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The preparation and cooking methods here specifically apply to all legumes, although kidney beans deserve special attention, since they have the highest concentration of lectins and are the most common cause of lectin toxicity.
What are lectins?
Lectins are a form of protein that bind to carbohydrates and are present in most plants and about 30% of the food we eat.
Most foods contain insignificant amounts (e.g. apples, bananas, cucumbers and sweet peppers) and the health benefits derived from the vitamins, minerals, fiber and phytochemicals contained in those foods outweigh the negative effects of this small amount of toxins. (3)
However, high (but varied) amounts of lectin called phytohaemagglutinin are present in in legumes, (especially red kidney beans, black beans, soybeans, lima beans, cannellini beans, and lentils) and some grain products. (4)
Many online and cook book recipes contain cooking methods that may not remove enough of those toxins, causing poisoning symptoms (such as severe vomiting, extreme nausea and diarrhea).
The most common source of poisoning from lectins occurs from beans, especially red kidney beans, when they are eaten undercooked or raw. (4)
Raw red kidney beans contain between 20,000-70,000 hau (hemagglutinating unit – measure unit of this toxin).
Fully cooking these beans reduces the amount of this toxin to only 200-400 hau.
Cannellini beans contain 70% less lectins than red kidney beans and broad beans contain about 90% less.
Cooking beans in low temperatures using slow cookers or crockpot pots is not recommend because temperatures of less than 176 °F (80%) do not destroy lectins.
Please note that currently FDA has no regulations or guidelines that would restrict the levels of lectins in food.
What happens if you consume a high amount of lectins?
- Vomiting (may be severe)
- Nausea (may be extreme)
- Diarrhea (develops from one to a few hours later)
- Abdominal pain (but not common)
Hospitalization in these cases is rare.
Frequent consumption of large quantities of lectins, however, may lead to:
- Increased permeability of the small intestine (8, 9)
- Disruption of the immune system, by releasing antigens to attack lectins, and of the tissues to which lectins are attached (10)
- Increased risk of autoimmune diseases (11)
- Lectins also reduce absorption of nutrients from the intestine by binding to mucosal cells. (4)
What do lectins do to your digestive tract?
The intestinal tract lining undergoes some natural “wear and tear”, caused by various stress factors, but it has the ability to repair itself quickly.
Lectins attach themselves to the intestinal wall, preventing the lining healing process, thereby causing more stress and damage.
Lectins also agglutinate many red blood cell types and interfere with the transport system of the cell membranes and with the metabolism of the cells. (12)
Cooking kidney beans in low temperatures is the most common reason of poisoning from lectins
The most common reports of poisoning from lectins are (12):
- Raw, soaked kidney beans, in salads or on their own
- Red kidney beans that have been cooked using low temperature cooking methods: slow cookers, crock pots and adding beans to casseroles cooked at low heat.
- Sous-vide, a method of cooking below boiling temperatures, is becoming very popular these days and is often mentioned in the recipes and sous-vide equipment sources as a new method of cooking beans. (13, 5, 7, 14)
Beans cooked at 80 degrees Celsius, which is common in some of these cooking methods, increase the toxicity potential 5 times higher than eating raw beans.
How to cook kidney beans (and other legumes) to make them safe to eat?
These preparation and cooking methods apply to all the legumes, although kidney beans deserve special attention since they have the highest concentration of lectins and are the most common cause of lectin toxicity.
Foods with high amounts of lectins, such as beans and grains, must be boiled in order to reduce these toxins to safe amounts.
- Soaking (soak beans for at least 5 hours prior to cooking and discard the water used for soaking)
- Fermenting (bacteria reduces the toxins by digesting them)
- Since most lectins are reduced by cooking in a moist heat, boiling is recommended.
- Cooking at high temperatures – cook on high heat for more than 10 minutes (reduces hau in beans by 200-fold)
NOTE: the canning processes use adequate heat to reduce the amount of lectins to insignificant amounts, so canned beans are safe to eat.
NUTRITION FACTS VS NUTRITION MYTHS
You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.