Isn't rhubarb poisonous?
Rhubarb leaves contain poisonous substances, including oxalic acid which is a nephrotoxic and corrosive acid that is present in many plants. Humans have been poisoned after ingesting the leaves; such poisoning was a particular problem in World War I, when the leaves were recommended as a food source in Britain.[18][19]
The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight,[20] or about 25 grams for a 65 kg (~140 lb) human. (Other sources give a much higher oral LDLo (lowest published lethal dose) of 600 mg/kg.[21]) While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%,[22] so a rather unlikely 5 kg of the extremely sour leaves would have to be consumed to reach an LD50 of oxalic acid. Cooking the leaves with soda can make them more poisonous by producing soluble oxalates.[23] However, the leaves are believed to also contain an additional, unidentified toxin,[24] which might be an anthraquinone glycoside (also known as senna glycosides).[25]
In the petioles (stalks), the amount of oxalic acid is much lower, only about 2-2.5% of the total acidity, which is dominated by malic acid.[26] This means the raw stalks may not be hazardous, although the tart taste of raw stalks is so strong as to be unpalatable to many.
