Carcinogens are any of a number of agents that can cause cancer, including chemicals, radiation, and viruses. Exposure to such agents, singly or in combination, can initiate cancer under conditions not wholly understood. When an interaction between a chemical carcinogen and DNA results in a mutation, the chemical is said to be a mutagen. Mutagens are also called genotoxins. Most known tumor initiators are mutagens.
Sources of Carcinogens
- Natural and polluted human environments
- Working conditions
- Tobacco smoking or chewing and the consumption of alcohol
- Intake of cytostatic agents in the course of chemotherapy (drugs designed to eliminate cancer cells can also generate lesions in normal cells)
According to rough estimation in a random population of people with cancer, one-third of all cancers are associated with tobacco smoking and one-third with different dietary factors.
The link between diet and cancer is rather complex. The majority of food products are neutral and do not have
carcinogenic properties. Obviously, an abundance of cancer causing food ingredients is as harmful as a lack of anti-cancer elements in the diet. However, such categorization is only true regarding a single chemical component, while food products are all complex mixtures.
Let us consider coffee, soy sauce, and red wine as examples:
Caffeine present in coffee beans belongs to a group of so-called oxypurines, capable of inhibiting the repair of damaged DNA. Methylglyoxal, another compound found in coffee beans, is the main mutagen in coffee. On the other hand, it has been shown that an extract from coffee beans exhibits antimutagenic activity against DNA damage.5
Red wine contains up to 18% of ethanol. Ethanol by itself is not mutagenic, but it can act as a cocarcinogen, increasing the toxicity of other substances. In addition, acetaldehyde, the first metabolite of ethanol, has the ability to interact with DNA.
Firthermore, many brands of wine and beer have been shown to contain traces of heterocyclic amines (HA) tipically present in temperature-processed meat and fish and known to be strong mutagens. On the other hand, some of the phenol and alcohol compounds present in red wine have been shown to reduce the DNA damage caused by oxidation and radiation. These well-documented protective properties along with the acknowledged beneficial influence on digestion are the reason why a moderate consumption of red wine is recommended by nuritionists.4
A wide range or carcinogens occur in food and they have been categorized in groups according to their origin:
Natural Food Carcinogens
Natural food carcinogens include genotoxins occurring in plants. Their role in nature is to protect plants against fungi, insects, or animals. Examples of such genotoxins are hydrazines (present in mushrooms),
safrole (spices in root beer), estragole (dried basil), and psoralens (celery).
The term pollution covers all types of contamination
emitted to the ground, surface waters, and atmosphere that could be further absorbed by plants, fish, and farm animals. The sources of pollution include industrial waste, diesel exhaust (mostly in the vicinity of roads), and pesticide residues in food products. An example of the effect of environmental pollution on nutrition is contamination of fish by polycyclic aromatic hydrocarbons. Fish living in a polluted water reservoir (in the vicinity of a oil refinery) use the contaminated water to rinse their gills; this results in the deposition of PAHs in the fish body.
Another example is contamination of foodstuffs by heavy metals. Although some metals are essential for human nutrition, others, including arsenic, cadmium, chromium, nickel, and lead, have been found to pose a potential carcinogenic threat to humans. Arsenic (its main dietary source is fish) and cadmium appear to be the most harmful.4
Food Storage and Conservation
Food storage and conservation as a source of carcinogens usually reflects improper storage conditions resulting in the occurence of mycotoxins such as aflatoxins, which are known to be liver carcinogens. On the other hand, a number of chemicals are added to food to extend its storage period. Also many of the are recognized as having carcinogenic properties, they are in common use. Example of additives with potentially carcinogenic properties include ethanol, saccharin, caramel, AF-2 (furylfuramide), nitrate (III), and nitrate (V). The latter two are used as coloring and preservation agents in meat products.
The compounds most recently causing cancers were three heterocyclic amines present in cooked meat. One of these compounds, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) has proven to be one of the most potent hepatocarcinogens in the history of the monkey project, inducing malignant liver tumors in 65% of animals over a 7-year period of exposure.2
All except two of the N-nitroso compounds were carcinogenic. Diethylnitrosamine, a compound commonly found in cured meats, was the most potent and predictable liver and nasal cavity carcinogen in studied monkeys. Procarbazine (MIH), an alkylating agents used to treat some kinds of cancer, caused acute nonlymphocytic leukemia in most of the cases.
Food can be processed in many ways, but the most frequently used is application of high temeperature (cooking). The main groups of food carcinogens resulting from high-temperature processing are polycyclic aromatic hydrocarbons (PAH), heterocyclic aromatic amines (HA), and N-nitrosamines (NOC).
Polycyclic Aromatic Hydrocarbons (PAH) are products of the incomplete combustion of organic matter and therefore occur commonly in temperature-processed foodstuffs. Direct contact with flames (grill, barbecue, broiling) drastically increases PAH content, while boiling in water appears to generate the smallest amount of PAH. PAHs are also found in smoked fish and meat. PAHs are considered to be the most potent carcinogens for humans, which require metabolic activation for becoming toxic.4
In a study covering a 32-year period of cancer incidence research in nonhuman primates, which was initiated by the National Cancer Institute in 1961, a large number of substances including a variety of food additives, food components, environmental contaminants, N-nitroso compounds, "classical" rodent carcinogens, and immunosuppressive agents have been evaluated for long-term carcinogenic activity.
The fungal food contaminants, aflatoxin B1 (AFB1) and sterigmatocystin (SMT), were found to be potent liver carcinogens. AFB1 also induced adenocarcinomas of the pancreas, osteosarcomas, and other tumors. Also, the aglycone of cycasin, MAM acetate, induced a variety of tumors, but primarily liver and kidney carcinomas.
Safrole, estragole, methyleugenol, and related compounds are present in many edible plants and are carcinogenic in rodents. Several of their metabolites have mutagenic potential. Black pepper contains a small amount of safrole and closely related compound piperine. Extracts of black pepper produce tumors in mice at various sites.1
Psoralen derivatives are potent light-activated carcinogens and mutagens that are widespread in celery, parsnip, figs, and parsley. Psoralens are activated by sunlight and then damage DNA, induce tumors, and produce oxygen radicals.
Solanine and chaconine, which are present in potatoes, are strong neurotoxins and potential teratogens (cause birth defects).6 Quinones produce superoxide radicals, which oxidize fat in cell membranes and generate mutagens and carcinogens. Plants such as rhubarb contain mutagenic quinone.1
1. Risk and Our Use of Pesticides. Richard H. Falk. Department Biologie, University of Hamburg
2. Tumor incidence in a chemical carcinogenesis study of nonhuman primates.
Thorgeirsson UP, Dalgard DW, Reeves J, Adamson RH. In: Regul Toxicol Pharmacol. 1994 Apr;19(2):130-51.
3. Natural and Synthetic Chemicals in the Diet: A Critical Analysis of Possible Cancer Hazards. Lois Swirsky Gold, Thomas H. Slone, and Bruce N. Ames. In: Food Safety and Food Quality. Issues in Environmental Science and Technology 15R. E. Hester & R. M. Harrison, eds.Cambridge, UK : Royal Society of Chemistry, pp. 95-128 (2001)
4. Carcinogenic and anticarcinogenic food components. Wanda Baer-Dubowska, Agnieszka Bartoszek, Danuta Malejka-Giganti
5. Caffeine inhibits gene-specific repair of UV-induced DNA damage in hamster cells and in human xeroderma pigmentosum group C cells. Charles J. Link, Jr, Michele K. Evans, John A. Cook, Rebecca Muldoon, Tinna Stevnsner and Vilhelm A. Bohr. In: Oxford University Press (1995)
6. Molecular mechanisms of toxicity of important food-borne phytotoxins.
Ivonne M. C. M. Rietjens, Martijn J. Martena, Marelle G. Boersma, Wim Spiegelenberg, Gerrit M. Alink (Review) Molecular Nutrition & Food Research
Volume 49 Issue 2, Pages 131 - 158