Oxalate, salicylate, lectins: hidden culprits or internet myth?

Healthy state

Plants make many defence compounds against predators. Three of them are currently often named as hidden causes of gut symptoms: oxalate Oxalate Plant compound found e.g. in spinach, rhubarb and nuts. Binds to calcium in the gut and is excreted. Too much in the urine can form kidney stones. , e.g. in spinach, chard, rhubarb, almonds and cocoa; salicylate, chemically related to the active ingredient of aspirin, in fruit, vegetables and spices; and lectins, sugar-binding proteins, especially in pulses and grains. A review of such "anti-nutrients" concludes that in usual amounts and properly prepared, the benefits of a plant-rich diet outweigh the risks.

In the gut, oxalate binds to calcium from the same meal. Calcium oxalate is insoluble and leaves with the stool. Only a small part of the free oxalate is absorbed and excreted by the kidneys. Some gut bacteria, such as Oxalobacter formigenes, break down oxalate. In one study, people with recurrent calcium oxalate stones carried this bacterium less often (17 percent) than people without stones (38 percent). After certain antibiotics it was found less often.

Only small amounts of salicylate reach us from food. In a study of people from four continents, everyone on a mixed diet excreted only about 10 micromoles of salicylate a day in urine, i.e. a few milligrams. A low-dose aspirin tablet contains many times that.

Lectins are inactivated by soaking and thorough cooking. In 2026, the European Food Safety Authority EFSA concluded that properly processed food poses no health risk. Pulses have one of the best reputations of all foods: in an observational study of 785 older people in four countries, the risk of death fell by 7 to 8 percent for every extra 20 g of pulses a day.

Disrupted state: causes and effects

Oxalate really matters for kidney stones. Most kidney stones are made of calcium oxalate. The common mistake: avoiding calcium. In a five-year study of 120 men with recurrent stones, 12 of 60 had new stones on a normal-calcium diet with little salt and less animal protein, compared with 23 of 60 on a low-calcium diet. On the low-calcium diet, urinary oxalate even rose, because less calcium was available in the gut to bind it.

A real and underestimated problem is enteric hyperoxaluria. When fat is not properly absorbed, for example in Crohn's disease affecting or removing the small intestine, short bowel syndrome, chronic pancreatic insufficiency or after certain gastric bypass operations, fatty acids bind the calcium. Oxalate stays free and is absorbed in larger amounts in the colon. The results are kidney stones and, in severe cases, kidney damage up to kidney failure. Treatment targets the underlying disease, with a low-oxalate, low-fat diet and calcium with meals. Whether this prevents stones and kidney damage is not yet well proven.

There is also too much of a good thing. There are case reports of acute kidney failure from drinking large amounts of green juices or smoothies every day. A 68-year-old man took in about 1500 mg of oxalate a day from spinach juices, roughly ten times a usual diet, and remained on dialysis afterwards. This is rare, but a reason to eat plants in rotation rather than one in concentrated form.

"Oxalate dumping", by contrast, is an internet term. It claims that when you cut out oxalate, stored oxalate is flushed from tissues and causes symptoms. A search of the Europe PMC research database found not a single study on it. A low-oxalate diet without kidney stones or a proven oxalate excess has no proven benefit but costs many healthy plants.

Salicylate intolerance is suspected mainly in people who react to aspirin and similar painkillers, for example with asthma and nasal polyps. Evidence for a diet is thin: in a randomised study with only 7 participants, nasal symptoms improved somewhat on a low-salicylate diet, but urinary salicylate did not differ. There is no reliable test. We found no controlled study on gut symptoms. A review warns that diets against several "food chemicals" easily lead to deficiencies.

Lectins are a real risk with raw or undercooked beans. Red kidney beans contain a lot of phytohaemagglutinin; just a few undercooked beans can cause violent vomiting and diarrhoea within one to three hours. In 2018, 200 people fell ill in a canteen in France after a chilli con carne with undercooked beans. EFSA sees a health risk precisely here, in undercooked beans.

The claim from books like "The Plant Paradox" that lectins from cooked pulses, whole grains or tomatoes make the gut leaky and trigger inflammation is not supported, however. The findings come from cell and animal studies with active lectins; EFSA names missing long-term human data as a gap. Avoiding pulses and whole grains for this reason means losing important sources of fibre.