102
I CASSAVA -
A VALUABLE FOOD BUT A POSSIBLE POISON
Cassava - a valuable food but a possible poison w. A. M. Cutting,
MB, ChB, MRCP, DCH
Ross Institute of Tropical Hygiene, London School of Hygiene and Tropical Medicine
TROPICAL DOCTOR,
1978,8,102-103
Nutritionists depreciate cassava (manioc) because as a dietary staple it is both low in protein (2"" dry weight) and relatively bulky. But the plant (Manihot esculenta or utilissima) has served man well for thousands of years. It is native to America where it has been cultivated by the Indians, and now it is grown right round the tropical world (Brouk 1975)· The assets of the plant are that it is quite easy to cultivate, produces good yields of food on soils too poor for other crops, and is resistant to locusts and other insect pests. The roots can be stored in the ground as a food reserve for up to two years. The parts of the plant generally used for food are tubers, swellings on the adventitious roots, but the leaves may also be eaten as a green vegetable. Flour from dried cassava contains 355 calories per 100 g, which is only 45 calories less than sugar. This flour combined with half its weight of groundnuts provides a very good nutritional mixture (McDowell 1976). Some forms of cassava may be eaten whole as sweet potatoes, but generally they are processed. "Garri" or "farinha" are the names for the crude flour preparation; "purupuru" is a less completely processed dish. Small starch particles that are made by boiling or drying cassava meal on hot plates is called "tapioca". The main problem about cassava is that it contains very poisonous hydrocyanic acid (HCN). Tubers with more HCN are "bitter", and those in which the HCN is only in the outer layers, the phelloderm, are described as "sweet". It is probably unwise to stress the two types of the plant, because there is no clear division between the bitter and sweet varieties. Some species of cassava can be "sweet" (low in HCN) on one soil, and "bitter" on another soil and in a different climate (Johnston 1958). When tubers are damaged, a glucosidase enzyme accelerates the release of HCN, and bruised tubers are known to be more toxic. The poisonous nature of the crop has long been recognized in every country where it is used. There are many traditional ways of preparing the tubers to decrease the HCN in food and these include soaking, washing in running water, boiling, roasting, and pounding. The highest concentration of HCN
Tropical Doclor,]uly 1978
reported is 245 mg. 100 g of dried root cortex (Montgomery 1969). The processing reduces this greatly, but some Nigerians are known to eat 750 g of cassava per day. If it" is in the form of garri (flour), he may ingest up to 8 mg of HCN, or if it is in purupuru, over 32 mg of HCN. These are significant amounts since the lower limit of the lethal dose is said to be 35 mg (Conn 1973). The acute toxic effects of cassava have been recognized for centuries (Clusius 1605). There is a dramatic onset of abdominal pain and vomiting, progressing to mental confusion, muscle pareses, and ultimately respiratory failure in fatal cases. This is rare in societies which prepare cassava in a very traditional way. Sometimes children eat inadequately processed tubers, or jungle cassava plants which have unexpectedly high levels of HCN (see p. 99 of this issue). Chronic toxicity due to prolonged exposure to low concentrations of HCN has only been recognized more recently. The cyanogen causes a metabolic block which results in elevated plasma thiocyanate. The clinical conditions attributed to chronic poisoning are tropical ataxic syndrome and possibly tropical amblyopia (blindness) (Montgomery 1965; Monekosso 1966; Osuntokun 1969). The raised thiocyanate causes a loss of iodine in the urine and a consequent antithyroid action. In areas where there is deficient dietary iodine, this mechanism may be the cause of goitre (Delange 1977). Cassava still has an important nutritional role because in the foreseeable future many people in tropical countries will depend on subsistence farming of poor soils. Cassava will be the crop of choice over wide areas, and when combined with nuts or legumes it provides a valuable mixed diet. To avoid toxicity,
Fig. I. A Nigerian woman grinding tubers of cassava. (Courtesy: World Health Organization)
Tropical Doctor, July 1978
CASSAVA - A VALUABLE 'FOOD BUT A POSSIBLE POISON
the traditional methods of preparing the food will have to be improved with the assistance of the appropriate village technology. REFERENCES
Brouk, B. (1975). Plants Consumed by Man. London: Academic Press. Clusius (1605). Liber Exoticum, Leyden. Cited in Bulletin of Imperial Institute, 1906, 4, 334. Conn, E. E. (1973). Toxicants Occurring Naturally in Foods. Washington, DC: Academy of Sciences.
I 103
Delange, F., et al. (1977). The toxic effect of cassava on human thyroid; in Proceedings of the 4th Symposium for Tropical Root Crops, Cook, J. and Macintyre, R. (eds.), p. 237. Ottawa: IDRC publications. Johnston, B. E. (1958). The Staple Food Economies of Western Tropical Africa. Stanford: Stanford University Press. McDowell, J. (1976). Tropical Doctor, 6, 37. Monekosso, G. L., and Wilson, J. (1966). Lancet, 1, 1062. Montgomery, R. D. (1965). Amer.]. din. Nutr., 17, 103. Montgomery, R. D. (1969). Toxic Constituents of Plant Food Stuffs. New York: Academic Press. Osuntokun, B. 0., et al. (1969). Brit. med. t.. 1, 547.
Book Reviews A Medical Biochemistry for the Tropics By J. K. Candlish. (London: Bailliere Tindall, 1977, pp. 262, 134 figs and tables, £5.50.)
This book aims to provide a basic text for medical students in the tropics. The emphasis is thus on simplicity of approach and relevance to the situation in developing countries. One of the important factors to be borne in mind in an English language book intended for use overseas is that, for the majority of students, English will not be their mother tongue. In this respect Professor Candlish has succeeded quite well in avoiding the linguistic pitfalls of a highly stylized "scientific" approach. However, on the whole I found the book rather disappointing. While the language of the book is simple and straightforward and there are some useful examples of relevance to the situation in the tropics, the way in which the material is presented is often illogical, sometimes confusing and occasionally positively misleading. Professor Candlish states in the preface that he has aimed "to transverse the chemistry of body constituents as rapidly as possible to allow the student to reach metabolism without being overburdened with formulae and structures". The result, unfortunately, is that complex biochemical concepts are often tossed out without any basic framework to enable the reader to grasp and understand their real significance. In this respect the chapter on Nucleic Acids, Protein Synthesis and Biochemical Genetics is
particularly disappointing. It deals with lactomlactim tautomerism in uric acid, differences in the effects of snake venom diesterase and spleen deoxyribonuclease, the cloverleaf structure of mRNA and the hyperchromic effect while largely ignoring or glossing over areas of fundamental importance like the biochemistry of base-pairing, the genetic code 8 triplet codons, mutations (insertions, deletions, etc.) and the specificity of the enzymes linking amino acids to tRNA. On the other hand, the chapters on nutrition were quite good; the examples were relevant to the tropics and the comparisons with the situation in developed countries useful. The chapter on Biochemical Estimations for the Hospital misses a good opportunity to give some sound information to "doctors capable of working in adverse conditions in rural areas, selfreliant in the absence of adequate laboratory facilities", to quote from the preface. The diagrams are virtually all line drawings and the majority were reasonably clear. However, an unfortunately large proportion are rather cramped and by no means easy ttr'lnterpret. The index is comprehensive. In conclusion, the approach adopted by this book is interesting and it can, and probably should, find its way into some libraries in developing countries, but I could not recommend it to students, particularly those with limited funds, for personal purchase. B. SEATON
I CASSAVA -
A VALUABLE FOOD BUT A POSSIBLE POISON
Cassava - a valuable food but a possible poison w. A. M. Cutting,
MB, ChB, MRCP, DCH
Ross Institute of Tropical Hygiene, London School of Hygiene and Tropical Medicine
TROPICAL DOCTOR,
1978,8,102-103
Nutritionists depreciate cassava (manioc) because as a dietary staple it is both low in protein (2"" dry weight) and relatively bulky. But the plant (Manihot esculenta or utilissima) has served man well for thousands of years. It is native to America where it has been cultivated by the Indians, and now it is grown right round the tropical world (Brouk 1975)· The assets of the plant are that it is quite easy to cultivate, produces good yields of food on soils too poor for other crops, and is resistant to locusts and other insect pests. The roots can be stored in the ground as a food reserve for up to two years. The parts of the plant generally used for food are tubers, swellings on the adventitious roots, but the leaves may also be eaten as a green vegetable. Flour from dried cassava contains 355 calories per 100 g, which is only 45 calories less than sugar. This flour combined with half its weight of groundnuts provides a very good nutritional mixture (McDowell 1976). Some forms of cassava may be eaten whole as sweet potatoes, but generally they are processed. "Garri" or "farinha" are the names for the crude flour preparation; "purupuru" is a less completely processed dish. Small starch particles that are made by boiling or drying cassava meal on hot plates is called "tapioca". The main problem about cassava is that it contains very poisonous hydrocyanic acid (HCN). Tubers with more HCN are "bitter", and those in which the HCN is only in the outer layers, the phelloderm, are described as "sweet". It is probably unwise to stress the two types of the plant, because there is no clear division between the bitter and sweet varieties. Some species of cassava can be "sweet" (low in HCN) on one soil, and "bitter" on another soil and in a different climate (Johnston 1958). When tubers are damaged, a glucosidase enzyme accelerates the release of HCN, and bruised tubers are known to be more toxic. The poisonous nature of the crop has long been recognized in every country where it is used. There are many traditional ways of preparing the tubers to decrease the HCN in food and these include soaking, washing in running water, boiling, roasting, and pounding. The highest concentration of HCN
Tropical Doclor,]uly 1978
reported is 245 mg. 100 g of dried root cortex (Montgomery 1969). The processing reduces this greatly, but some Nigerians are known to eat 750 g of cassava per day. If it" is in the form of garri (flour), he may ingest up to 8 mg of HCN, or if it is in purupuru, over 32 mg of HCN. These are significant amounts since the lower limit of the lethal dose is said to be 35 mg (Conn 1973). The acute toxic effects of cassava have been recognized for centuries (Clusius 1605). There is a dramatic onset of abdominal pain and vomiting, progressing to mental confusion, muscle pareses, and ultimately respiratory failure in fatal cases. This is rare in societies which prepare cassava in a very traditional way. Sometimes children eat inadequately processed tubers, or jungle cassava plants which have unexpectedly high levels of HCN (see p. 99 of this issue). Chronic toxicity due to prolonged exposure to low concentrations of HCN has only been recognized more recently. The cyanogen causes a metabolic block which results in elevated plasma thiocyanate. The clinical conditions attributed to chronic poisoning are tropical ataxic syndrome and possibly tropical amblyopia (blindness) (Montgomery 1965; Monekosso 1966; Osuntokun 1969). The raised thiocyanate causes a loss of iodine in the urine and a consequent antithyroid action. In areas where there is deficient dietary iodine, this mechanism may be the cause of goitre (Delange 1977). Cassava still has an important nutritional role because in the foreseeable future many people in tropical countries will depend on subsistence farming of poor soils. Cassava will be the crop of choice over wide areas, and when combined with nuts or legumes it provides a valuable mixed diet. To avoid toxicity,
Fig. I. A Nigerian woman grinding tubers of cassava. (Courtesy: World Health Organization)
Tropical Doctor, July 1978
CASSAVA - A VALUABLE 'FOOD BUT A POSSIBLE POISON
the traditional methods of preparing the food will have to be improved with the assistance of the appropriate village technology. REFERENCES
Brouk, B. (1975). Plants Consumed by Man. London: Academic Press. Clusius (1605). Liber Exoticum, Leyden. Cited in Bulletin of Imperial Institute, 1906, 4, 334. Conn, E. E. (1973). Toxicants Occurring Naturally in Foods. Washington, DC: Academy of Sciences.
I 103
Delange, F., et al. (1977). The toxic effect of cassava on human thyroid; in Proceedings of the 4th Symposium for Tropical Root Crops, Cook, J. and Macintyre, R. (eds.), p. 237. Ottawa: IDRC publications. Johnston, B. E. (1958). The Staple Food Economies of Western Tropical Africa. Stanford: Stanford University Press. McDowell, J. (1976). Tropical Doctor, 6, 37. Monekosso, G. L., and Wilson, J. (1966). Lancet, 1, 1062. Montgomery, R. D. (1965). Amer.]. din. Nutr., 17, 103. Montgomery, R. D. (1969). Toxic Constituents of Plant Food Stuffs. New York: Academic Press. Osuntokun, B. 0., et al. (1969). Brit. med. t.. 1, 547.
Book Reviews A Medical Biochemistry for the Tropics By J. K. Candlish. (London: Bailliere Tindall, 1977, pp. 262, 134 figs and tables, £5.50.)
This book aims to provide a basic text for medical students in the tropics. The emphasis is thus on simplicity of approach and relevance to the situation in developing countries. One of the important factors to be borne in mind in an English language book intended for use overseas is that, for the majority of students, English will not be their mother tongue. In this respect Professor Candlish has succeeded quite well in avoiding the linguistic pitfalls of a highly stylized "scientific" approach. However, on the whole I found the book rather disappointing. While the language of the book is simple and straightforward and there are some useful examples of relevance to the situation in the tropics, the way in which the material is presented is often illogical, sometimes confusing and occasionally positively misleading. Professor Candlish states in the preface that he has aimed "to transverse the chemistry of body constituents as rapidly as possible to allow the student to reach metabolism without being overburdened with formulae and structures". The result, unfortunately, is that complex biochemical concepts are often tossed out without any basic framework to enable the reader to grasp and understand their real significance. In this respect the chapter on Nucleic Acids, Protein Synthesis and Biochemical Genetics is
particularly disappointing. It deals with lactomlactim tautomerism in uric acid, differences in the effects of snake venom diesterase and spleen deoxyribonuclease, the cloverleaf structure of mRNA and the hyperchromic effect while largely ignoring or glossing over areas of fundamental importance like the biochemistry of base-pairing, the genetic code 8 triplet codons, mutations (insertions, deletions, etc.) and the specificity of the enzymes linking amino acids to tRNA. On the other hand, the chapters on nutrition were quite good; the examples were relevant to the tropics and the comparisons with the situation in developed countries useful. The chapter on Biochemical Estimations for the Hospital misses a good opportunity to give some sound information to "doctors capable of working in adverse conditions in rural areas, selfreliant in the absence of adequate laboratory facilities", to quote from the preface. The diagrams are virtually all line drawings and the majority were reasonably clear. However, an unfortunately large proportion are rather cramped and by no means easy ttr'lnterpret. The index is comprehensive. In conclusion, the approach adopted by this book is interesting and it can, and probably should, find its way into some libraries in developing countries, but I could not recommend it to students, particularly those with limited funds, for personal purchase. B. SEATON
