Survival of the Sickest: Chapter 4 Summary & Analysis

In ancient Greece, a variety of sources warned against eating fava beans, from Greek scholar Pythagoras to a cult called the Orphics (who believed the beans contained the souls of the dead). Moalem writes that these superstitions were not without merit. Favism is an inherited enzyme deficiency carried by 400 million people—the most common enzyme deficiency in the world. In extreme cases, people who have favism and eat fava beans experience “rapid, severe anemia that can often lead to death.”

Scientists first recognized some truth behind the aversion to fava beans during the Korean War. American soldiers were prescribed drugs to combat malaria, including one called primaquine. About 10 percent of African American soldiers developed anemia while taking primaquine, and other soldiers (mostly of Mediterranean descent) experienced an even more severe version called hemolytic anemia.

Three years later, scientists discovered the cause of these side effects. The soldiers who reacted to the drug lacked an enzyme called G6PD, which clears out elements that are harmful to the cell. These elements include free radicals, which are molecules with unpaired electrons that can disrupt cellular chemistry. Primaquine is thought to stop the spread of malaria by preventing malaria-causing parasites from invading the body’s red blood cells. But with a lack of G6PD to maintain the cell’s defense systems, the cells become even more vulnerable. Hemolytic anemia can then lead to kidney failure, heart failure, and death.

This lack of G6PD is known as favism, and it is named that way because when people who have favism eat fava beans, they undergo a similar reaction to the one that occurs after taking primaquine because of the free radicals in fava beans. The irony of favism is that the people who have favism are more likely to live in places where fava beans are cultivated: North Africa, Southern Europe, and around the Middle East (the countries that line the Mediterranean). Moalem again questions why humans would have evolved a mutation that causes problems specifically when eating a staple of their local diet.

Moalem then shifts to the broader connection between evolution in the animal kingdom and evolution in the plant kingdom. He notes that plants that produce edible fruit evolve that way for their own benefit: animals learn to pick the fruit, eat it, and deposit the seeds somewhere else so that the plants spread and reproduce.

Plants have also evolved defense mechanisms so that the fruit is the only thing the animals eat. These mechanisms include thorns, but also chemicals and toxins that they have developed. Clover, sweet potato, and soy all belong to a group of plants that contain chemicals called phytoestrogens. These chemicals mimic the effects of sex hormones like estrogen, and the compounds diminish animals’ ability to reproduce. This ultimately proved useful to humans, however, as chemist Carl Djerassi based his development of the birth control pill on these hormones—specifically phytoestrogens produced by the Mexican yam.

Plants also produce poisons to prevent predators from eating them. For instance, the cassava plant contains a precursor to cyanide, while the Indian vetch contains a neurotoxin that can cause paralysis. Plant toxins can also have milder effects, like interfering with digestions or burning lips. Raw habanero peppers have a chemical called capsaicin, which causes burning sensations in mammals but not in birds. This is because mammals’ digestive systems destroy the peppers’ seeds, while birds’ digestive systems do not.

Humans will frequently still eat plants with toxins—between 5,000 and 10,000 every year—because we have also evolved mechanisms to manage those toxins. Tasting bitterness is one mechanism, as this ability allows us to detect toxins in plants and avoid eating them. However, not all compounds that are linked to bitterness are toxic—likely because most of the plants’ chemical weapons are aimed at insects, bacteria, fungi, and herbivores.

In the past, humans have tried to increase plants’ natural defenses in order to protect them from other predators like insects—sometimes with results that backfire. A potato was bred with more solanine (its natural poison), and it became so poisonous it was inedible. Celery defends itself with a toxin called psoralen, which causes sensitivity to sunlight in humans. It produces more psoralen when it feels under attack. For farmers who use pesticides, fewer insects attack the celery and it produces less psoralen. But organic farmers don’t use these pesticides, and ironically, refraining from external poison actually results in more internal poison in the celery plant.

Moalem returns to the connection between fava beans and favism. People who have favism and lack the G6PD enzyme cannot mop up free radicals, which results in anemia in more than 400 million people around the Mediterranean. However, this gives people an advantage over another common disease around Africa and the Mediterranean: malaria.

Malaria infects almost 500 million people and kills more than 1 million people every year. Malaria causes joint pain, vomiting, and anemia. Ultimately, it can lead to coma and death, especially in children and pregnant women. It was initially thought that malaria was caused by unhealthy vapors emanating from still water (malaria is old Italian for “bad air”). This theory was incorrect, but it led to the development of air conditioning by Dr. John Gorrie, hoping to eliminate that “bad air.”

In reality, malaria is caused by parasitic protozoa which are deposited in the human bloodstream through mosquitoes. The most dangerous protozoan is called Plasmodium falciparum. Scientist J.B.S. Haldane was one of the first people to understand that certain groups with anemia have better natural resistance to malaria. Today, many scientists believe that a G6PD deficiency (favism) provides even more resistance because the red blood cells are less hospitable to the protozoa, and because they are taken out of circulation more frequently, disrupting the parasite’s life cycle.

Moalem then explores why the same populations with favism might have cultivated fava beans: he explains that favism is only passed on the X chromosome, and so many women only have one copy of the G6PD mutation. When someone with a partial or mild G6PD deficiency, or without the deficiency at all, eats fava beans, it makes the red blood cells less hospitable. Only women with two copies of the gene, or men with one copy, might have an extreme reaction to fava beans.

Humans have long been relying on herbal remedies, but the first antimalarial medicine came from George Cleghorn in the 19th century, who found a remedy in the bark of the cinchona tree. A century later, French chemists found the beneficial compound, quinine, and made a tonic from it to prevent malaria. However, over time, nearly every strain of malaria has become resistant to quinine. Fava beans, on the other hand, are still effective because they alter human body chemistry, not the protozoa’s chemistry.

Many plant toxins can be harmful but also come with benefits: phytoestrogens can stop the growth of prostate cancer cells and may ease the effects of menopause. Capsaicin stimulates the release of endorphins, increases your metabolic rate, and can also serve as a natural painkiller. Psoralen in celery can cause skin damage but can also help psoriasis. Even common medications liked aspirin or taxol (an anti-cancer drug) are derived from plants. It’s helpful, Moalem concludes, to recognize the benefit that plants can afford to us.