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.
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!
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
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).
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.
- The Poison Garden – Rhubarb; http://www.thepoisongarden.co.uk/atoz/rheum_x_hybridum.htm
- Wikipedia – Oxalic Acid; https://en.wikipedia.org/wiki/Oxalic_acid
- The Chemistry of Rhubarb; http://www.compoundchem.com/2015/04/16/rhubarb/
- 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