Toxicity
The toxic compound phytohaemagglutinin, a lectin, is present in many common bean varieties, but is especially concentrated in red kidney beans. White kidney beans contain about a third as much toxin as the red variety; broad beans (Vicia faba) contain 5 to 10% as much as red kidney beans.
Phytohaemagglutinin can be deactivated by boiling beans for ten minutes; the ten minutes at boiling point (100 °C (212 °F)) are sufficient to degrade the toxin, but not to cook the beans. For dry beans, the U.S. Food and Drug Administration (FDA) also recommends an initial soak of at least 5 hours in water, which should then be discarded.
If the beans are cooked at a temperature below boiling (without a preliminary boil), as in a slow cooker, the toxic effect of haemagglutinin is increased: beans cooked at 80 °C (176 °F) are reported to be up five times as toxic as raw beans. Outbreaks of poisoning have been associated with cooking kidney beans in slow cookers.
The primary symptoms of phytohaemagglutinin poisoning are nausea, vomiting, and diarrhea. Onset is from one to three hours after consumption of improperly prepared beans, and symptoms typically resolve within a few hours. Consumption of as few as four or five raw, soaked kidney beans can cause symptoms.
Beans are high in purines, which are metabolized to uric acid. Uric acid is not a toxin as such, but may promote the development or exacerbation of gout. For this reason, persons with gout are often advised to limit their consumption of beans. Uric acid is an important antioxidant in humans and, according to cohort studies, might be neuroprotective in cases of multiple sclerosis and Parkinson's disease.
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