Maize our Staple Food : Should Toxins Concern us?

by Bradley Flett

ARC-Grain Crops Institute, Potchefstroom

South Africans from all walks of life consume maize in some form or other. Maize meal is eaten as a staple food by the majority of South Africans. Alternately it is very popular as a breakfast porridge or as "pap" with a "braai". Many other everyday commodities such as pharmaceuticals, confectionary, toothpastes, popcorn, soups, etc. include maize in various forms. Most of our livestock are fed on maize which are then consumed by humans as meat, dairy products, cheeses or eggs. Maize is more often than not infected with various fungi. The most important are Fusarium moniliforme, Fusarium graminearum, Diplodia maydis and Aspergillus flavus. These fungi all produce potentially dangerous mycotoxins when consumed by humans or animals. The most important mycotoxins are Fumonisins, Deoxynivalenol, Nivalenol, Zearalenone, Aflatoxin and an unknown, yet to be identified, toxin caused by Diplodia maydis. The question is "what are the effects of these mycotoxins on animal and human health?"

Symptoms

Fumonisin mycotoxins caused by Fusarium moniliforme, are highly toxic to horses, donkeys and mules causing hole in the head syndrome (leucoencephalomalacia) which is a necrosis of the brain. These mycotoxins also cause liver cancer in rats and lung diseases in pigs. Fumonisins have also been implicated in the occurrence of esophageal cancer of humans and have been categorized by the International Agency for Research on Cancer as Group 2B carcinogens i.e. possibly carcinogenic to humans. Substances and activities included in the category 2A carcinogens include coffee, saccharum, welding fumes, bracken ferns, manmade mineral fibres, etc. These are possible human carcinogens. So to date fumonisins are considered safer than these substances and activities. Further research may show they are not carcinogenic or they may move into "higher" categories. Fusarium graminearum infected maize may be contaminated with Deoxynivalenol, Nivalenol and Zearalenone. These mycotoxins are a major problem for pig farmers where Deoxynivalenol and/or Nivalenol lead to feed refusal and Zearalenone causes an estrogenic syndrome which reduces reproductive performance. Feed refusal is caused by unpalatability of feed and vomiting may result. Maize containing more than 5% infected kernels should not be included in rations for pigs. Cattle are more resistant to these effects and chickens do not seem to be affected. No reported effects on humans have yet been published. Aflatoxins are possibly the best understood and most researched mycotoxins, produced by the fungus Aspergillus flavus. Aflatoxin B1 is the most carcinogenic substance known to man. As recently as 4 July 2001 an article appeared in the Beeld newspaper with the heading "Kankerbom tik in skole" which reported the high incidence of aflatoxin in peanut butter in school feeding programs. This was cheaply produced inferior peanut butter when compared to that commercially available in the local supermarkets which underlines the importance of grading systems for various crops. Animal feed may also be contaminated with Aflatoxins and some such as Aflatoxin B1 are converted to aflatoxin M1 in cows which then contaminates milk. Diplodiosis is a disease of cattle caused by the yet to be isolated mycotoxin produced by Diplodia maydis. Diplodiosis symptoms in cattle include nervous system defects, coordination loss, paralysis and finally death. Diplodiosis of sheep leads to abortions. Poultry are also particularly susceptible to Diplodia toxins resulting in poor broiler growth and reduced egg laying. Diplodia toxins are yet to be linked to detrimental effects in humans. It is evident that mycotoxins are species and organ specific and that a mycotoxin causing a condition in rats is apparently harmless to some other animal or humans. This means that mycotoxins detrimental to animals may have no effect on humans. On the other hand, it may manifest itself in a completely different symptom and affect a different organ in humans. The only maize mycotoxins directly linked to humans to date are the Aflatoxins. Fumonisins have been circumstantially linked to human esophageal cancer by correlation. However, vitamin and trace element deficiencies are also correlated strongly to esophageal cancer raising the question as to the actual cause of esophageal cancer. 

Mycotoxins produced by Fusarium graminearum and Diplodia maydis have to date not been linked to human diseases or disorders but may well be in future. The effect of mycotoxins on humans requires further funding and research.

Legal limits

Legal limits for Aflatoxins require that food for human consumption must contain less than 10mg/kg (parts per billion) of which only 5 ppb may be aflotoxin B1. The Fertilizers, Farm Feeds, Agricultural Remedies and Stock Remedies Act restricts Aflatoxins to a maximum of 50 ppb in animal feed. Exceptions are 20 ppb for lactating cows, calves, pigs, lambs and poultry. Piglets, chickens under laying age and lambs younger than 4 months have a limit of 10 ppb. Feed for cats, dogs, horses and ostriches must have less than 20 ppb. The aflotoxin problem has been extensively researched and these limits have been based largely on research results. Legal limits for fumonisins have not been introduced locally and vary from one country to another. Switzerland has established a provisional tolerance level of 1 ppm (parts per million) for maize and maize based products for human consumption. Locally a provisional tolerance level of 100-200 ppb total fumonisins has been suggested for maize and maize based products intended for human consumption. This lower tolerance level (RSA vs Switzerland) is based on the higher human consumption of maize, locally. Unofficial animal feed tolerance levels of Fumonisins B1 are less than 5 ppb for horses, 10 ppb for pigs, 50 ppb for beef cattle and poultry. Legal limits for Deoxynyvalenol have been implemented in five countries and vary from 500 to 1 000 ppb in foods and 1 000 ppb to 10 000 ppb in animal feed. Zearalenone levels in a number of countries vary from 30 to 1 000 ppb in food with none for animal feed. The implementation of tolerance levels must be determined objectively and without emotional hysteria. Research, as indicated above has played a large role in the implementation of tolerance levels for Aflatoxin. Unfortunately not enough conclusive evidence for the effect of Fumonisins on humans has been collected. The International Agency for Research on Cancer has classified fumonisins as Group 2B carcinogens i.e. possibly carcinogenic to humans. Other terms such as provisional tolerance levels indicate that there is still a great deal of uncertainty regarding the effect of these toxins on human health. It is essential that researchers provide statistical, substantial evidence regarding the dangers of fungal toxins as the economic implications of tolerance levels based on maybe's, are far reaching. Starving people will not ask "How many toxins are in my food?" prior to satisfying a principle basic need of satisfying their hunger.  Tolerance levels must be determined scientifically and high risk practices, farming systems and localities must be identified. Production systems differ drastically as maize is grown over a range of climatic conditions for a wide range of uses (own use, commercially, animal feed, etc.). It is however important that when definitely linked to human diseases that information be disseminated by education programs and the dangers of eating infected grain be pointed out to all consumers (those buying or producing their own grain). Implementation of legal tolerance levels will not prevent subsistence farmers using mouldy grain for brewing beer. It will however prevent commercial grain with high levels of mycotoxins being used by consumers. Different strategies for different risk situations need to be developed. For implementation of tolerance levels quick, cheap toxin detection techniques and statistically based sampling procedures are required.  The questions that need to be answered when setting mycotoxin tolerance levels are: 1. Is there a need for such tolerance levels? 2. What concentration must such levels be set at? 3. What affect would this have on global and local staple food supplies? 4. How will the levels be enforced i.e. sampling and measurement techniques? 5. Should we be importing maize with a potentially higher risk? The good news for South African maize consumers is that Aflatoxins are virtually absent in locally produced maize but are frequent in imported USA and Argentinean maize. Fumonisins are always present but generally lower than 250 to 700 ppb (Switzerland's tolerance level is 1 000 ppb) in local maize with much higher levels in USA maize. The moral of the story is "Eat Local Maize".

 

November 2001