By Kamal Naser and Suhail Abdul Wahab-
Recently the President of Government Medical Officers Association (GMOA) claimed in electronic media that beef is a carcinogenic food, which means that it is a causative agent of cancer. (NewsWire, 2020) We as responsible medical professionals would like to analyse this statement in the light of science and give a balanced, neutral opinion on “What causes cancer.” In this article we will be presenting a broad overview of cancer-causing foods, food habits and other lifestyle factors backed by scientific evidence.
What is cancer?
Cancer is a frightening word for many of us. In fact, it is a generic term for a large group of diseases that can affect any part of the body. Causative factors and behaviour of different cancers can greatly vary. Cancer is a result of the interaction between a person’s genetic factors and categories of external agents: physical, chemical and biological carcinogens. Rapid proliferation of abnormal cells beyond their boundaries is the characteristic feature of any cancer. Transformation of normal cells into cancer cells occurs in multiple stages. (World Health Organisation, 2018)
Who is most affected by cancer?
Cancer is one of the leading cause of death globally and is second only to cardiovascular diseases. It was responsible for 9.6 million deaths in 2018. More than two thirds of these deaths occurred in low and middle-income countries. (World Health Organisation, 2018)
Five leading cancers in the world are lung, breast, colorectal, prostate and skin cancer. (World Health Organisation, 2018) In 2014 the commonest cancer in Sri Lanka was breast cancer followed by oral, thyroid, bowel (colorectal) and oesophageal (food pipe) cancer. (National Cancer Control Programme, 2014) (National Cancer Control Programme, 2014) Generally, the incidence of cancer increases with age. Different cancers affect males and females differently. For instance, the breast cancer affects females mostly whereas most sufferers of lung cancer are males. (National Cancer Control Programme, 2014) (World Health Organisation, 2014)
Environmental factors contribute for many types of cancers. For instance,, tobacco smoking is strongly implicated in lung cancer and betel chewing is associated with oral cancer. Certain chronic viral infections also can cause cancer. (World Health Organisation, 2018) Some cancers run in families indicating genetic susceptibility. Familial breast cancer and bowel cancer syndromes are good examples for this. (Cogliano, et al., 2011)
How scientific evidence is collected?
Scientific evidence are collected in many different ways. One of the methods is “epidemiological studies” which means observing disease patterns in different population and determining potential causes. Good example of an early epidemiological study is about chimney sweeping and scrotal cancer. Percival Pott (1714-88), a British surgeon observed that an alarming number of former chimney sweeps developed a skin cancer on their scrotal skin. His observation lead to the conclusion that the chimney soot was getting deposited in the scrotal skin which eventually lead to the cancer. (CHIMNEY-SWEEPS AND MALIGNANT DISEASE, 1890)
Another way of collecting evidence is using “animal studies.” If a substance is thought to be causing a disease, that substance will be introduced to animals. Those animals will be observed for emergence of the disease concerned.
When it comes to carcinogenic substances there are two notable organisations which carry out experiments to determine if a substance is carcinogenic. They are the International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP). IARC is a part of the World Health Organisation (WHO). IARC has tested more than thousand substances and classified them under one of the following categories:
1 Carcinogenic to humans
2A Probably carcinogenic to humans
2B Possibly carcinogenic to humans
3 Not classifiable as to its carcinogenicity to humans
Out of the 1020 substances tested by the IARC only 120 were classified as “Carcinogenic to humans” and nearly half (n=499) were in category 3. Although this broad classification is useful in identifying potential carcinogens, it falls short in determining how carcinogenic each substance is. It is understood from various research that cigarette is way more carcinogenic than processed meat although both are in the same IARC category. (International Agency for Research on Cancer, 2020)
Alcohol and cigarettes
Tobacco smoking has been implicated in 17 different cancers. In addition to that there is limited evidence of causing breast cancer and leukaemia in smoker’s children. Even second-hand smoking (that is, people who passively inhale the air contaminated with tobacco smoke) is a well-recognised cause of lung cancer. Tobacco chewing is known to cause oral, oesophageal and pancreatic cancer. Areca nut, commonly chewed with betel and tobacco induces oral lesions with high propensity to progress to cancer. (International Agency for Research on Cancer, 2019) Tobacco use is the single most important risk factor for cancer and is responsible more than one fifth of all cancer deaths. (World Health Organisation, 2018)
Consumption of alcoholic beverages is associated with breast (female), colorectal, laryngeal, liver, oesophageal, oral and pharyngeal cancers. (Cogliano, et al., 2011) (International Agency for Research on Cancer, 2019) According to WHO data, on average a Sri Lankan male (≥15 years of age) consumes 18.9 litres of alcohol and a female consumes 6.7 litres of alcohol per year. These figures have steadily increased over the past 40 years. (World Health Organisation, 2016)
Does red meat cause cancer?
Meat is an important source of protein, some vitamins (e.g. vitamin A and B) and minerals (e.g. zinc, iron) . Amino acids are organic compounds that combine to form proteins. They are the building blocks of life. Out of 28 amino acids found in humans, 9 cannot be synthesised in our body. They are known as essential amino acids. Meat is an important source of essential amino acids. Therefore, they should be provided with diet. (Rohrmann & Linseisen, 2016) (A.D.A.M Inc, 2020)
According to IARC, consumption of red meat is classified as “Probably carcinogenic to humans” Which means that unprocessed red meat is not a confirmed causative factor of cancer. Red meat refers to all types of mammalian muscle meat and this includes beef, lamb and pork. Chicken, turkey, duck, goose and rabbit do not fall under the category of red meat. The processed meat like; sausages, salami, ham and bacon are categorised as “carcinogenic to humans” (category 1), i.e., proven to cause cancer, but it is dependent on quantity eaten. Although there is no universally agreed definition for processed meat; it generally refers to any meat that has been transformed through salting, curing, fermentation, smoking or other processes to enhance flavour or to improve preservation. The only cancer which has strong evidence for increased incidence with high consumption of red or processed meat is bowel cancer. There is inconclusive data about its role in other cancers. (Cogliano, et al., 2011) (World Cancer Research Fund/American Institute of Cancer Research, 2018) (Zhao, Yin, & Zhao, 2017)
The other important factor to consider is the amount of red meat or processed meat consumed. It appears that higher amount of their intake is associated with cancer. On the other hand, people who consume large amount of red or processed meat tend to eat less amount of poultry, fish and vegetables. It is clearly shown that increased intake of fruits and vegetables protects against cancer. The World Cancer Research Fund recommends that red meat intake should be limited to <500 g/week and avoid processed meat. (Rohrmann & Linseisen, 2016) (World Cancer Research Fund/American Institute of Cancer Research, 2018) This figure could be different for Asians since genetic and environmental factors play a role in cancer. There is no clear data about safe limit of red meat for Sri Lankans.
There is conflicting reports regarding the role of fish and sea food consumption in cancer prevention and causation. (Azeem, et al., 2015) There is evidence of Cantonese-style salted fish (dry fish/ ‘Karawala’) causing nasopharyngeal cancer specially if consumed during young ages. Studies have indicated that hard dry fish is not associated with cancer whereas soft dry fish is associated with cancer. Both varieties are consumed widely in Sri Lanka. The reason for their distinct carcinogenic profiles is due to the difference in their preparation method. Soft dry fish is allowed to ferment and decompose before drying. During decomposition there are some chemicals generated in the dead fish which are carcinogenic to humans. (Barrett, et al., 2019) (Takachi, et al., 2010)
In summary, processed meat has been shown to be definitely a cancer-causing food while red meat is categorised as potentially carcinogenic, but only if consumed in large quantities. The soft dry fish is also proven to be definitely cancer-causing, however quantity and duration of consumption are important factors.
Dairy products, sugar, fats, and cancer risk
Adequate consumption of milk, cheese and other dairy products are associated with decreased risk of bowel cancer. Likewise, they may have a protective role in breast cancer as well. There seems to be an increased risk of prostate cancer despite lacking evidence. (World Cancer Research Fund/American Institute of Cancer Research, 2018)
Preserved foods, saturated/animal fats, cholesterol, high sugar foods, spicy foods and refined carbohydrates have a positive association with bowel cancer. Vegetables and fruits play a protective role against bowel cancer. (Azeem, et al., 2015)
Sugar and sugar sweetened beverages promote weight gain. Obesity is associated with certain types of cancers including bowel and breast cancer. Therefore, free sugars should constitute no more than 5% of our calorie intake and consumption of sugar sweetened beverages should be minimised. (Albert, 2019)
Trans-fat is an important cancer-causing agent. Many routinely consumed food items have high concentration of trans-fat. When unsaturated oil or fat such as sunflower oil, corn oil or margarine is used for deep frying or baking, it leads to formation of transfat. Re-using oil for frying increases transfat concentrations. Frying with coconut oil is less likely to produce trans-fat, but when it is re-used, the production of transfat will increase. Many fast foods, short eats, biscuits and bakery items are sources of trans fat. Excessive transfats in the diet is a significant risk factor for cancer, cardiovascular disease and diabetes. (Islam, et al., 2019) In contrast, polyunsaturated fatty acid (PUFA) consumption was found to be inversely associated with cancer relate death. (DD, et al., 2016)
Other factors (Obesity, diabetes and sedentary lifestyle and cancer)
Routine physical activity is linked to reduced incidence of many types of common cancers. Additionally, it reduces the risk of death among patients with breast and bowel cancer. (Lugo, et al., 2019)Long term, uncontrolled diabetes is a risk factor for cancers. Obesity is also proven to be a risk factor for cancer.
Carcinogenicity of a substance is determined according to currently available data. With emergence of new information this classification is revised periodically. Therefore, a substance from one group may be escalated or de-escalated to another group. Carcinogens do not necessarily cause cancer in every person, every time a person is exposed to it. Some may only be carcinogenic if a person is exposed in a certain way (e.g. drinking alcohol as opposed to touching it). Some may only cause cancer in people who have a certain genetic makeup. Some of these agents may lead to cancer after only a small exposure, while others might require intense exposure over many years. Even if a substance or exposure is known or suspected to cause cancer, this does not necessarily mean that it can or should be avoided at all costs. For example, sunlight is a major source of ultraviolet (UV) rays, which are a known to cause skin cancer. Meanwhile UV rays are essential for synthesis of vitamin D in our skin. Therefore, it is neither practical nor advisable to completely avoid the sun. (American Cancer Society, 2019)
Although tobacco, diet and infectious agents are responsible for a substantial fraction of cancers at some sites, it is also likely that many human carcinogens remain to be identified. This is suggested by the continuing identification of carcinogenic agents throughout the 40-year history of the IARC. (Cogliano, et al., 2011)
The causal relationship with processed meat or red meat and bowel cancer should be understood in the correct context. Moreover, there are many other foods and lifestyle factors which can lead to cancer. Therefore, it is important to strike a balance between necessity and harm when making food choices and following different lifestyles. Making healthy lifestyle changes is the most effective way to reduce cancer risk. These include regular exercise, eating a healthy diet, avoiding smoking, and not drinking too much alcohol.
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Authors: Dr Kamal Naser, MBBS (Jaffna), MD (Colombo), FRCP(Lond), FRCP(Glasg), FACE – Consultant Physician, Sherwood Forest NHS Hospitals Trust, UK
Dr Suhail Abdul Wahab MBBS (Peradeniya), MRCP(UK) – Sherwood Forest NHS Hospitals Trust, UK