Cassava: bad for thyroid, etc.

December 25, 2018



Cassava (manioc) often appears in health food stores as a gluten-free, healthy alternative – often in the form of chips. It is native to South America. It is often dried to create tapioca.


The plant contains very high amounts of cyanogenic glycosides. If these are not detoxed in the body, they can bring cyanide poisoning to various degrees. Even if they are detoxed, they can weaken the thyroid:


"Study of the effects of hydrogen cyanide exposure in Cassava workers…Chronic exposure to HCN may cause neurological, respiratory, cardiovascular and thyroid defects. Onset of symptoms depends on dose and duration of exposure. Large scale of Cassava processing could be disastrous due to discharge of hydrocyanic acid into the air…An increase in TGL and AIP shows a higher degree of cardiovascular risk. A decrease in T4 suggests an insufficient iodine uptake by thyroid gland…Cassava (Manihot esculenta) is an important raw material for starch and sago industries…Cassava roots and leaves cannot be consumed as they contain two cyanogenic glycosides - linamarin and lotaustralin…processing may not ensure the complete removal of cyanide and the left out last traces are removed during digestion by detoxification…"
(Indian J Occup Environ Med. 2011 Sep-Dec; 15)

The cyanogenic glycosides that are ingested in the body are detoxed into thiocyanate, which hinders the thyroid, iodine, etc. This detoxification saves the body from direct cyanide poisoning (in the presence of sulfur, protein). But the detoxification of cyanide, in turn, becomes toxic to the thyroid:


"HCN is detoxified with rhodanase forming a goiterogenic compound thiocyanate which is excreted in urine. This process requires sulfur donors such as dietary sulfur-containing amino acid…the detoxification of HCN is influenced by the nutritional status, such as B vitamins like B12, folic acid and essential sulfur-containing amino acids…Systemic effects notably on the nervous system, digestive tract and thyroid have been reported after repeated occupational exposure to cyanide…This compound is known to limit the ability of thyroid glands to store and process iodine. The goiterogenic effect of thiocyanate may also lead to endemic goiters. The effect of iodine deficiency can be devasting robbing the next generation of brain, power and productivity…"
(Indian J Occup Environ Med. 2011 Sep-Dec; 15)


"A cyanogen is a substance [in cassava] that induces cyanide production…Chronic, low-level cyanide exposure is associated with the development of goiter and with tropical ataxic neuropathy, a nerve-damaging disorder that renders a person unsteady and uncoordinated…People who get little or no protein in their diets are particularly susceptible to cyanide poisoning, as they lack the proper amino acids necessary to help detoxify the poison…Linamarin is converted to cyanide when eaten…"
(Science Daily, May 14, 2003)


Preventing outright goiter should not be the only concern. Inhibition of the thyroid to any degree can bring many problems:


"The ingestion of fresh or processed cassava based diets causes reduced growth rates in rats, pigs, African giant rats, sheep and goats (Tewe et al., 1977; Tewe and Maner, 1981; Tewe, 1983). The animals also have increased serum and urinary levels of thiocyanate, which is a continuous cause of depletion of sulphur containing amino acids...The thiocyanate also inhibits the intra-thyroidal uptake of iodine, causes an increase in secretion of thyroid stimulating hormone (TSH) and causes a reduction in thyroxine level which is necessary for growth. It is thus a goitrogenic factor, which was demonstrated by Tewe et al. (1984), who reported a significant reduction in serum thyroxine levels in growing pigs fed cassava peel diets containing 96 ppm total cyanide (Table 4)...In rats and pigs consuming inadequate amounts of protein and sulphur amino acids, the serum thiocyanate concentration becomes lower as the animals become unable to adequately detoxify cyanide. Additionally, this condition can also aggravate deficiencies in selenium, zinc, copper and vitamin A. Even with sufficient protein intake, consumption of cassava flour based rations can result in parakeratosis in pigs, attributable to zinc deficiency, aggravated by the cyanide in cassava diets. Other features include paralysis of the hind limbs and muscular weakness."
(O.O. Tewe, Detoxification of cassava products and effects of residual toxins on consuming animals)


A study in the Journal of Food Composition and Analysis (February 2012) found that cassava chips in markets in Australia, had enough cyanide to expose children to lethal doses if they ate 40-270 g.




"Reduction of the cyanide content during fermentation of cassava roots and leaves to produce bikedi and ntoba mbodi, two food products from Congo...cassava roots and leaves contain some cyanide in the form of cyanogenic glucosides, notably the linamarine, which can constitute a poison for the consumers...Cassava roots and leaves processing in Congo, as in most central African countries, involve fermentation...The fermentation of the cassava roots is a lactic fermentation...The cyanide content decreases during the fermentation of cassava roots and leaves by more than 70% through the activities of the bacterial produced linamarase, allowing the hydrolysis of cyanogenic glucosides...cassava roots and leaves are also rich in cyanide in the form of cyanogenic glucosides, linamarine and lotaustraline (Montgomery 1980; Dunstan et al., 1996) in a ratio of 93:7 (Butler et al., 1965)...the chronic exposure to cyanide due to the consumption of nondetoxified cassava products is associated to a certain number of diseases inferred by the cyanide, including goitre, dwarfism and the tropical ataxic neuropathy...fermentation is thus a detoxication process...The cyanide content varies very little during the first 24 h of fermentation, but decreases drastically from 1158 to 339.6 mg/kg after 48 h of fermentation (Figure 9), which corresponds to 70.67% reduction....The cyanhydric acid rate becomes zero after one minute of cooking of fermented cassava leaves...After the traditional fermentation of roots and leaves of cassava, the cyanogenic glucosides content is reduced significantly by 70 to 75% (Louembe et al., 1997; Kobawila et al., 2003). Our results confirm those obtained by Agbor-Egbe et al. (1995) confirming fermentation is then a very effective process for elimination of endogenous cyanic compounds from cassava roots."

(African Journal of Biotechnology, Vol. 4 (7), pp. 689-696, July 2005)


"Soaking followed by boiling is better than soaking or boiling alone in removing cyanide. Traditional African food products such as gari and fufu are made by a series of operations such as grating, dewatering, fermenting, and roasting. During the various stages of gari manufacture, 80 to 95% cyanide loss occurs. The best processing method for the use of cassava leaves as human food is pounding the leaves and cooking the mash in water. Fermentation, boiling, and ensiling are efficient techniques for removing cyanide from cassava peels."
(Crit Rev Food Sci Nutr. 1995 Jul)


"Fermentation of cassava pulp for 96 hours during cassava processing for gari reduced the HCN by 22 ppm (52.4 percent) and 20 ppm (57.3 percent) for 30572 TMS and 30555 TMS respectively...Soaking of the sliced cassava tissue for 24 hours in cassava flour production prior to sundrying resulted in 16 ppm (38.1 percent) and 15 ppm (38.4 percent) HCN reduction for 30572 TMS and 30555 TMS respectively..."
(Plant Foods for Human Nutrition, December 1995, Volume 48, Issue 4)


"Soaking of cassava roots normally precedes cooking or fermentation. It provides a suitably larger medium for fermentation and allows for greater extraction of the soluble cyanide into the soaking water. The process removes about 20% of the free cyanide in fresh root chips after 4 hours, although bound cyanide is only negligibly reduced. Bound cyanide begins to decrease only after the onset of fermentation (Cooke and Maduagwu, 1978). A very significant reduction in total cyanide is achieved if the soaking water is routinely changed over a period of 3–5 days. A variation to the soaking technique known as retting, was described by Ayenor (1985). This process involves prolonged soaking of cassava roots in water to effect the breakdown of tissue and extraction of the starchy mass. A simulation of the technique, followed by sundrying showed a reduction of cyanide of about 98.6% of the initial content in the roots..."
(O.O. Tewe, Detoxification of cassava products and effects of residual toxins on consuming animals)


It appears that cassava requires an amazing amount of processing time and labor in an attempt to avoid thyroid inhibition (or worse, a degree of cyanide poisoning). Certainly, the older ways of preparation - still preserved in various cultures around the world - are valuable with foods that contain only a minor amount of anti-nutrients. In nations where quality, nutritional foods are scarce, it is understandable that they must make-do with such extensive labor.


In the United States, many "foods" are regularly eaten that should be totally discarded as a part of the diet. In other cases, the foods (various grains or vegetables) should be properly prepared with soaking, fermenting and cooking to reduce mineral-binding anti-nutrients, cyanogenic glycosides, etc. The thyroid, and all it controls, will be happy with the extra efforts!




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