Lectins: What They Are, Which Foods Have Them and What Cooking Does

Lectins are proteins that bind to carbohydrates, and they are found in many plant foods, with the highest amounts in raw beans, lentils, peas, soybeans, peanuts and whole grains such as wheat. One lectin, phytohaemagglutinin in raw or undercooked red kidney beans, causes real food poisoning, which is why the FDA tells consumers to cook kidney beans well. Most other lectins are largely switched off by soaking and boiling, and canned beans are already cooked. Below is what lectins are, which foods contain them, what cooking does, and what the evidence says about lectin-free eating.

Peptide Labs does not sell beans, grains or lectin products. This article is general nutrition and food safety information, not medical advice.

What are lectins?

The FDA describes lectins as widely occurring, sugar-binding proteins that perform many functions in plants and animals, including humans. Some of them can become toxic at high levels. In plants, lectins help defend seeds against pests and pathogens. Because some lectins make red blood cells clump together, they are also called hemagglutinins, and one of them is used in laboratories to stimulate immune cells in tests.

Harvard’s Nutrition Source groups lectins with other so-called anti-nutrients, compounds that can interfere with digestion or nutrient absorption when eaten in large amounts in their active form. The key word is active. Lectins are proteins, and like many proteins they lose their shape and activity with wet heat. That is the main reason the lectin story looks very different for a raw bean than for a bowl of cooked chili.

Foods high in lectins

According to Harvard, the highest amounts are in raw legumes (beans, lentils, peas, soybeans and peanuts) and in whole grains like wheat. Lectins occur in smaller amounts in many other plant foods. The table shows what the main legume sources supply once they are cooked, the way people actually eat them. Values come from USDA FoodData Central SR Legacy entries, with the percent Daily Value (%DV) based on FDA label values.

These are some of the richest everyday sources of plant protein, fiber and folate. A cooked cup of lentils, for example, covers about 90% of the folate Daily Value in USDA data. Our guides to lentils and fiber-rich foods go deeper on these foods, and our guide to refined carbs explains why beans count as complex carbohydrates.

Raw kidney beans: the one lectin problem that is well documented

The FDA Bad Bug Book, the agency’s handbook of foodborne illnesses, has a chapter on phytohaemagglutinin, the kidney bean lectin. Its key points:

• Small amounts matter: as few as four or five raw beans can trigger symptoms.
• Fast onset: extreme nausea and vomiting usually begin within 1 to 3 hours, with diarrhea later and abdominal pain in some people. Recovery is usually rapid, within 3 to 4 hours, although some cases have needed hospital care.
• Red kidney beans are highest: raw kidney beans contain about 20,000 to 70,000 hemagglutinating units, while fully cooked beans contain about 200 to 400. White kidney beans have about one-third the amount of the red variety, and broad beans about 5% to 10%.
• How it happens: outbreaks usually involve raw, soaked beans in salads or casseroles, and several have been linked to beans cooked in slow cookers that never got hot enough.

The FDA consumer advice is specific. British studies suggest soaking the beans in water for at least 5 hours, pouring that water away, and boiling them in fresh water for at least 30 minutes. One study found that 10 minutes of boiling destroyed the toxin, but the FDA advises the longer time so the beans reach a high enough temperature throughout. Slow cookers should not be used to cook raw kidney beans or dishes that contain them; casseroles tested in slow cookers often reached only about 75 degrees Celsius inside. Canned kidney beans are already cooked.

This is also why white kidney bean extract supplements are processed products rather than raw bean powder; our white kidney bean extract guide covers what those labels say.

What cooking, soaking and sprouting do

Harvard’s summary of the research is that wet, high-heat cooking such as boiling or stewing, or soaking in water for several hours, can inactivate most lectins. Sprouting grains and beans and removing outer hulls also lower lectin activity. Fermentation is another traditional preparation step for soybeans and grains, discussed in our fermented foods guide.

• Boiling or stewing: the most reliable way to inactivate lectins in dried beans.
• Soaking: helps, and is the first step in the FDA kidney bean advice, but soaking alone is not cooking.
• Canned beans: already cooked during canning, so low in active lectins.
• Low-temperature cooking: slow cookers and casseroles may not get dried kidney beans hot enough; boil them first.
• Dry heat: roasting and baking are less studied for lectins than boiling.

Are lectins bad for you? What the lectin free diet evidence shows

Popular diet books argue that lectins in beans, grains and some vegetables damage the gut and drive weight gain or inflammation, and they recommend a lectin free diet. The evidence behind those claims is thin. Harvard notes that there is very limited research in humans on the amount of active lectins people actually eat and their long-term health effects. Much of the concern comes from laboratory and animal studies using raw or purified lectins; the FDA chapter notes that in several animal species, eating lectins reduces intestinal absorption.

Meanwhile, the foods richest in lectins have a large body of observational research behind them. Harvard points out that eating legumes, whole grains and nuts is associated with lower rates of cardiovascular disease and type 2 diabetes. Observational studies cannot prove cause and effect, but they make it hard to argue that cooked beans and grains are harmful for most people. Harvard’s bottom line is that the health benefits of these foods outweigh the potential harm of lectins, especially when they are cooked or soaked.

Cutting out every lectin source also means cutting out beans, lentils, peanuts and whole grains, which can lower fiber intake considerably. Our gut health guide explains why fiber variety matters.

When beans and grains may still be a problem

Some people do feel better eating fewer beans or wheat, but lectins are not the only possible reason. Beans contain fermentable carbohydrates that cause gas, wheat contains gluten, and peanuts and soy are major food allergens. Our low FODMAP diet guide covers fermentable carbohydrates, and our quinoa guide covers a naturally gluten-free grain. If you have ongoing digestive symptoms, talk to your clinician or a registered dietitian before removing whole food groups, so the cause can be identified and your diet stays balanced.

Frequently Asked Questions

Questions about a label? Contact us or read our Buyer’s Guide. Food comes first; follow the label and ask a healthcare provider before starting a supplement, especially if you take medicines, are pregnant or have a food allergy. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Last reviewed October 2026.

Related guides: Black Beans: USDA Nutrition per Cup, Protein, Fiber and Canned vs Dried, Nightshade Vegetables: List, Nutrition and Are Nightshades Bad for You?, Oxalates: High Oxalate Foods, Low Oxalate Foods and Kidney Stones.