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Carcinogens are chemicals or other factors that can cause, promote or increase the risks of cancer. They are usually regarded as substances that damage DNA directly or through the formation of oxidising free radicals causing rearrangements and mutations of the genes. If this happens, the genes that cause cancer (Oncogenes), get separated from the genes that stop them activating (suppressor genes) and placed next to genes that stimulate them (promoter genes). The activation and promotion of these genes then trigger harmful biochemical pathways leading to cancer. 

Carcinogens, however, also have other complex direct and indirect modes of actions which affect the risk of cancer development and promotion. In summary, the mechanism of action of carcinogens in include:

  • Genetic damage

    • Via free radicals that directly damage DNA

    • Inhibit DNA repair

    • Alter expression of genes (epigenetics)

  • Reduce immunity

  • Promote chronic inflammation

  • Stimulating hormone sensitive cells

The International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to carcinogen exposures is about 15%. This number is likely to be higher as we are exposed to a substantial number of chemicals, on a daily basis, and for long periods of our lives and only about 50% of chemicals have undergone even minimal testing for carcinogenicity by the Environmental Protection Agency (EPA). We are exposed to a wide array of these potential dietary and environmental chemicals but many are completely safe, others are known to be definitely carcinogenic, others are have uncertain potential or are under investigation. IARC categorises them into five groups

  • Group 1: Carcinogenic to humans

  • Group 2a: Probably carcinogenic to humans

  • Group 2b: Possibly carcinogenic to humans

  • Group 3: Unclassifiable as to carcinogenicity risk

  • Group 4: Probably not carcinogenic to humans

The EPA, part of the American Food and Drug Administration (FDA)test for their ability to cause cancer using cultured cells in the laboratory and in animals looking for the degree of carcinogenicity, based on four criteria; gene mutation; clastogenicity; DNA damage and cell transformation. They then share their data with other countries in accord with the Cooperative Research and Development Agreement (CRADA).

Why do carcinogens matter after a cancer diagnosis?

Although patients with established cancer have already sustained the initial DNA damage in order to mutate from benign to malignant cells, the progress from an early cancer to one that has spread around the body can be fuelled by continued consumption of carcinogenic foods. Further DNA damage encourages the cancer to mutate into a more aggressive type or develop mechanism to hide from the body’s immunity or become resistant to medical treatments. Carcinogens can also have a direct effect on the biological processes that cause cancers to become more harmful via a number of mechanisms, which encourage cancer cells to:

  • Grow faster (proliferate)

  • Not die when they have reached the end of their life cycle (apoptosis),

  • Stimulate blood vessels to feed the rapidly growing cells (angiogenesis),

  • Loose their stickiness to the site of origin (loss of adhesion),

  • Grow into adjacent organs (invasion)

  • Spread through the body (metastasise).

Avoiding carcinogens after cancer is also a benefit by reducing the risk of developing further new cancers that are more likely in patients who have a greater susceptibility from a pre-existing genetic vulnerability or acquired genetic damage from chemotherapy or radiotherapy.  This hypothesis is supported by a study from Newfoundland, which evaluated the eating habits of  patients with colorectal cancer. They found that between the highest and lowest quartile of processed meat intake there was a two-fold increase in relapse and overall death rate. The carcinogens within processed meat particularly those smoked, fried or high-temperature cooked include N-nitroso compounds, heterocyclic amines and polycylic aromatic hydrocarbons and the mechanisms of harm are explained later. Other trials have demonstrated an association between dietary carcinogens and relapse  for ovarian cancer and breast cancer.

The effect of diet on individuals more susceptible to cancer was demonstrated by a study conducted by the Radiation Effects Research Foundation (RERF) involving people surviving the Hiroshima and Nagasaki bombings. They showed that those who undertook regular exercise and had a higher intake of fruit and vegetables and had a low meat intake had a significantly lower risk of cancers despite their acquired susceptibility. Furthermore, a study evaluating the eating habits of people with the genetic susceptibility Lynch syndrome showed that diets that featured a major unhealthy snack component – chips, fast food and sweets – had an increased risk of developing colorectal adenomas. Likewise, lifestyle habits were significant factors in the development of breast disease in women presenting at a young age who had strong family histories.

How much of a carcinogen exposure is harmful?

It is impossible to live a life free of carcinogens, which is also unnecessary as the body’s natural anti-oxidant defences are able to cope with a certain amount. It’s only when excess levels are consumed over long periods of time that these defences can be overloaded. The harmful effect of carcinogens, very much depends on the amount taken over time and whether they are consumed with other carcinogens (total carcinogenic load) or combined with natural anticancer phytochemicals. A good example is coffee, which contains moderate levels of acrylamide (a probable carcinogen) but is also loaded with healthy polyphenols – the net result is that studies show that regular coffee intake is associated with a lower risk of cancer. Likewise, a study from Maryland, USA eloquently demonstrated that levels of the carcinogen in barbequed meat known as polycyclic hydrocarbons is significantly lower if it had been marinated in polyphenol-rich rosemary and eaten with a salad.

So the risk of a carcinogen depends on other dietary or environmental factors which may enhance or negate and the underlying susceptibility of the individual exposed. Nevertheless, for ease of explanation, this section tries to quantify levels and provided advice on how to avoid them within separate categories separately although of course there are considerable crossover and synergy effects between them.

Natural antidotes to carcinogens: As well as avoiding carcinogens it is important to concentrate on lifestyle strategies which prevent cancer either on their own or by acting as antidotes to carcinogens. These are addressed in their own sections and include regular exercise; having a healthy profile of gut bacteria, adopting an anti-inflammatory lifestyle; ensuring good levels of vitamins and minerals and ensuring a high intake of polyphenol rich foods.

Categorisation of carcinogens

Lifestyle factors such as smoking, sedentary behaviour, alcohol, obesity, sun burning exposure and processed sugar intake although increasing the risk of cancer are not usually classified as carcinogens and are described separately in other lifestyle habit sections.

  • Dietary carcinogens (acrylamides, nitrosamines and polycyclic hydrocarbons) - read more

  • Pesticides and herbicides - read more

  • Infective agents - read more

  • Naturally occurring exposures (ultraviolet light, cosmic radiation and radon gas) - read more

  • Medical treatments (radiation, chemotherapy, hormone drugs, immune suppressants) - read more

  • Electromagnetic waves - read more

  • Industrial and environmental exposures - read more

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