The cassava plant carries two cyanogenic glucosides, linamarin and lotaustralin, in its edible roots and leaves. The amounts of these potentially toxic compounds vary considerably, according to cultivar and growing conditions. "Sweet" varieties usually have such small amounts as to be innocuous, whereas "bitter" varieties have sufficiently high levels to require domestic processing to remove most of the toxins.
In situations where famine or extreme poverty may force a population to eat poorly processed cassava in a diet that is also deficient in nutrients such as protein, the plant's cyanogenic glucosides can lead to poisoning. A classic case was the infantile kwashiorkor epidemic in famine-stricken Biafra in 1968, but there have also been recent examples of spastic paraparesis, or konzo, in drought-stricken regions of Mozambique and Tanzania.Detoxifying cassava
Farming populations who cultivate cassava have developed many methods of detoxifying cassava. Boiling and drying are sufficient to make low-cyanogen cultivars safe for consumption, but more rigorous procedures such as grating, fermenting, and sun-drying, are necessary to effectively remove cyanogens from cultivars of higher toxicity (see Table).
The protein link
Whenever a chronic disease has been linked to cassava consumption, the victims have also been found to suffer from protein deficiency, suggesting a relationship between the two.
Protein is essential for all the body's vital functions, and for eliminating certain dietary toxins. With the help of the enzyme rhodanese, the human body detoxifies cyanide by forming thiocyanate. When the body is regularly exposed to cassava cyanogens, the increased synthesis of rhodanese makes extra demands on the body's reserves of amino acids, the building blocks of proteins. To detoxify 1.0 mg hydrocyanic acid (HCN), the body also needs a daily supply of about 1.2 mg of dietary sulfur (S) from S- containing amino acids (SAA). If the demand for rhodanese and SAA is prolonged, as in the regular consumption of cassava, and the diet is inadequate, the synthesis of many proteins vital for bodily functions may be impaired, leading to the development of protein deficiency diseases.
Cassava - low protein source
Cyanogens alone cannot be blamed for toxicity because other cyanogenic crops, such as sorghum and Lathyrus bean, which are widely used as food, cause few toxicity problems. But the protein contents of these two crops (11.0% and 18.7%, respectively) are higher.
Many cassava products contain very low amounts of cyanogens, which can be efficiently eliminated by the body, if the protein intake is adequate. Cassava roots, being bulky and rich in carbohydrates, free dietary proteins from having to meet the body's energy needs, thus allowing them to be used more efficiently. However, the level of protein in cassava lags far behind the levels found in rice, wheat, and tuber crops (Figure 1). An adult consuming 1 kg of cassava has to ingest 52 g of protein from other sources to obtain the U.S. recommended daily allowance (RDA) of 65 g protein per adult. In contrast, 1 kg of wheat supplies 121 g of protein and rice, 61 to 64 g of protein.
If protein intake is more than adequate for both general metabolic requirements and cyanide elimination, toxic effects are lessened or even eliminated, even if cassava is improperly processed. (Fatal poisoning can result from ingestion of large amounts of unprocessed or poorly processed high-cyanogen cassava.) Hence, the lack of protein in cassava roots is probably responsible for most non-fatal cases of cyanide poisoning associated with cassava.
Developing new cassava varieties
To help resolve the cassava toxicity-protein deficiency problem, biotechnology can contribute to the breeding of cassava varieties that are acyanogenic and/or have protein-rich roots.(CIAT: Cassava Newsletter 19/2)
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