Oxalic Acid (The painful fate of that tasty antifreeze)

The previous entry on raspberry ketones got me thinking about supplements in general. For the most part they don't cause people problems - which is pretty remarkable, considering a lot of them do have real druglike molecules in them. Most people are taking these completely unsupervised, with only a label to guide them.

There's lots of toxic stuff in plants, too, though. One such compound is oxalic acid, which plants tolerate fine, mammals less so.


The big problem with oxalate is that it does a great job coordinating divalent metal cations, making insoluble complexes. Calcium oxalate can precipitate in your kidney, forming kidney stones.

The same metabolic pathway in your body that processes ethanol will process ethylene glycol - antifreeze - to oxalic acid - hence the toxicity. For this reason, some people have started using propylene glycol, which is metabolized to the blameless pyruvate (which actually provides some nutriment!).

Your alcohol processing enzymes do a much better job on ethanol than anything else (we evolved in the presence of small quantities of ethanol). Because of this, ethanol is an antidote to ethylene glycol poisoning - and ER docs used to get you drunk in just this situation. A superior antidote exists, however, so I have a feeling you don't see free ER booze much anymore.

Check out that old entry for more information on oxalate and EtGly metabolism.


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It's always fascinated me that we make rhubarb stems into tasty pies but the leaves just above those stems are chock full of oxalic acid.

Just curious, does all this mean that denatured ethanol (with a small amount of methanol) shouldn't be that toxic because the ethanol binds the pathways more tightly than the methanol?

The problem is that in order to prevent methanol from being metabolized into formaldehyde (and subsequently to formate), you have to completely saturate the alcohole dehydrogenase system. If your consumption of denatured alcohol was accidental, you're very unlikely to have consumed a large enough dose to get enough ethanol to saturate the system and the methanol will be converted (and even if you were deliberately consuming it in an attempt to get completely blotto, the non-instantaneous absorption means that plenty of methanol will have gotten into the bloodstream and been metabolized before the system saturates).

This also invalidates the anti-aspartamista "argument" that the small amount of methanol released through the metabolism of aspartame is more dangerous than the much larger amount of methanol found in fruit juices because the juices also contain ethanol which has a protective effect. The thing is, they don't have anywhere near enough of it to be protective.

The US Army Survival Manual suggests sorrel (a leafy green with a distinctive acidic taste) is unsafe in large quantities raw but safe when cooked because the oxalic acid is destroyed. In my experience cooking leaves the distinctive taste intact and only serves to alter the texture. Does anyone know a chemical basis for the army's claims?

Jared: Hawaiian poi is made from the Taro root. The leaves of Taro contain an especially high amount of oxalic acid, too, and uncooked root contains enough to make your mouth feel like it's closing up and burning. Thoroughly cooking Taro (leaves and root) makes it delicious and perfectly safe.

Though I don't know for sure, I'd imagine that cooking would turn oxalic acid into two molecules of carbon dioxide...oxalate looks like it's just dying to fall apart into the highly stable high-oxidation-state CO2.

By Vince Noir (not verified) on 26 Jan 2008 #permalink

re chris-rhubarb: i was told there is some of the oxalic thing in the stems too, that is why we should not raw rhubarb. wikipedie says some is also found in strawberries )the quantity must be eally small though - there are not many strawberry deaths are there?)

"Thoroughly cooking Taro (leaves and root) makes it delicious and perfectly safe."

As an aside, traditionally Poi is made by pounding the Kalo (Taro) corm (sort of like a root); it is not cooked. Somehow, the beating of the poi smashes up the calcium oxalate crystals up enough that it won't shred your mouth when you eat it. Fresh poi is very fluffy and gummy, like pounded rice mochi. Old poi turns sour, but it is still edible (but I think it tastes gross.)

Kalo (Taro) corms must be cooked to make poi. The cooked corm is pounded or you can put it in the food processor to get it to the consistancy of poi.

By Mariaricardo (not verified) on 29 Mar 2009 #permalink