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Meat Consumption and Cancer Risk

The consumption of meat has been associated with cancer risk, especially with an increased risk of colorectal cancer. There seems to be a consensus in the recognition that processed meats (and other processed and ultra-processed foods) increase the risk of cancer, and the same happens with some food preservation and cooking methods, such as the use of nitrites and high temperature cooking.

However, there has also been some controversy regarding what type of meats are associated with increased cancer risk and if animal raising practices and preparation methods make a difference. Another important point is that most studies conducted towards evaluating the effect of meat consumption and cancer are epidemiological, with evident limitations in terms of identifying and separating the effect of different variables that may play a role in cancer development.

In the light of these difficulties, we have tried to analyze the most recent findings and recommendations, along with the most up to date research, to give you some clarity regarding meat consumption, especially in what concerns cancer prevention and treatment.

Let’s start by analyzing 2 important Expert Reports

1.  International Agency for Research on Cancer (IARC) – Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 114 – Red Meat and Processed Meat – 20181, 2

2.  American Institute for Cancer Research (AICR) in collaboration with the World Cancer Research Fund International (WCRF) – Diet, Nutrition, Physical Activity and Cancer: a Global Perspective (Third Expert Report) – 20183

Both these reports gathered a multidisciplinary panel of independent experts and specialists that analyzed and reviewed hundreds of scientific studies released within the past decades related to cancer risk. For example, the IARC Working Group only, considered more than 800 different studies on cancer in humans, in which a total of more than 700 epidemiological studies provided data on red meat and more than 400 epidemiological studies provided data on processed meat. The AICR Report was produced over a ten year period from a massive database of scientific literature, providing evidence on how lifestyle factors such as diet, nutrition, body weight, and physical activity can impact cancer risk and survivorship.

Considering the amount of research and data analyzed by these reports, we consider them to be a good source of information, therefore we are going to address our topic on meat consumption having them as base.

Question: Is red meat and processed meat consumption associated with increased cancer risk?

According to both IARC and AICR, the answer is YES.

IARC Monographs on Carcinogenic Classification (2018)

  • Processed meat – Carcinogenic to humans – Group 1 (same group as smoking and alcohol)

  • Red meat – Probably carcinogenic to humans – Group 2A (same group as night shift work)

This classification of carcinogenic effects of meat consumption was based on a compilation of epidemiology data and mechanistic evidence from animal and human studies.

Fresh white meat, such as chicken and fish, was not considered to be linked with an increased risk of cancer.

What Type of Cancer?

Both working groups considered the consumption of processed meat and red meat to be associated mainly with an increased risk of colorectal cancer. They also found a limited association for pancreatic cancer and prostate cancer with red meat and gastric cancer with processed meat.

What is Processed Meat?

Processed meat: meat that has been transformed through one or several of the following processes: salting, curing, fermentation, smoking, or other processes to enhance flavor or improve preservation, or addition of chemical preservatives.

Processed Meats: sausage, bacon, deli meats, hot dogs, ham, pepperoni, salami, corned beef.

What is Red Meat

Red meat: fresh unprocessed mammalian muscle meat which may be minced or frozen and is usually consumed cooked.

Red Meats: beef, veal, pork, lamb, mutton, horse, or goat meat.

Why do Processed Meats Increase Cancer Risk?

A possible explanation is that many of the chemicals formed during processing or cooking of these meats may form DNA adducts, some of which can induce mutations and initiate carcinogenesis. Both IACR and AICR outline some possible mechanisms in which chemicals present mostly in processed meat may induce DNA damage and lead to carcinogenicity, including:

  • Nitrates/Nitrites: These are added to processed meats to preserve color and prevent spoilage. Curing meats with nitrite can produce carcinogenic N-nitroso compounds (NOCs).
  • Smoking: Smoking meat produces polycyclic aromatic hydrocarbons (PAHs), substances that are formed at high-heat and considered carcinogenic.
  • Cooking at high temperatures: Meats cooked at high temperatures (like grilling or barbequing) can also contain PAHs and heterocyclic amines (HCAs), which can damage DNA.
  • Heme iron: Ingestion of heme iron found mostly in red meat can catalyze the formation of NOCs and lipid peroxidation products (LPOs) in the digestive tract, which may damage the lining of the colon.

What about Red Meat?

There have been many theories trying to explain the association between red meat consumption and disease risk. Some agents have been linked with red meat, such as saturated fat, high salt intake, Trimethylamine-N-oxide (TMAO) generation by microbiota and accumulation of environmental pollutants and hormone disruptors in meat. However, these are not exclusive to red meat and the same happens with some of the mechanisms mentioned above, such as processing and cooking of meats (which can be associated with any type of food). Since many of the proposed theories are not specific for red meat (even heme iron is found in other animal foods, such as poultry, seafood and fish), there are likely to be multiple mechanisms operating simultaneously for risk causation4.

More recently, an international team of researchers identified a direct molecular link between meat and dairy consumption and the development of antibodies in the blood that increase the chances of developing cancer. Antibodies against the red meat-derived carbohydrate N-glycolylneuraminic acid (Neu5Gc) exacerbate cancer in “human-like” mice. Human anti-Neu5Gc IgG and red meat have been both independently proposed to increase cancer risk. In this study published in September 2020, researchers found a significant correlation between high consumption of Neu5Gc from red meat and cheeses and increased development of antibodies that heighten the risk of cancer. This connection may explain the high incidence of cancer among those who consume large amounts of dairy products and red meat5.

  • The non-human sialic acid present in red meat, N-glycolylneuraminic acid (Neu5Gc), may become incorporated in the cell membrane, triggering the immune response with associated inflammation and reactive oxygen species, which can contribute to DNA damage, tumor promotion, and cancer5,6.

Is the Gut Microbiome (which is highly determined by dietary patterns) Important?

Most definitely.

The role of gut microbiota is of major importance since these live microbial populations play a critical role in immune function and homeostasis. Their ability to metabolize endogenous and xenobiotic chemicals, including the bioactivation and detoxification of carcinogens, justifies further study of their importance in cancer development, especially in what relates to colorectal cancer6.

Conclusion

The conclusion is that pro-oxidants in cooked red meat and processed meats, including ingested fats and lipid peroxidation products, heme and Neu5Gc, and chemicals added to meats, may contribute to inflammation and tumor promotion, leading to the development of cancer, namely colorectal cancer1-6. The role of the microbiome in cancer seems to be of high significance, and further investigation a priority. However, this is a huge topic in and of itself, and we will leave its discussion for another post.

How Much Meat is Too Much?

The report by AICR/WCRF found that eating even small amounts of cold cuts or other processed meats on a regular basis increases the risk of colorectal cancer. Experts recommend avoiding processed meats, as the risk rises with increased consumption3.

The IARC working group concluded that for every 50 grams of processed meat or 100 grams of red meat eaten, the relative risk of colon cancer was increased by about 18% compared to those individuals who ate the least meats1-2(Note that relative risk compares the risk of a particular outcome in two different groups and is considered a descriptive statistic, not an inferential statistic, meaning it does not determine statistical significance).

In a 2018 follow up of the IARC report, the author explains that this relative risk can be considered modest when compared to the relative risk of developing lung cancer from smoking cigarettes (risk between 1000–3000%!!!), however the author also highlights that exposure to genotoxicants in the diet, such as those present in processed meats, should be avoided6.

Is Red Meat Bad?

Many studies also show that red meat is a nutritious food and may be an important source of protein with all essential amino acids. Rich in B vitamins such as B6, B12, niacin, and thiamine and one of the richest sources of minerals such as iron or zinc and phosphorus. Red meat is also a good source of long‐chain omega‐3 polyunsaturated fats, riboflavin, pantothenic acid and selenium. It is also a source of a range of endogenous antioxidants and other bioactive substances, including taurine, carnitine, carnosine, ubiquinone, glutathione and creatine7.

How to Reduce Risk

When the intake of animal foods is advised, we recommend wild caught fish and organic eggs from free range chickens.  We do not recommend the consumption of any other animal products.

However, if you do eat meat, there are healthier meat products, and you should buy organic pasture raised, grass fed and finished meats. Avoid the consumption of all processed meats, and especially those treated with nitrite. Do not overcook or charr the meat (or any foods). Cooking at high temperatures or with the food in direct contact with a flame or a hot surface, as in barbecuing or pan-frying, produces more carcinogenic chemicals (PHAs and HCAs).

Limitations of these type of studies and risk association

Most studies used in dietary risk association are epidemiologic or observational. Epidemiologic studies have linked consumption of processed meat or red meat with cardiovascular diseases, obesity, type 2 diabetes, and certain cancers. For most cases, observational studies have reported small increased relative risks. However, both epidemiologic and observational studies have certain limitations, and that is the reason why they usually establish “association” and not “causation”.

In the 2015 article “Research gaps in evaluating the relationship of meat and health.” the author describes some of the limitations in these types of studies8:

  • inability to accurately estimate intake
  • lack of prespecified hypotheses
  • multiple comparisons
  • confusion between many variables (including body weight, fruit/vegetable intake, physical activity, smoking, and alcohol) that correlate significantly either positively or negatively with meat intake and limit the reliability of conclusions from these studies
  • observational studies are heterogeneous and do not fulfill many of the points proposed considered necessary for inferring causality

These limitations are often brought to light when debate on meat consumption comes to the public. Therefore, care should be taken when interpreting the data. If for one side, accepting small, statistically significant risks as “real” from observational associations might lead to scientific failure, large populational studies are also valuable and should be followed up and hypotheses confirmed.

References

1.  IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Red Meat and Processed Meat. Lyon (FR): International Agency for Research on Cancer; 2018. PMID: 29949327.

2.  Bouvard V, Loomis D, Guyton KZ, Grosse Y, Ghissassi FE, Benbrahim-Tallaa L, Guha N, Mattock H, Straif K; International Agency for Research on Cancer Monograph Working Group. Carcinogenicity of consumption of red and processed meat. Lancet Oncol. 2015 Dec;16(16):1599-600. doi: 10.1016/S1470-2045(15)00444-1. Epub 2015 Oct 29. PMID: 26514947.

3.  World Cancer Research Fund/American Institute for Cancer Research. Diet, Nutrition, Physical Activity and Cancer: A Global Perspective. Continuous Update Project Expert Report, 2018.

4.  Alisson-Silva F, Kawanishi K, Varki A. Human risk of diseases associated with red meat intake: Analysis of current theories and proposed role for metabolic incorporation of a non-human sialic acid. Mol Aspects Med. 2016 Oct;51:16-30. doi: 10.1016/j.mam.2016.07.002. Epub 2016 Jul 12. PMID: 27421909; PMCID: PMC5035214.

5.  Bashir, S., Fezeu, L.K., Leviatan Ben-Arye, S. et al. Association between Neu5Gc carbohydrate and serum antibodies against it provides the molecular link to cancer: French NutriNet-Santé study. BMC Med 18, 262 (2020). https://doi.org/10.1186/s12916-020-01721-8

6.  Turesky RJ. Mechanistic Evidence for Red Meat and Processed Meat Intake and Cancer Risk: A Follow-up on the International Agency for Research on Cancer Evaluation of 2015. Chimia (Aarau). 2018 Oct 31;72(10):718-724. doi: 10.2533/chimia.2018.718. PMID: 30376922; PMCID: PMC6294997.

7.  Williams P. Nutritional composition of red meat. Nutrition & Dietetics. 2007; 64:S113-S119. https://doi.org/10.1111/j.1747-0080.2007.00197.x

8.  Klurfeld DM. Research gaps in evaluating the relationship of meat and health. Meat Sci. 2015 Nov;109:86-95. doi: 10.1016/j.meatsci.2015.05.022. Epub 2015 May 23. PMID: 26043666.