Evidence continues to mount that red meat, and especially processed meat cause cancer. A new study, published online on October 26 in the journal The Lancet adds to that evidence. The France-based International Agency for Research on Cancer (IARC), a component of the World Health Organization (WHO), placed processed meat such as like hot dogs, ham, and bacon in its group 1 list, which already includes tobacco, asbestos, and diesel fumes, for which there is “sufficient evidence” of cancer links.
In October, 2015, 22 researchers from ten nations met at the IARC in Lyon, France to evaluate the carcinogenicity of the consumption of red meat and processed meat. The authors defined red meat as unprocessed mammalian muscle meat (e.g., beef, veal, pork, lamb, mutton, horse, or goat meat, including minced or frozen meat. Processed meat refers to meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavor or improve preservation. Most processed meats contain pork or beef, but might also contain other red meats, such as chicken, liver, or meat byproducts such as blood.
The authors explain that red meat contains high biological-value proteins and important micronutrients such as B vitamins, iron, and zinc. The fat content of red meat varies depending on animal species, age, sex, breed, and feed, as well as the cut of the meat. Meat processing, such as curing and smoking, can result in formation of carcinogenic chemicals, including N-nitroso-compounds (NOC) and polycyclic aromatic hydrocarbons (PAH). Cooking improves the digestibility and palatability of meat; however, it can also produce known or suspected carcinogens, including heterocyclic aromatic amines (HAA) and PAH. High-temperature cooking by pan-frying, grilling, or barbecuing generally produces the highest amounts of these chemicals.
Depending on the nation, the proportion of the population that consumes red meat varies worldwide from less than 5% to up to 100%, and from less than 2% to 65% for processed meat. The mean intake of red meat by those who consume it is about 50–100 g per person per day, with high consumption equaling more than 200 g per person per day. Less information is available on the consumption of processed meat.
For the study, the IARC Working Group evaluated more than 800 epidemiological studies that investigated the association of cancer with consumption of red meat or processed meat in many nations, from several continents, with diverse ethnicities and diets. For the assessment, the greatest weight was given to prospective (forward-looking) studies conducted in the general population. High quality population-based case-control studies (studies with a control group) provided additional evidence. For both designs, the studies judged to be most informative were those that considered red meat and processed meat separately, had quantitative dietary data obtained from validated questionnaires, a large sample size, and controlled for the major potential confounders (factors that could skew the data) for the cancer sites concerned.
The largest body of epidemiological evidence involved colorectal cancer. Data on the association of red meat consumption with colorectal cancer were available from 14 studies. Positive associations were seen with high versus low consumption of red meat in 50% of those studies, including a study from ten European nations spanning a wide range of meat consumption and other large studies in Sweden and Australia. Of the 15 informative case-control studies reviewed, seven reported positive associations of colorectal cancer with high versus low consumption of red meat. Positive associations of colorectal cancer with consumption of processed meat were reported in 12 of the 18 studies that provided relevant data, including studies in Europe, Japan, and the USA. Supporting evidence came from six of nine informative case-control studies. A meta-analysis of colorectal cancer in ten studies reported a statistically significant dose–response relationship, with a 17% increased risk per 100 grams (3.5 ounces) per day of red meat and an 18% increase per 50 grams (1.75 ounces) per day of processed meat.
Data were also available for more than 15 other types of cancer. Positive links were found between consumption of red meat and cancers of the pancreas and the prostate (primarily advanced prostate cancer), and between consumption of processed meat and stomach cancer. Based on the large amount of data and the consistent associations of colorectal cancer with consumption of processed meat across studies in different populations, which make chance, bias, and confounding factors unlikely as explanations, a majority of the Working Group concluded that there is sufficient evidence in humans for the carcinogenicity of the consumption of processed meat. Chance, bias, and confounding could not be ruled out with the same level of confidence for the data on red meat consumption, because no clear association was seen in several of the high quality studies, and residual confounding from other diet and lifestyle risk is difficult to exclude. Thus, the Working Group concluded that there is limited evidence in humans for the carcinogenicity of the consumption of red meat.
In addition, the researchers found that there was inadequate evidence in experimental animals for the carcinogenicity of consumption of red meat and of processed meat. However, in rats treated with colon cancer initiators and promoted with low calcium diets containing either red meat or processed meat, an increase in the occurrence of colonic precancerous lesions was reported in three studies.
Evidence for carcinogenicity was assessed as strong for red meat and moderate for processed meat. This evidence is primarily available for the digestive tract. A meta-analysis published in 2013 reported a modest but statistically significant association between consumption of red or processed meat and adenomas (precancerous lesions) of the colon and rectum that was consistent across studies. For genetic toxicity and oxidative stress, evidence was moderate for the consumption of red or processed meat.
Significant supporting evidence was available for multiple meat components (NOC, haem iron, and HAA). Consumption of red meat and processed meat by humans induces NOC formation in the colon. High red meat consumption (300 or 420 grams/day; 10.6 or 14.1 ounces/day) increased levels of DNA material derived from NOC in shed colon cells or rectal biopsies in two studies. Limited human data, especially from intervention studies, were available for processed meat. Haem iron stimulates the formation of NOC, and of lipid oxidation products in the digestive tract of humans and rodents. Meat smoked or cooked over a heated surface or open flame contains PAH. These chemicals cause DNA damage; however, only a small amount of direct evidence exists that this occurs following meat consumption.
Overall, the Working Group classified consumption of processed meat as “carcinogenic to humans” (Group 1) on the basis of sufficient evidence for colorectal cancer. In addition, a positive association with the consumption of processed meat was found for stomach cancer. The Working Group classified consumption of red meat as “probably carcinogenic to humans” (Group 2A). In making this evaluation, the group took into account all the relevant data, including the substantial epidemiological data showing a positive association between consumption of red meat and colorectal cancer. Consumption of red meat was also positively associated with pancreatic and with prostate cancer.