The toxic compound phytohaemagglutinin, a lectin, is present in many varieties of common bean but is especially concentrated in red kidney beans. Phytohaemagglutinin can be deactivated by cooking beans at 100 °C (212 °F) for ten minutes. However, for dry beans the U.S. Food and Drug Administration (FDA) also recommends an initial soak of at least 5 hours in water; the soaking water should be discarded.
The ten minutes at 100 °C (212 °F) is required to degrade the toxin is much shorter than the hours required to fully cook the beans themselves. However, lower cooking temperatures may have the paradoxical effect of potentiating the toxic effect of haemagglutinin. 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 the use of slow cookers, the low cooking temperatures of which may be unable to degrade the toxin.
The primary symptoms of phytohaemagglutinin poisoning are nausea, vomiting, and diarrhea. Onset is from 1 to 3 hours after consumption of improperly prepared beans, and symptoms typically resolve within a few hours. Consumption of as few as four or five raw kidney beans may be sufficient to trigger symptoms.
Beans are high in purines, which are metabolized to uric acid. Uric acid is not itself considered a toxin, but it 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 also an important antioxidant in humans and, according to cohort studies, might be neuroprotective in cases of multiple sclerosis and Parkinson's disease.