The toxic compound in rhubarb leaves that can damage the kidneys is oxalic acid. In the body, oxalic acid can bind calcium and form insoluble calcium oxalate, contributing to kidney stones and, in severe poisoning, impaired kidney function.
Rhubarb stalks are widely eaten after cooking, but the plant’s large triangular leaves are considered inedible. Their oxalic-acid content is much higher than that of the edible petioles, and the leaves also contain other potentially harmful compounds, including anthraquinone glycosides. The stalk’s characteristic sourness, however, comes mainly from malic acid rather than oxalic acid.
Historical warnings about rhubarb leaves became especially important during World War I, when people in Britain were mistakenly encouraged to eat them as a vegetable. A later wartime rediscovery of that advice contributed to further poisonings. Baking soda is not a safe remedy: it can increase the formation of soluble oxalates.