How carcinogens are studied and identified
Chart: Cai Huawei
The International Agency for Research on Cancer has attracted widespread attention after it classified processed meat products and red meat as Class 1 and Class 2A carcinogens respectively. Meat associations and meat processing companies in countries and regions such as South Korea, North America and China do not agree with the above conclusions.
In response, the WHO representative office in China responded on the afternoon of the 30th that the risk of carcinogenicity of processed meat is actually a reaffirmation of the report made in 2002. The original intention is not to make people stop eating meat, but to remind people to eat meat reasonably and reduce possible dietary risks. It also emphasized that the results of the International Agency for Research on Cancer's investigation were not affected by non-scientific factors, and the conclusions drawn were only findings, not recommendations.
In an interview with our reporter, the International Agency for Research on Cancer emphasized that people have misunderstandings about the research conclusions. Their research only reflects whether a substance is carcinogenic, not the size of the substance's carcinogenic risk or carcinogenic ability. The fact that processed meat products are classified as Class 1 carcinogens only indicates that "there is sufficient medical evidence and animal experiments to prove that it is carcinogenic." In fact, the risk or ability of meat to cause cancer is very low, and it is enough to control the intake.
Experts of the IARC project team reviewed academic research results in multiple fields and identified them in combination with epidemiology
The IARC is a subsidiary of the World Health Organization. It is composed of experts from multiple disciplines and combines epidemiology, laboratory science and biostatistics to identify the causes of cancer.
The IARC monograph project team is responsible for the research on carcinogens. It is this project team that listed red meat and processed meat products as carcinogens. Their task is to identify which environmental factors can cause cancer.
Kurt Streif, head of the IARC monograph project, introduced that since the establishment of the project in 1971, experts from various countries have identified nearly a thousand environmental factors including chemical agents, occupational habits, organisms, and lifestyles to determine the carcinogenicity classification. In the study, experts mainly reviewed the research results in four fields: first, studying the environment that people are exposed to that may contain carcinogenic factors; second, conducting epidemiological studies on cancer patients who have been exposed to the factor; third, exposing animals to the factor in the laboratory to see if it will induce cancer; and fourth, studying the development of cancer under the influence of the factor.
For example, in this meat study, "our study is based on published academic papers, so it is not a new study, but a conclusion drawn from all the research results." Kurt Streif said that in the past year, the project team reviewed all the published academic papers on the carcinogenicity of red meat and processed meat products, and identified 22 experts after repeated discussions and contacts with experts from all parties. They drafted documents based on existing scientific evidence, and then held an 8-day seminar in Lyon, France, to finally determine the carcinogenicity of red meat and processed meat products.
"We reviewed more than 800 studies in total to find the relationship between eating red meat and processed meat products and cancer, and determine the final classification. Among them, it was found that there was a clear correlation between eating processed meat products and suffering from colorectal cancer. More importantly, the results obtained by various studies were consistent. Therefore, the expert group believes that eating processed meat products can cause cancer." Kurt Streif said.
Among the more than 800 studies reviewed this time, the European Prospective Investigation into Cancer and Nutrition was included. The survey involved 10 European countries, and more than 500,000 respondents were followed for 15 years. In addition, the expert group also reviewed research data and observations from all continents and conducted a rigorous case-control analysis.
"For example, to study the relationship between smoking and lung cancer, we selected smokers and non-smokers respectively, observed them for several years, and calculated the probability of cancer from smoking based on the number of people with lung cancer. We also used the same method in this study of the carcinogenicity of red meat and processed meat products." Kurt Streif said.
The classification is based on whether there is sufficient evidence for the carcinogenicity of the substance. The more sufficient the evidence of carcinogenicity, the higher the level. It is not a classification of the carcinogenicity or carcinogenic risk of the substance
Currently, in the classification of carcinogens, the only one that is considered to be non-carcinogenic to humans is caprolactam (the main use is to generate polyamide slices through polymerization, which can be processed into nylon fibers, engineering plastics, plastic films, etc.), which is classified as Level 4. According to reports, the classification of caprolactam is mainly based on white mouse experiments, and the experiment found that contact with caprolactam does not cause lesions.
The reason why arsenic is classified as a Class 1 carcinogen is that there is enough medical evidence and animal experimental results to prove its carcinogenicity. Since people may be exposed to arsenic in industrial exhaust gases, automobile exhaust, water, and soil, there are observations and studies on long-term intake of arsenic by people all over the world. In the late 1950s, in the southwestern coastal areas of Taiwan, local people drank "arsenic water" for a long time and developed "black foot disease". The research report of Chinese scholars on this issue became one of the important evidences for the study of the carcinogenicity of arsenic. Cohort studies and case analyses in northern Chile, the Argentine city of Cordoba, India and Bangladesh have found that long-term exposure to arsenic may cause lung cancer, skin cancer, kidney cancer and other problems. White mice that drank water containing arsenic for two years were also observed to have cancer and the tumors grew significantly. Therefore, the project team classified the carcinogenicity of arsenic as Class 1.
The research results are evidence-based and scientifically credible, but the classification of carcinogens is only one aspect of the research
"Bacon, ham, and sausage are all delicacies that have been passed down for thousands of years. It is really incredible that they suddenly become "carcinogens". But with the development of science, it is normal for some unknown risks in the past to be revealed." said Zhong Kai, an associate researcher at the National Food Safety Risk Assessment Center.
"There have always been different voices on the research and classification of carcinogens. Carcinogens are not simply black and white, and it is normal to have doubts." Chen Wanqing, deputy director of the China Cancer Prevention and Control Office, said, "The International Agency for Research on Cancer is the most authoritative cancer research institution in the world, and its research results are evidence-based and credible."
But Chen Wanqing also believes that the classification method of carcinogens mainly considers single factors, while cancer is the result of the combined action of multiple factors. At present, the medical community has not figured out the cause of cancer, and the classification of carcinogens is only one aspect of the research. Regarding the meat identification results, Chen Wanqing said that although we should try not to eat too much meat, it would be too absolute to only eat vegetarian food. Amino acids and proteins should be obtained from animal proteins. A balanced diet and a healthy lifestyle are the basis of health.