3.5

Skip to 0 minutes and 2 secondsAs we have already mentioned, one major health concern for several food contaminants is that they may cause cancer.

Skip to 0 minutes and 12 secondsCancer takes years to develop and many environmental chemicals can contribute to it. Therefore, it is often difficult to determine the extent to which food contaminants cause human cancer. However, if a chemical is known to cause cancer in animals, it is assumed that it may be carcinogenic to humans and efforts must be made to limit human exposure. Let us consider how toxins in food may cause cancer. Firstly, what do we mean when we say something causes cancer? That means it's a carcinogen. Evidence is gathered from my range of sources, including experimental evidence from animals exposed to the chemicals and epidemiological evidence that looks at the patterns of cancer in relation to exposure of the chemical of food in question.

Skip to 1 minute and 8 secondsThe World Health Organisation includes a special section dedicated to the study of causes of cancer called the International Agency for Research on Cancer, IARC.

Skip to 1 minute and 20 secondsOne of the things that IARC does is to assess the evidence that a chemical or environmental exposure causes cancer. They have developed a four-part classification systems which is applied after a panel of experts carefully review all of the available scientific evidence. Group 1, carcinogenic to human. That's sufficient evidence from both human and animal studies.

Skip to 1 minute and 47 secondsGroup 2A, probably carcinogenic to humans. Means limited evidence in human, sufficient evidence in animal. Group 2B, possible carcinogenic to humans. Limited evidence in human, and less than sufficient evidence in animal. Group 3, not classified as to its carcinogenicity to humans. Inadequate evidence in human and animal. Group 4, probably not carcinogenic to humans. Evidence suggesting a lack of carcinogenicity.

Skip to 2 minutes and 20 secondsWhat is cancer? Cancer is a disease in which damaged cells are freed from normal controls that prevent unwanted growth and division. All cells grow or divide at some point in their life, but this occurs in careful control to maintain the correct the structure and function of the body's tissues.

Skip to 2 minutes and 44 secondsCarcinogenic chemicals, such as aflatoxins, cause cancers by inducing mutations in DNA. A mutation is a change to the sequence of bases in DNA. This sequence of bases in DNA determines the protein produced by genes. And if it is altered by a mutation, it can lead to the production of a faulty protein or block the production of that protein altogether.

Skip to 3 minutes and 11 secondsWhen mutations occur in the number of key genes that are involved in controlling how cells grow and divide, cells can become cancer cells, and gradually multiply to form a tumour. The pathway to cancer, carcinogenicity.

Skip to 3 minutes and 28 secondsCarcinogenesis refers to the ability of a toxin to cause cancer. The first step is often the formation of DNA adducts that leads to mutation.

Skip to 3 minutes and 39 secondsAfter a person has been exposed to a carcinogen, that carcinogen, such as aflatoxin, will be much metabolised in the body. Some metabolites may be chemically reactive and damage DNA by forming adducts that can be mutagenic.

Skip to 3 minutes and 57 secondsAn accumulation of mutations in genes that control growth can lead to a tumour being formed, a process which often taken many years from the first exposure.

Skip to 4 minutes and 9 secondsDamage to the DNA is usually repaired, but if the exposure is at high levels and for a long time, then some damaged DNA will slip through the controls to lead to mutations. The higher the level of exposure, the more likely such mutations are to occur.

Skip to 4 minutes and 28 secondsChemical carcinogens occur as a variety of different structures, such as those shown here. These chemicals can all be found in certain foods. Aflatoxins can be found in mouldy peanuts or maize. Acrylamide can be found in fried or baked foods such as chips and biscuits. PhIP can be found in grilled or fried meat. Although they have different structures, they can all be metabolised in the body to a reactive intermediate that binds to DNA to form DNA adducts. If these adducts are not repaired, they can interfere with normal DNA replication leading to mistakes that are mutations in the DNA sequence. For example, the carcinogen benzopyrene, which is formed in burned food, is metabolised in the body.

Skip to 5 minutes and 20 secondsThe parent compounds does not react with DNA, but enzymes modify the structure to make it more soluble in water and so easier to excrete. The drawback is that for some chemicals, such as benzopyrene, aflatoxin, or PhIP. The metabolites produced are also more likely to react with DNA. In the last part of the figure, you can see the bulky benzopyrene adducts covalently bind to the exocyclic nitrogen of guanine.

Skip to 5 minutes and 52 secondsAlthough DNA adducts can be repaired, the more adducts that form, the more likely it is that some will give rise to mutations when the DNA is replicated.

Skip to 6 minutes and 6 secondsSo chemicals, such as aflatoxin, PhIP, acrylamide, or benzopyrene that can be present or be formed in certain food types pose a danger to humans who eat the food. Long term exposure to these chemicals increase the risk of cancers such as stomach cancer, liver, or colon cancer. Not all carcinogens act through the formation of DNA adducts. Fumonisin is another type of mycotoxin that interferes with lipid metabolism and which may also increase oxidative damage in cells.

Food contamination and the risk of cancer

What are carcinogens?

A carcinogen is a substance that can cause cancer. Carcinogens can cause cancer by affecting the structure of the DNA - causing DNA damage - or through changing the cell metabolism, accelerating the development of cancer.

This video looks at carcinogens in more detail.

Categories of Carcinogens

The World Health Organisation (WHO) and International Agency for Research on Cancer (IARC) have classifed carcinogens as follows:

  • Group 1: Carcinogenic to humans
  • Group 2A: Probably carcinogenic to humans
  • Group 2B: Possibly carcinogenic to humans
  • Group 3: Not classifiable as to its carcinogenicity to humans
  • Group 4: Probably not carcinogenic to humans

Definitions and examples

Group 1: Carcinogen to humans

Studies have put forward sufficient evidence to show that these agents, or toxins, are carcinogenic to both human and animals.

An example of group 1 is Aflatoxin B1.

Group 2A: Probably carcinogenic to humans

There is limited evidence that these agents are carcinogenic to humans but sufficient evidence in animal experiments.

An example of group 2A is Acrylamide.

Group 2B: Possibly carcinogenic to humans

There is limited evidence that these agents are carcinogenic to humans and less than sufficient evidence that they are carcinogenic to animals.

Example agents include Fumonisin B1 and Ochratoxin A.

Group 3: Not classifiable as to its carcinogenicity to humans

There is limited evidence that these agents are carcinogenic to humans or animals.

An example is Arsenobetaine.

Group 4: Probably not carcinogenic to humans

Evidence would suggest that these agents are not carcinogenic to humans.

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This video is from the free online course:

Tackling Global Food Safety

Queen's University Belfast