Sorrel earns its place in the garden with a flavour nothing else quite matches: sharp, lemony, faintly green-apple, and capable of lifting a bowl of new potatoes or a cream soup in seconds. That tang comes from oxalic acid, the same compound that gives spinach and rhubarb their slightly chalky reputation. So is sorrel safe to eat? For most people, in ordinary kitchen amounts, yes. But oxalic acid is worth understanding, especially if your kidneys have ever given you trouble.

Oxalic acid: what gives sorrel its bite

Oxalic acid is a natural compound found in a long list of everyday foods: spinach, chard, beetroot leaves, rhubarb, parsley, almonds, cocoa, tea. Plants make it as part of their tissue chemistry, and it helps defend them from being eaten. When we eat it, it can bind to minerals such as calcium, magnesium and iron in the gut, which means those minerals are less available to the body.

The form that matters most for health is soluble oxalate, because that is the portion absorbed into the bloodstream and eventually filtered by the kidneys. Sorrel (Rumex acetosa) sits fairly high on the list — in the same broad territory as spinach. Sheep's sorrel, the smaller wild relative often picked for salads, is similar. French sorrel (Rumex scutatus) is usually described as milder. Wood sorrel, which looks comparable but belongs to an entirely different plant family, contains oxalates too.

Oxalates and kidney stones: the genuine concern

The most common kidney stones are made of calcium oxalate, which is why high-oxalate foods attract attention. It isn't a simple equation, though. Your body produces oxalate of its own, and whether a stone forms depends on how much you drink, how much calcium your diet supplies, your genetics, your gut health and other medical conditions. Someone who eats spinach most days may never form a stone; someone else may form one after far less.

One pattern that does seem to matter is eating a lot of high-oxalate food while taking in very little calcium. Calcium in the same meal binds oxalate in the gut, so less of it reaches the kidneys. That is an argument for eating sorrel with yoghurt, cream, cheese or a milky pudding rather than on its own — which happens to be how most traditional sorrel recipes work anyway.

If you have had a calcium oxalate stone, or you have kidney disease, general advice about oxalates is not enough. Ask your GP or a registered dietitian what suits your situation; a low-oxalate diet is usually tailored, not copied from a website.

Who should be more careful with sorrel?

Sorrel is not off-limits for most people. It is a food to treat a little like rhubarb or spinach — enjoyable, but not the only green thing on the plate, every day, in large bowls. Extra care is sensible if you fall into any of these groups:

• Anyone with a history of calcium oxalate kidney stones. Moderating high-oxalate foods is often part of the plan.
• People with reduced kidney function or kidney disease, whose kidneys clear oxalate less efficiently.
• People with conditions that cause fat malabsorption, such as some forms of inflammatory bowel disease or following certain bowel surgery. When fat is not absorbed properly, oxalate absorption can rise.
• Anyone already following a low-oxalate diet for medical reasons.
• Young children eating large quantities of it — smaller bodies, proportionally bigger portions.

If that list includes you, get individual advice rather than guessing from general guidance.

Can cooking reduce the oxalic acid in sorrel?

Heat does not destroy oxalate, but the compound is water-soluble. That means cooking methods which use plenty of water and then discard it will pull some of the oxalate out of the leaves.

1. Shred or roughly chop the sorrel.
2. Drop it into a pan of boiling water for about a minute.
3. Drain it well and throw the cooking water away — do not add it to the soup or stock.
4. Taste before you season. Much of the sharpness will have gone, so you may want a squeeze of lemon or a splash of vinegar to bring the flavour back.

The trade-off is obvious: sorrel's tang is the point. Blanch it hard and bin the water and you lose some of what makes it special. Steaming, stir-frying and folding raw leaves into a sauce keep the flavour, but they keep the oxalate too. For most people, using a modest amount of raw sorrel in a dish is a perfectly reasonable compromise.

How much sorrel is sensible?

Think of sorrel the way cooks traditionally have: as a herb or a flavouring rather than a main vegetable. A small handful — a few leaves shredded into a salad, a couple of tablespoons of sorrel sauce with fish, a bowl of sorrel soup made with a moderate quantity of leaves and plenty of cream or stock — is how the plant has been eaten for generations in French and Eastern European cooking.Photo: Jana Ohajdova / Pexels