enough to compare foods, for at least four reasons:

1. The kind of oxalic acid: as one study reported, "The higher oxalate absorption from [food #1] than from [food #2] suggests that the relative amount of soluble and insoluble oxalate in food has an important role in the determination of oxalate absorption [emphasis added]. Simple percent-composition lists make no such distinctions.

2. The actual food on your dish: it has been shown that the oxalic-acid content of foodstuffs can vary substantially depending on their growth environment (for example, plants with lots of ammonia available to them when growing had substantially lower oxalic-acid contents).

3. Again the actual food on your dish: how was it prepared? One tabulation shows "boiled spinach" at 0.60% oxalic acid but "frozen spinach" at 0.75% oxalic acid, a 25% increase.

4. Different numbers from different sources: perhaps the differences only reflect the two factors cited above, but they are real and sometimes substantial. The table we had formerly presented here was from a 1984 USDA publication, which is the ultimate source of most oxalic-acid tabulations to be found on the web; but there are other, newer, and apparently quite trustworthy tabulations (see the list below) that often disagree with the USDA-table data, and with each other as well. Such a situation is, to use a technical term of the medical profession, insane.

So any simplistic, straightforward tabulation of the "oxalic-acid content" of foodstuffs is thus a tricky thing at best. One has to look at such listings with an understanding that they are likely to be only broad-brush indicators--what is relatively high, what is relatively low--not exact, reliable numbers.

For lack of a better idea, we present here, side by side, oxalic-acid data from three sources:

1. USDA Agriculture Handbook No. 8-11, Vegetables and Vegetable Products, 1984

2. Phytochemical and Ethnobotanical Databases, Agricultural Research Service of the National Genetic Resources Program (most data taken from Dr. James Duke's 1992 manual, Handbook of phytochemical constituents of GRAS herbs and other economic plants, Boca Raton, FL; CRC Press)

3. LithoLink Corporation, a metabolic testing and disease management service for kidney stone patients (founded by Dr. Fredric Coe, a University of Chicago Medical School Professor)

Both Duke and Litholink give concentrations to much more than two decimal places, but we have rounded off their data to match the low precision offered by the simplistic USDA, to make comparison simpler. As you can see, sometimes they are all in good agreement, as with corn; sometimes they are not together but not wildly apart, as with beans; and sometimes they are on different planets, as with carrots. What the consumer (or, in this case, the gardener) is supposed to make of such an insane mish-mash is beyond our ken.

We have fallen back to using light red to highlight those foodstuffs that seem worth paying attention to if one has a susceptability to oxalic acid, but that is only a rough guide. Those in that condition should get medical advice and follow it.

Summing Up

We think it is nuts for anyone not already under medical advice to select this leafy green over that based on oxalic-acid content.

--Site Directory--

Search this site, or the web

	Web growingtaste.com

• Background Information about the purposes and design of this site
  • Site Front Page
  • an Apologia: why one should cultivate one's garden   » First-time visitors: please, start here! «
  • Introduction
    • Our Climate--
      • Our Location (USDA Zone Map)
      • Daily Weather Table - Note! 300K file
      • Our Freeze Risks
      • Our Garden Design
    • Deep-Bed Gardening (forthcoming)
    • Vegetarian and Organic Considerations (forthcoming)
  
  
  
• Recommended Crops for a home garden, by variety
  • Vegetable Varieties
  • Herb and Spice Varieties
  • Fruit and Berry Varieties
  
  
  
• Gardening information and aids
  • Tools and Equipment
    • Garden Tools (forthcoming)
    • Seed-Starting (forthcoming)
    • Colored Plastic Mulches
  • A Garden Calendar
  • Suppliers for Recommended Varieties
  • Estimated Seed Life in Storage
  
  
  
• Miscellaneous Information of interest to the home gardener
  • Links:
    • Vegetable-Gardening Links
    • On-Line Vegetable Seed & Plant Sellers
  • About Oxalic Acid in Vegetables
  • About Induction Cooking [now at a site of its own: The Induction Site]
  • So Who Are We?
  
  
  
• Books on "Vegetables" and vegetable gardening, both new and used
  In reality, we have six bookshops, one for each of the six national divisions of Amazon books; the links farther below are to the U.S.-based shop, but you can visit any of the six:
  

  ·  U.S. ·  Canada ·  U.K. ·  Germany  (only books in English listed) ·  France  (only books in English listed) ·  Japan  (only books in English listed)

  
  In our U.S.A. Bookshop:
  • About Our Bookshop - including many specific book recommendations
  • "Vegetables" Books Available New Today:
    • "Vegetables" Books By Title:
      (vegetables book titles beginning with the word "vegetables" are broken out separately in the alphabetical title lists below)
      A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | non-letters
    • The "Vegetables"-Books "Master List" (a very large file!)
  • Search For Any New Book at Amazon
  • Search For Any Used Book at Abebooks

(What do you know about OmniKnow?)

Since you're growing your own vegetables and fruits, shouldn't you be cooking them in the best way possible? Visit The Induction Site to find out what that best way is!

This site is one of The Owlcroft Company family of web sites. Please click on the link (or the owl) to see a menu of our other diverse user-friendly, helpful sites.

Like all our sites, this one is hosted at the highly regarded Pair Networks, whom we strongly recommend. We invite you to click on the Pair link (or their logo) for more information on getting your site or sites hosted on a first-class service.

Click here to send us email.

And why not look in at Is it a blog yet?

---

So that you need not be a victim of the "Browser Wars," we have taken the trouble to assure that
this web page is 100% compliant with the World Wide Web Consortium's
XHTML Protocol v1.0 (Transitional).
You can click on the logo below to test this page!

Not every browser renders proper HTML correctly (Internet Explorer famously does not);
so, if your browser experiences any difficulties with this page (or, really, even if it doesn't),

(It's free!)

You loaded this page on Friday, 9 May 2008, at 15:57 EDT.;
it was last modified on Sunday, 11 March 2007, at 23:34 EDT.

All content copyright ©1999 - 2008 by The Owlcroft Company