Kidney beans contain a plant protein called lectin, which has shown toxicity in animal studies. Reports of food poisoning caused by kidney beans have come from the United States, Canada, Australia and the United Kingdom, according to the study "Red Kidney Bean Poisoning in the UK" published in 1990 in the journal, "Epidemiology and Infection." Proper food preparation neutralizes the lectins in kidney beans, making them safe to eat.
The Facts
Lectins are a diverse group of glycoproteins found in plants, fungus and lichen. They play a role in the defensive ability of the plant against attack by insects and microorganisms. Lectins have the ability to bind to the sugars found in animal cell membranes, with different lectins binding to different sugars. Not all lectins are toxic to humans. However, high levels of some plant lectins cause a form of food poisoning. These lectins also cause biochemical changes in the structure of blood cells, causing them to clump together or agglutinate. Some lectins have therapeutic potential. The lectin jacalin, found in jackfruit, has shown an ability to block infection from HIV type 1 in laboratory studies.
Kidney Beans
Kidney beans, or Phaseolus vulgaris, contain a lectin called phytoheamagglutinin, which is classified as orally toxic. Rats fed nothing but raw kidney beans will show signs of toxicity and die within one week. Kidney bean lectins are resistant to the digestive process and to gastric juices. Therefore, these beans may cause symptoms of food poisoning if eaten raw, dried or insufficiently cooked, according to the study published in "Epidemiology and Infection." Symptoms start one to three hours after eating and include nausea, vomiting, diarrhea and abdominal pain. Recovery is usually rapid with no lasting effects.
Avoiding Symptoms
Boil or cook canned kidney beans in a sauce to remove lectins, since moist heat destroys the glycoprotein. If you are preparing the beans from the dried state, soak them overnight in water and then boil the kidney beans for a minimum of 10 minutes. Kidney beans cooked at temperatures below boiling have shown an increase in lectin levels, according to a study published in 1990 in the journal "Epidemiology and Infection." Lectin levels vary considerably even among the same vegetable. If you are lectin sensitive, you may find that vegetables harvested from some locations cause less of a reaction. Choose roasted over fresh nuts as the roasting process minimizes lectins.
Some lectins show reactivity only with specific blood types, according to the study, "Lectins in the United States Diet, published in "American Journal of Clinical Nutrition" in 1980. For example, pomegranate lectins are most reactive with blood type B, while string bean lectins are most reactive with blood type A. The blood type diet theory suggests that avoiding specific lectins based on your blood type will reduce bowel symptoms from these proteins.
Kidney Bean Nutrients
One cup of cooked, canned kidney beans is rich in nutrients. It is a good source of protein, with 15 g, and cholesterol-free. It is very low in simple sugars, with less than 0.5 g. It contains 48 mg of calcium, 222 micrograms of folate and 690 mg of potassium. The cup of kidney beans also provides iron, selenium, zinc, choline, vitamin K and vitamin C.
References
- Cornell University Department of Animal Science: Plant Lectins
- "American Journal of Clinical Nutrition"; Lectins in the United States Diet: A Survey of Lectins in Commonly Consumed Foods and a Review of the Literature; M.S. Nachbar; 1980
- "Epidemiology and Infection"; Red Kidney Bean Poisoning in the UK: An Analysis of 50 Suspected Incidents Between 1976 and 1989; J.C. Rodhouse; July 1990
- U.S. Department of Agriculture: What's in the Foods You Eat Search Tool
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