Will Oxalic Acid in Rhubarb Leaves Harm You?

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Robert Pavlis

Rhubarb is a favorite vegetable of gardeners in temperate climates since it is so easy to grow. We eat the stems, and know that you should never eat the leaves since they are poisonous due to high levels of oxalic acid.

I’ve known this fact since I was a kid so you can imagine my surprise when I learned a few weeks ago that this is all a big myth. Lets dig into the truth.

Will the Oxalic Acid in Rhubarb Leaves Harm You?
Will the Oxalic Acid in Rhubarb Leaves Harm You?

Is Oxalic Acid Poisonous?

Oxalic acid is a natural chemical produced by many plants. It is a nephrotoxin (a poison that affects the kidneys) and a corrosive acid. The LD 50 (median lethal dose) for humans is estimated to be 385 mg/kg. A 65 kg (143 lb) human would need to ingest 25 g to be lethal.

Clearly oxalic acid is lethal, but 25 grams is quite a bit.

Oxalic Acid in Rhubarb

The oxalic acid in rhubarb leaves (blade + petiole) is about 0.5 g/100 g, based on fresh weight. To reach the lethal dose of 25 grams, a 65 kg human would need to eat 5 kg of leaves. That is a pretty big salad!

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What about the stalks? How much oxalic acid is found in the stalks? I found lots of references that said they contain much less oxalic acid, but only one reference gave a value of  0.4 to 0.5 g/100 g which is only a bit less than the leaves. It is odd that everyone says the value is lower but nobody reports a value.

Update: A new study has looked at oxalic acid in stalks for 71 cultivars and found an average soluble oxalate of 3 g/100 g (1.5 – 6%) and total oxalate of 6 g/100 g (3 – 9%), based on dry weight. Leaves are 90% water so on a fresh weight basis soluble oxalate is 0.3 g/100g and total oxalate is 0.6 g/100g. Another review reported the oxalic found 0.65 g/100 g for rhubarb leaves and 0.46 g/100 g for stalks.

Oxalic Acid in Vegetables

The reason for writing this post is that I came across a list showing the oxalic acid content of other vegetables and it was a real eye opener. Here are the values for some common vegetables (ref 2). These values are similar to those found in the oxalic database.

Carrot – 0.5 g/100 g

Chives – 1.48 g/100 g

Parsley – 1.70 g/100 g

radish – 0.5 g/100 g

Rhubarb leaves – 0.5 g/100 g

Spinach – 0.97 g/100 g

If rhubarb leaves are too toxic to eat because of the oxalic acid, why do we eat these other vegetables? Why are we not warned that carrots are as poisonous as rhubarb leaves, and that spinach is twice as poisonous? The reality is that oxalic acid is not as poisonous as people believe.

Some other foods that have high levels of oxalic acid include tea, coffee, draft beer, chocolate, berries and tofu.

What is Oxalic Acid?

Oxalic acid is the acid form of oxalate, the latter being more common in plants and animals. When gardeners talk about a concern for oxalic acid, they are usually talking about oxalate. Oxalate forms soluble salts with potassium, sodium and magnesium. It forms insoluble salts with calcium and iron and these usually pass through our body without being absorbed.

Are Rhubarb Leaves Poisonous?

There are stories on the net about rhubarb leaves being eaten during the first world war and some people died from eating them. These stories may or may not be true. People may have died from something else and rhubarb was blamed. Or the leaves might have been sprayed with toxic pesticides which were in common use at time. Postmortems did not find oxalic crystals in the bodies suggesting that death was due to other reasons (ref 3 and 4).

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Plants make thousands of chemicals and many are toxic. Rhubarb makes anthraquinone glycosides which have been proposed as the likely candidate for deaths. The reality is that there are almost no reported deaths due to eating rhubarb leaves and the science on what is the most poisonous thing in the leaves is inconclusive.

The bottom line is that the leaves may be poisonous, if you eat enough – so don’t do that! But oxalic acid is not the culprit.

References:

  1. The Poison Garden – Rhubarb; http://www.thepoisongarden.co.uk/atoz/rheum_x_hybridum.htm
  2. Wikipedia – Oxalic Acid; https://en.wikipedia.org/wiki/Oxalic_acid
  3. The Chemistry of Rhubarb; http://www.compoundchem.com/2015/04/16/rhubarb/
  4. Toxicants Occurring naturally in Foods; https://books.google.ca/books?id=lIsrAAAAYAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

32 thoughts on “Will Oxalic Acid in Rhubarb Leaves Harm You?”

  1. Great post! Tiny issue for me – WHICH “spinach” was used in the reporting of oxalic acid – garden spinach (Spinacia oleracea) in the Chenopodiaceae or New Zealand spinach (Tetragonia tetragonioides) in the Aizoaceae? Not closely related at all! Both common in supermarkets.

    Why do I ask? Idiot botanists made massive phylogenetic trees that were WRONG because they simply bought stuff in the supermarket. Science needs to be replicatable. I was derided at a BSA meeting where I pointed this out and have never been back. The red-faced idiots had to republish everything 8 years later.

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  2. I believe the issue with oxalic acid is more the structure of the compound. As I understand it, oxalic acid forms small sharp crystals. In plants that have high amounts, such as some of the aroids like dieffenbachia, if a person or pet chews on the plant, crystals become lodged in the throat causing it to swell, and resulting in possible suffocation. So oxalic acid can be a problem; perhaps the level in rhubarb is enough to cause the tart flavor but not enough to cause medical issues.

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    • I have read that of aroids, but have not looked into it. Jack-in-the-pulpit plants contain calcium oxalate crystals which have the effect you describe. Rhubarb does not seem to have these.

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  3. So interesting! I’ll have to tell my family about this. They’ve been carefully throwing away all of the leaves for 60 years. 🙂 I don’t like rhubarb at all, and never eat any part of it, but I eat lots of carrots and the other things listed here, so am glad that they are not poison either.

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  4. I love the way you challenge gardening myths and if not debunk them at least give us some research as the basis for your views. Power to your elbow!

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  5. Well that is a surprise! I always thought the same and like most people in the UK after the war, rhubarb was widely grown and eaten. We grew up eating it and we knew about the poisonous leaves!
    That being said, there are home recipes incorporating the leaves for a pesticide spray on other plants. However my rhubarb leaves get munched on by insects or critters, leaving holes in them, and that always left me wondering why – if they are so poisonous!

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  6. A crazy mountain man type told me years ago that “twice boiling” rhubarb leaves made them quite edible. Twice boiling meant cooking the leaves in two changes of water.
    So I tried it. I am still alive. Based on taste, there is no chance that anyone will ever eat too many rhubarb leaves.

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  7. Grew up in the country in the U.K. Rhubarb pies were the weekly thing while in season. In the 1950’s, what was considered to be an old wives tale today would have been considered close to gospel back then. Hot Rhubarb pies with lashings of hot ‘Birds Custard’, still available today, was about as close to Heaven as us country folks could get. Almost as good as pigeon casserole. As kids we’d stand in the vegetable garden bushes with the wind at our backs, and shoot Grey Pigeons as the flocks flew in to land on the Brussel Sprouts. Field clean (remove the breasts) and after a couple of hours you had enough breasts to feed our family of five. The art was to chew gently initially, checking for shot. Absolutely delicious (the pigeon, not the shot..!!)
    Nuncle H.

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  8. Very eye opening! I always wondered exactly how poisonous the leaves are, but like you said there is little mention out there about specific values. Once again, you stumped an ongoing garden myth! Well done! Julia

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    • Very interesting! Thank you for the info; it gives me things to think about. Our leaves are so big, perfect and beautiful – too lovely not to eat! People should bear in mind that tomatoes were regarded as being deadly poisonous until one man convinced everyone by eating them in public. Perhaps rhubarb leaves will eventually become a valued food. I appreciate your research and for writing this article.

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