Without iodine, the thyroid is unable to regulate important metabolic processes in adults and trigger key stages of fetal and infant development. Children born to even moderately iodine-deficient mothers risk irreversible brain damage and physical stunting. One study found that the mean IQ of iodine deficient communities was a full 13.5 points lower than that of their iodine-sufficient neighbors.

Goitrogens are so called because by blocking iodine absorption, they encourage the development goiter, the bulbous swelling of the thyroid gland that is the most visible sign of iodine deficiency. Cassava is by no means the only goitrogenic food—lima beans, almonds and bamboo shoots have similar properties, but they are rarely eaten consistently enough or in large enough amounts to pose a health risk, whereas many people eat cassava for several meals a day.

Cassava is prized for its hardiness, but it is the times when its stamina matters most that eating it is most likely to become hazardous. Cassava is often the only crop to survive a drought, but cyanide levels in its roots rise considerably in dry conditions, doubling the toxin levels that remain in processed cassava flour. The high cyanide content can lead to far more immediate effects than iodine deficiency, such as the neurological disorder konzo, which causes irreversible paralysis of the legs. High-cyanide cassava was blamed for a konzo outbreak during a 2005 drought in Mozambique.

Detoxifying cassava usually requires several days of grating, pressing and drying, and in times of war or famine refugee populations may have no choice but to eat the roots before they have been fully treated. . During the country’s prolonged civil war, thousands have been paralyzed by konzo after eating unprocessed cassava.

But as cassava spreads to new regions, the knowledge of proper processing techniques needs to come with it. “If the people get the right kind of varieties and the right kind of knowledge in handling it, there is no problem at all, even though it may be a new crop in the area,” Asiedu said. The IITA, FAO and other research organizations have spent years developing varieties with lower cyanide content, as well as processing techniques that make the crop both safer and more profitable.

Even after most of the plant’s toxins have been removed, the traces of cyanide that are left are known to have a harmful effect on the thyroid function of severely iodine-deficient populations.  Eating detoxified cassava poses no threat to those who were well-iodized to begin with. “The effect on the thyroid gland is likely only in the case of pre-existing iodine deficiency,”

It is cassava’s impact on the populations in between—communities whose diet is low in iodine but who do not show a high enough incidence of goiter to alert health workers to iodine deficiency—that worries some researchers.

“Clearly, there are health consequences of relying on cassava as a staple food in areas where there is coexisting iodine deficiency,” Gallat said, but she added that “more research needs to be conducted into the link between cassava consumption and iodine deficiency.” Scientists still aren’t sure how much cassava it takes to alter thyroid function, and little is known about what happens when people who are only slightly iodine deficient start eating more cassava.

The point is hardly academic. Forty percent of Africans rely on cassava as a major source of calories. Forty-two percent do not get enough iodine.

“If cassava consumption is going to be encouraged and it’s going to be done in areas where iodine supply in marginal.

“It would be nice to see in a study of mild to moderate iodine deficiency if chronic daily high levels of consumption of cassava would make a difference in terms of thyroid function,” Zimmerman said. “That hasn’t been done.”

In any case, “ensuring adequate iodine intake in vulnerable communities appears to be a key intervention in preventing the occurrence of goiter related to cassava consumption,” Gallat noted.

 “The most effective way to deliver iodine to every human being every day in exactly the right amount is through iodized salt,”

Salt is one of the only substances consumed in predictable amounts by nearly every person on earth, and it is cheap and easy to iodize—if salt production is centralized and regulated. Iodized table salt has virtually eliminated iodine deficiency in many developed countries.

The areas that remain iodine deficient tend to be the world’s poorest, where salt is mined or collected by small-scale producers and iodization is difficult.