Cassava’s link to iodine deficiency requires further study
It has been called King Cassava, but with roots that resemble long hairy potatoes, naturally-occurring toxins that can sicken unwitting consumers, and a reputation for under-appreciation, cassava is perhaps best described as the frog prince of agricultural development.
What this hardy crop lacks in glamour, it makes up for in sheer usefulness, and in a time of widespread food shortages, its calorie-rich roots are considered a key to global food security. But some scientists worry that cassava’s growing popularity could lead to increases in iodine deficiency, a micronutrient shortage that the World Health Organization (WHO) calls the world’s leading cause of mental retardation.
Cassava’s popularity is on the rise, particularly in Africa, where production has tripled in the past 20 years. “Cassava is being transformed—not only as a food security crop. It is becoming a cash crop,” NeBambi Lutaladio, the Roots and Tubers Specialist at the Food and Agriculture Organization’s (FAO) headquarters in Rome, told MediaGlobal. “You see places like Ghana and Nigeria—cassava production is increasing dramatically, because you have all these small entrepreneurs in the rural areas processing cassava and being able to provide to the domestic and regional market.”
While it drives rural development, cassava is rarely traded internationally. This makes it immune to the kind of market fluctuations that sent prices of wheat and corn skyrocketing last year, Lutaladio said. Countries that currently depend on imported grain are taking note, and some governments are encouraging farmers to plant more cassava.
“[It’s] a growing industry,” Dr. Robert Asiedu, Director of Research for Development at Nigeria’s International Institute for Tropical Agriculture (IITA), said in an interview with MediaGlobal. “Over the past few years it has been transforming from a crop that is known as a poor man’s food into a crop that provides a lot of food for the urban population.”
Nearly a billion people around the world rely on cassava as a dietary staple, and in Africa, only maize provides more calories. Exceptionally productive, it can be harvested year-round, weathers drought and requires little fertilizer—it’s own nutrient-rich leaves fertilize the soil as they drop. When the leaves and roots are eaten together, cassava provides balanced nutrition and a range of vitamins.
But cassava roots also contain varying amounts of cyanide, a potent poison. Most of the toxins can be eliminated though proper processing, but “whichever method of detoxification is used, it is difficult to remove the last traces of cyanide from the cassava root, especially from the bitter, high cyanide varieties,” Stephanie Gallat, a post-harvest management officer at the FAO’s Rural Infrastructure and Agro-Industries Division, told MediaGlobal. The body is able to detoxify the remaining traces of cyanide during digestion, but as they break down, a goitrogenic compound called thiocyanate is produced. This compound is known to limit the thyroid gland’s ability to store and process iodine.
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. And the bulk of cassava consumers live in the world’s poorest countries, where iodine deficiency is highest.
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. “[If] the people are under pressure because they are moving from place to place, they are not able to process cassava the way they normally do,” Asiedu, the IITA scientist, said, noting that ongoing conflict in the Democratic Republic of Congo has made proper detoxification difficult or impossible for many people. During the country’s prolonged civil war, thousands have been paralyzed by konzo after eating unprocessed cassava.
But Asiedu noted that, under normal conditions, people who are familiar with the crop know how to process correctly. “We have, over the years, analyzed this in a lot of detail and come up with the conclusion that if people use the cassava in the way that they normally do, and use the right varieties of cassava, you will not have this problem at all,” he said.
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. But most experts, including Annika Wennberg of the FAO’s Nutrition and Consumer Protection Division, are confident that 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,” she told MediaGlobal.
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, then I think [the relationship] should be better appreciated,” Dr. Michael Zimmermann, a senior scientist at the Laboratory for Human Nutrition at the Swiss Federal Institute of Technology in Zurich, said in an interview with MediaGlobal.
What information does exist may be out of date. Zimmerman observed that much of what is known about cassava’s role in iodine deficiency comes from a few studies conducted in central Africa in the 1970s and ’80s.
“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.
Public health officials have known just how to do this for nearly a century. “The most effective way to deliver iodine to every human being every day in exactly the right amount is through iodized salt,” David Haxton, Executive Director of the International Council for the Control of Iodine Deficiency Disorders (ICCIDD), said in an interview with MediaGlobal.
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. These are the areas most in need of economic development, where the devastating effects of iodine deficiency help drive cycles of poverty. They are also the regions where cassava is most likely to be a major, or even sole, staple food.
Combating iodine deficiency requires cooperation, and there is often little communication between those involved in cassava development projects and those monitoring iodine deficiency or promoting salt iodization. Ghana, for example, has one of the world’s highest rates of iodine deficiency—the effects of which cost the country an estimated $22 million each year in productivity losses. And cassava is Ghana’s most important food crop—production tripled between 1961 and 1999. Yet the World Food Programme’s Ghana office, which oversees a major salt iodization initiative there, informed MediaGlobal that no research had been done on the impact of cassava consumption on iodine deficiency.
“I would say that communication has been minimal as these issues are handled by different ministries—agriculture and health—who generally don’t spend a lot of time talking to one another,” Gallat said. “The link between food security and public health certainly needs to be strengthened.”
“This is not a sectoral problem. It is not a health problem or a nutrition problem alone,” Haxton said. The effects of iodine deficiency, particularly in the first months of life, can be devastating, robbing the next generation of brain power and productivity. “You’re talking about a national security issue here when you’re talking about baby’s brains,” he said.
As climate change prompts governments in drought-prone areas to encourage farmers to plant cassava, and high import prices transform the locally-grown roots into a lucrative industry, “the consumption of cassava products is only going to increase,” said Lutaladio, the FAO’s roots and tubers specialist.
“So many questions remain to be answered,” Gallat noted, “but as this issue is one that predominantly affects poor African communities, it has not received priority attention or adequate funding.”
While more research needs to be done, some things are certain. A meal containing thoroughly detoxified cassava is healthier if it is seasoned with iodized salt. And that’s a meal that the health and agriculture sectors will have to work together to create.