explainer — Blog — Penn Science Policy and Diplomacy Group

by Neha Pancholi

Note: Here at the PSPG blog, we like to feature writing from anyone in the Penn community interested in the science policy process or science for general interest. This is the 1st in a series of posts from new authors. Interested is writing for the blog? Contact us!

The daily meat consumption in the United States exceeds that of almost every other country¹. While the majority of meat consumed in the United States is red meat², the consumption of certain red meats has decreased over the past few decades due to associated health concerns, such as heart disease and diabetes^1,2. In October, the World Health Organization (WHO) highlighted another potential health concern for red meat: cancer.

The announcement concerned both red and processed meat. Red meat is defined as unprocessed muscle meat from mammals, such as beef and pork³. Processed meat– generally red meat –has been altered to improve flavor through processes such as curing or smoking³. Examples of processed meat include bacon and sausage. The WHO confirmed that processed meat causes cancer and that red meat probably causes cancer. Given the prevalence of meat in the American diet, it was not surprising that the announcement dominated headlines and social media. So how exactly did the WHO decide that processed meat causes cancer?

The announcement by the WHO followed a report from the International Agency for Research on Cancer (IARC), which is responsible for identifying and assessing suspected causes of cancer. The IARC evaluates the typical level of exposure to a suspected agent, results from existing studies, and the mechanism by which the agent could cause cancer.

After a review of existing literature, the IARC classifies the strength of scientific evidence linking the suspected cancer-causing agent to cancer. Importantly, the IARC determines only whether there is sufficient evidence that something can cause cancer. The IARC does not evaluate risk, meaning that it does not evaluate how carcinogenic something is. The IARC classifies the suspected carcinogen into one of the following categories⁴:

Group 1 – There is convincing evidence linking the agent to cancer in humans. The agent is deemed carcinogenic.
Group 2A – There is sufficient evidence of cancer in animal models, and there is a positive association observed in humans. However, the evidence in humans does not exclude the possibility of bias, chance, or confounding variables. The agent is deemed as a probable carcinogen.
Group 2B – There is a positive association in humans, but the possibility of bias, chance, or confounding variables cannot be excluded. There is inadequate evidence in animal models.
This category is also used when there is sufficient evidence of cancer in animal models, but there is not an association observed in humans. The agent is a possible carcinogen.
Group 3 – There is inadequate evidence in humans and animals. The agent cannot be classified as carcinogenic or not carcinogenic.
Group 4 – There is sufficient evidence to conclude that the agent is not carcinogenic in humans or in animals.

The IARC reviewed over 800 studies that examined the correlation between consumption of processed or red meat and cancer occurrence in humans. These types of studies, which examine patterns of disease in different populations, are called epidemiological studies. The studies included observations from all over the world and included diverse ethnicities and diets. The greatest weight was given to studies that followed the same group of people over time and had an appropriate control group. Most of the available data examined the association between meat consumption and colorectal cancer, but some studies also assessed the effect on stomach, pancreatic, and prostate cancer. The majority of studies showed a higher occurrence of colorectal cancer in people whose diets included high consumption of red or processed meat compared to those who have low consumption. By comparing results from several studies, the IARC determined that for every 100 grams of red meat consumed per day, there is a 17% increase in cancer occurrence. For every 50 grams of processed meat eaten per day, there is an 18% increase.The average red meat consumption for those who eat it is 50-100 grams per day.³

The IARC also reviewed studies that examined how meat could cause cancer. They found strong evidence that consumption of red or processed meat leads to the formation of known carcinogens called N-nitroso compounds in the colon. It is also known that cooked meat contains two types of compounds that are known to damage DNA, which can lead to cancer. However, there is not a direct link between eating meat containing these compounds and DNA damage in the body.³

Based on the strong evidence demonstrating a positive association with consumption of processed meat and colorectal cancer, the IARC classified processed meat as a Group 1 agent³. This means that there is sufficient evidence that consumption of processed meat causes cancer.

There was a positive association between consumption of red meat and colorectal cancer in several epidemiological studies. However, the possibility of chance or bias could not be excluded from these studies. Furthermore, the best-designed epidemiological studies did not show any association between red meat consumption and cancer. Despite the limited epidemiological evidence, there was strong mechanistic evidence demonstrating that red meat consumption results in the production of known carcinogens in the colon. Therefore, red meat was classified as a probable carcinogen (Group 2A)³.

It will be interesting to see how the WHO announcement affects red meat consumption in the United States and worldwide. But before swearing off processed and red meat forever, there are a few things to consider.

First, it is important to bear in mind that agents classified within the same group have varying carcinogenic potential. Processed meat was classified as a Group 1 agent, which is the same classification for tobacco smoke. However, estimates by the Global Burden of Disease Project attribute approximately 34,000 cancer deaths per year to consumption of processed meat⁵. In contrast, one million cancer deaths per year are due to tobacco smoke⁵. While the evidence linking processed meat to cancer is strong, the risk of cancer due to processed meat consumption appears to be much lower than other known carcinogens. Second, the IARC did not evaluate studies that compared vegetarian or poultry diets to red meat consumption⁵. Therefore, it is unknown whether vegetarian or poultry diets are associated with fewer cases of cancer. Finally, red meat is high in protein, iron, zinc, and vitamin B12³. Thus, while high red meat consumption is associated with some diseases, there are also several health benefits of consuming red meat in moderation. Ultimately, it will be important to balance the risks and benefits of processed and red meat consumption.

¹http://www.npr.org/sections/thesalt/2012/06/27/155527365/visualizing-a-nation-of-meat-eaters

²http://www.usda.gov/factbook/chapter2.pdf

³Bouvard et al. Carcinogenicity of consumption of red and processed meat. The Lancet Oncology, 2015. 16(16): 1599-1600.

⁴http://www.iarc.fr/en/media-centre/iarcnews/pdf/Monographs-Q&A.pdf

⁵http://www.who.int/features/qa/cancer-red-meat/en/