NewsColumnThe Human Factor - Peanuts, Turkeys and Cancer

The Human Factor – Peanuts, Turkeys and Cancer

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So, tell me, what is the relationship between mouldy corn and Otto Preminger’s last movie? The film we’re talking about is The Human Factor, an adaptation of Graham Greene’s 1978 spy novel. In a nutshell, the story is about a suspected “mole” in the British Foreign Office who is thought to be leaking information to the Russians. A decision is taken to eliminate the traitor without any adverse publicity by poisoning him. Of course, a poison that works swiftly would raise suspicion, so a physician working for the foreign office decides on aflatoxin B, a chemical that triggers cancer of the liver. Pretty ingenious and at least in theory, feasible!

Aflatoxins are naturally occurring toxins produced by moulds of the genus Aspergillus flavus, which can infect nuts and grains, particularly corn when these are stored under warm, humid conditions. The aflatoxin story is an interesting one because these compounds were the first to be recognized as dietary carcinogens. They were not the first carcinogens to be identified; that honour goes to substances found in snuff. In 1761, John Hill, an English physician, noted that snuff users were more likely to develop nasal tumours. Then in 1775, Percival Potts, a surgeon, discovered that chimney sweeps had an unusually high incidence of skin cancer on their scrotum. The sticky material inside chimneys, known as creosote, was the likely candidate. Researchers began to explore this possibility by painting creosote on the skin of animals to see if tumours could be induced. For about 150 years these experiments went on sporadically and unsuccessfully.

Dr. Joe Schwarcz

Then along came Katsusaburō Yamagiwa, a medical researcher in Japan. He had something that apparently his predecessors in this research area did not have. Patience. Yamagiwa carefully painted the ears of 137 rabbits every day for over a year. Finally, tumours began to appear on seven of the rabbits. This could not happen by chance; Yamagiwa had found a carcinogen.

By the time Yamagiwa made his discovery, epidemiologists had realized that some occupations other than chimney sweeps also were associated with higher rates of certain cancers. By the 1930s the existence of a link between certain industrial chemicals and cancer had become clear. Nobody, though, thought that food could also be a factor. At least not until 1960, when turkeys began to die on mass on poultry farms in England.

Within a few months, over 100,000 birds perished from what came to be called “Turkey X Disease.” When ducks and pheasants began to die as well, scientists looked for a common cause and found it in the feed the birds were given. The turkeys, ducks and pheasants all had been fed on Brazilian peanut meal. Within a year, the culprit was identified as the mould Aspergillus flavus, which produced the appropriately named “aflatoxins.”

The turkey episode raised questions about a potential risk to humans who consumed aflatoxin contaminated food. The concern became very real when it became apparent that crops damaged by drought, not an unusual occurrence, were particularly prone to contamination. And the red flag was raised high when laboratory studies showed that aflatoxins could induce cancer. It didn’t take long for scientists to discover that this was happening outside the laboratory as well. Epidemiological surveys confirmed a high rate of liver cancer in areas of high aflatoxin exposure, particularly China, where rice often is contaminated by moulds. As many as one in ten Chinese adults develop liver cancer. Studies in Africa confirmed the theory. Both in Kenya and Mozambique peanut meal is a staple of the diet, but storage conditions differ. Calculations show that in Kenya the average daily intake of aflatoxins is 3.5 nanograms per kg of body weight (a nanogram is a billionth of a gram) while in Mozambique it is 220 nanograms per kg. The liver cancer incidence in Mozambique is thirteen times higher than in Kenya.

In North America stored grains are routinely assayed for aflatoxins. For corn, a common method is to shine greenish-yellow light on the kernels which will then fluoresce if they are contaminated with Aspergillus. A fascinating new technique to detect the mould uses sound. Corn kernels are heated by infrared radiation and are then quickly cooled. Those that are contaminated make a different sound when they contract, and computer analysis of the frequencies produced can determine the extent of contamination. If Aspergillus is found, a laboratory analysis is carried out, and corn with aflatoxin levels greater than 20 parts per billion is deemed unfit for consumption. Of course, this does not mean that we have no exposure because obviously not every lot of every grain can be tested. Nobody knows how many cases of liver cancer in North America may be attributed to aflatoxins. Judging by a fascinating case of attempted suicide, probably not many. A young American woman tried to kill herself by ingesting 5.5 mg of aflatoxin B (a high dose) that she had stolen from a laboratory. Nothing happened. Six months later, she tried again with a total of 35 mgs over two weeks. Again, nothing. She then decided life was worth living after all, and 14 years later she was still well with no liver problems. Oh well, it worked in The Human Factor.

 

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