The Truth About Lectins: Are They a Concern for Plant-Eaters?

Beans and grains are staples of a plant-based diet, but these foods are high in lectins, plant proteins that some say can be harmful to your health. Should you cut these foods from your diet?

The short answer, after doing much research on the topic: Probably not.

Lectins are proteins found in all plant foods—and also in animals and bacteria. They’re especially abundant in beans, peanuts, nightshade vegetables such as tomatoes, and grains, including wheat, barley, quinoa, and rice, as well as in dairy products. In plants, high levels of lectins function as natural insecticides that protect the plant from being eaten. In our bodies, they react differently.

The bad reputation of lectins comes in part from the deadly poison ricin, made from a lectin found in castor beans. This plant is grown as a showy garden ornamental, but the seeds aren’t edible. Even though gardeners are warned not to grow this plant if small children can get at it, actual cases of castor bean poisoning are practically nonexistent.

The other part of the bad rap comes from the lectins found in some legumes, particularly red kidney beans. While most of the lectins in plant foods are harmless and pass through you without being digested, the lectins in these beans can indeed do some damage in the intestines—if they’re eaten raw or undercooked. Symptoms include abdominal pain, vomiting, and diarrhea. In some cases, the symptoms can be severe enough to require hospitalization. 

There’s a simple way to avoid this problem: Don’t eat raw or undercooked beans of any kind. When beans, grains, and other lectin-containing foods such as grains are cooked, processed for canning, sprouted, or fermented, the harmful lectins are deactivated. For most of us, deactivation means they pass right through you, undigested and unnoticed. Some people are unusually sensitive to lectins, however, and may find that beans and other high-lectin foods, even when cooked, cause gas, bloating, and digestive upsets. And people with existing inflammatory bowel disease, such as Crohn’s disease, might be more sensitive to high-lectin foods.

What about the lectins in nightshade vegetables, including tomatoes, peppers, eggplant, and potatoes? Cooking will destroy the lectins in eggplant and potatoes, but raw tomatoes and peppers may be a problem for very sensitive people. The lectins in raw tomatoes can be reduced by removing the seeds, but nothing can be done about the lectins in raw peppers. If you’re very sensitive to lectins, raw peppers may have to come out of your diet. Because raw corn is relatively high in lectins, avoid it if you’re very sensitive to lectins. 

The most controversial claim about high-lectin foods is that they cause weight gain. The reasoning for this is convoluted, but it comes down to the idea that one particular type of lectin found in grains, called wheat germ agglutinin (WGA), binds to insulin receptors in your cell walls, according to a study. This keeps insulin from carrying blood sugar into your cells to be burned as energy, so it ends up getting stored as fat instead. The evidence for this claim is weak—it’s mostly based on test tube and animal studies. On the other hand, lots of studies with real people show that a plant-based diet with plenty of beans helps people lose weight. 

The best way to avoid lectins isn’t to stay away from foods containing them—that really restricts your plant-based diet in unhealthy ways. Instead, cook high-lectin foods to deactivate the protein. 

Because beans are a major source of lectins, preparing them properly deactivates the lectins. Not surprisingly, preparing dried beans to remove lectins is exactly the same as preparing them in general: soak and then cook. 

Useful rules of thumb: One cup of dried beans works out to about three cups of cooked beans. When you soak dried beans, they will double or more in volume—use a big bowl or pot. 

Some beans don’t need preparation. Canned beans are already cooked, so their lectins are deactivated. Fresh string beans and pole beans are harvested before the seeds get large enough to contain lectins, so they don’t need any preparation to remove them. Lentils and dried peas don’t need to be soaked before cooking. 

If you don’t have the time to give your dried beans a long soak, try the quick method:

To prepare dried beans in a pressure cooker or instant pot, check the manufacturer’s guidelines. The beans will take less time and be more digestible if you soak them first as above. 

To cook the beans after soaking, place them in a pot and cover with water to the depth of 2 inches. Add salt and any aromatics you like (bay leaf, onion, bouquet garni, whatever). Simmer, partly covered, until the beans are cooked through but not mushy. Let them cool in their cooking liquid, then drain and rinse. 

Alternatively, you can add the soaked beans to whatever stew or soup you’re making. Just be sure there’s enough liquid in the pot to cover the beans to a depth of at least 1 inch. Simmer gently to keep the beans from getting mushy. If the liquid is absorbed and the beans still aren’t cooked through, add more liquid and keep simmering until they are.