The Common Bean That Can Be Deadly For Millions Of People With This Genetic Trait
Humans have been growing fava beans, also known as broad beans, for almost as long as we've been farming, but for some people, eating them can be deadly.
The strange specificity has roots in the tangled co-evolutionary history between our bodies, the plants we eat, and the environments we share. Fava beans are unusually rich in two compounds, vicine and convicine, which aren't found in other types of bean. These are converted into highly reactive substances during digestion, putting red blood cells under oxidative stress. G6PD is part of a cellular mechanism that helps those cells withstand oxidative damage, and without enough of the enzyme, that defense can be overwhelmed, causing irreversible damage to red blood cells.
The ensuing condition is called hemolytic anemia. During an acute episode, damaged blood cells can turn urine dark, and the loss of functioning red blood cells can cause weakness, breathlessness, pallor, or jaundice. None of that is fun, but a severe favism reaction can be life-threatening, and calls for urgent medical treatment.
To bean or not to bean
The bean itself, whether fresh, canned, or dried, isn't a universal poison, and the condition isn't technically an allergy. Why this vulnerability to an ancient staple food is shared by hundreds of millions of people, though, requires a look outside the bean pod, at the environments in which human genetics evolved.
G6PD deficiency is the world's most common enzyme deficiency, and it's particularly prevalent in teh Mediterranean, as well as parts of Africa and Asia. That geographic distribution strikingly overlaps with regions where malaria has been historically endemic. Researchers believe that certain G6PD variants may have persisted because they offered some protection against malaria, possibly by making red blood cells a less hospitable environment for the parasite. In other words, the same genetic variation that leaves those cells vulnerable to the oxidative stress of fava beans may have offered an advantage against a far greater historical killer.
Having G6PD deficiency doesn't mean a plate of favas will inevitably trigger favism, and if you can eat them, fava beans make an easy side dish, tossed on the grill and dressed simply with olive oil, garlic, lemon and salt. But only a quarter of people with the deficiency will have any reaction, and severity can vary with the degree of enzyme deficiency, age, quantity eaten, and even the beans themselves. Many people with the deficiency have no symptoms until something triggers an episode. If they never eat a fava bean, they might never know they have the enzyme deficiency. The compounds in question are relatively heat resistant, and because the consequences can be serious, people diagnosed with G6PD deficiency are generally advised to avoid fava beans rather than rely on cooking or processing methods to make them safe.
