The green and red stalks are often combined with fruit and berries in European cuisine to make sweet desserts, but botanically they are a vegetable. They contain up to 95% water and are correspondingly low in sugar, fat and calories, but they are rich in vital minerals (e.g. potassium) and vitamins.
Oxalic acid interferes with calcium absorption
So is rhubarb the perfect slimmer’s snack that invites self-indulgent feasting? Not quite. It is not for nothing that old folk wisdom insists rhubarb harvested after 24 June (Midsummer’s Day) is bad for you: indeed, the fruity acidic taste of rhubarb is due to a not inconsiderable oxalic acid content, which can interfere with calcium absorption, cause the formation of kidney stones and, in extreme cases, lead to poisoning if consumed in large quantities.
Young rhubarb is safe to eat
It is rather unlikely, however, that the consumption of rhubarb dishes alone will make you ill. Since most of the acid is in the skin of the rhubarb, its content also can be markedly reduced by peeling or boiling the stalks before consuming them, although this also results in water-soluble vitamins and minerals being lost.
Preference should also be given to young rhubarb, which is harvested at the start of the season, because this contains much less oxalic acid than older, more mature stalks towards the end of the season. So if you keep to the old principle of harvesting rhubarb as early in the season as possible (from beginning of April to mid-June) and not consuming it raw in significant quantities, then it is basically safe to enjoy.
Exception: expectant mothers and infants
To be on the safe side, people with an increased need for calcium, such as pregnant and breastfeeding women, babies and infants and people predisposed to kidney stones, should only partake of rhubarb in small quantities and ideally combine it with calcium-rich foods such as yoghurt or quark in order to prevent a possible loss of calcium.