Animal Protein and Cancer Risk

Do all animal foods increase the risk for cancer?  What factors within animal food are problematic?

Claim: A plant-based diet with minimal to no animal protein is the best diet to lower cancer risk.   

Overview

In this article we will be reviewing various animal proteins and if the consumption of these foods is associated with an increased risk for cancer.  We will also explore the effects of preparation and processing of the animal protein compared to the protein itself.  Our goal is to evaluate and decide whether or not it is safe to eat an omnivorous diet and to discuss specific guidelines as to how much, how often, and with what overall dietary pattern we should be aiming for.

There have been several decades of research on this topic.  We have limited our review to the last 10 years to try and get the most recent data on this subject.  We will spend quite a bit of the discussion looking at the mechanisms as to why some animal proteins may be more problematic than plant sources of protein. 

Below is a list of the animal proteins we will be researching.

Animal Protein

Examples

Red Meat

Beef, veal, lamb, pork

Processed Meat   

Red meat that has been preserved (curing, smoking, salting, drying)

Poultry

Chicken, turkey, duck

Dairy

Milk, yogurt, cheese

Eggs

 

Fish

Fin and shellfish

 

Cooking and Processing Methods

Cooking animal foods at high-heat will create carcinogens.  The by-products include Heterocyclic Amines (HCAs) and Polycylic Aromatic Hydrocarbons (PAHs).  HCAs are made when creatines and amino acids (both found in meats) react together with heat.  This reaction starts to occur at 212 F (100 C) but happens more rapidly from about 572 F (300 C) and higher.  PAHs include over 100 different compounds formed by the incomplete burning of organic matter at temperatures in excess of 392 F (200 C).  Essentially the hotter and longer a meat is cooked, the more HCAs and PAHs produced.  Direct heat methods like grilling and frying produce more HCA’s and PAH’s than indirect-heat methods like stewing, steaming, or poaching.

Processing meats add chemicals, like nitrites, that generate N-Nitroso Compounds (NOCs); another class of carcinogenic compounds.

Nutrients Within

Does the problem lie with the nutrients within the animal food?  Some of the problems that will be discussed further relates to the fat content, specifically cholesterol content of the food.  Another concern is heme iron, which is particularly high in red meat, liver, and processed meat.  Yet another concern lies with choline content, which is high in eggs, liver, red meat, and milk.  The potential concern with choline is that it is eventually converted in the liver to trimethylamine oxide, which increases inflammation in the body. 1

Industry Practices

Yet another potential problem is how the industrialized husbandry practices affect the quality of the animal food we are eating.  Pesticides used on feed and/or medications or hormones given to the animal can create toxic residues left in the meat that we then consume.  In addition to whether pesticides are being used on the feed, what the animal is eating is equally important.  As discussed in Important Foods for Cancer Patients, there is concern regarding the inflammation producing properties found in meat from animals fed grain-based diets.  According to a Review written in Nutrition Journal in 2010,2 grass-based diets produce higher contents of omega-3s and CLA in the beef as well as higher levels of antioxidants.  We’ve discussed the anti-inflammatory benefits of Omega-3s throughout the website.  Conjugated Linoleic Acid (CLA) is a naturally occurring fatty acid made from the omega-6 essential fatty acids found in meat and dairy.  The isomers of CLA have been studied for their potential benefit with immune support and in weight loss.  Research suggests that pastured cows produce up to 500% more CLA than grain-fed cows2.

 

What is the Evidence?

Before we get into the details of the research we reviewed we think it’s important to point out some of the limitations of the research in this area.  Almost all of the studies we reviewed were either pre-clinical animal model studies, which means that those results may or may not translate to human populations.  Most of the rest of the studies were epidemiological, which is not possible to be conclusive regarding cause and effect. It’s also important to point out that none of the studies commented on whether the animal protein consumed was organic or grass-fed.  Based on what we know about the inflammatory markers alone this is a very important factor when making correlations between exposure and disease risk.

Red Meat:

In the text above we described how cooking methods are one of the factors contributing to the concern with meat and cancer. The by-products of high heat cooking have been shown to be carcinogenic.   According to a number of pre-clinical animal studies, HCAs have been shown to increase occurrence of tumors in multiple sites; mammary, lung, colon, stomach, prostate, pancreas, esophagus 3.  It’s important to keep in mind that any animal protein can create HCAs if cooked at a high enough heat.

Although the studies we reviewed showed a positive association with high meat intake and the following cancers: esophageal, lung, pancreatic, colorectal, breast, and stomach, there were mixed results seen for a correlation between meat intake and endometrial, bladder, or ovarian cancer. 

Among over 60,000 women in the Swedish Mammography Cohort, a modest elevation in risk for endometrial cancer was shown with those with the highest intake of heme iron, especially from consuming liver 4. There was no statistically significant association observed for intakes of other red or processed meats.  The pooled results of 10 cohort studies and 11 case-control studies indicate that overall meat intake was not related to risk of bladder cancer, however those with the highest intake of red and processed meats were at a 17% or 10% increased risk respectively 5. A dose-response meta-analysis of 8 cohort studies concluded that there was no association seen between the intake of red and processed meats and the risk for ovarian cancer 6.

According to the results of 4 different meta-analysis7-10, the combined results of 7 cohort and over 30 case control studies, concluded that individuals with the highest intake of red meat, which was measured at 3.5-4.2 ounces of red meat per day, have an increased risk for developing esophageal cancer; ranging from 26-57% higher risk depending on the study.  Most concern seems to lie in the high amount of heme iron found in meat as well as the HCAs and PAHs produced from cooking meat at high temperatures. 

A number of studies in the literature have examined the association between red meat intake and colorectal cancer.  The potential mechanisms may be due to the production of HCAs secondary to high-heat cooking.  According to 1 meta-analysis of 16 case-control and 5 cohort studies, for every 100 gram (3.5 oz) per day increase in red meat intake there is a 36% increased risk of colorectal cancer 11.  According to another meta-analysis of 3 case-control and 21 cohort studies, the risk of colorectal cancers significantly increases linearly with increasing intake of red meat up to 140 grams (5 oz) per day, where the curve approaches its plateau 12.

For breast cancer the association between red meat intake and increased risk seems to be due from a combination of factors including heme iron content, by-products of cooking, exogenous hormones used to treat commercially raised cattle, and an animal sugar known as NEUR5gc, which can be absorbed by human tissue and leads to inflammation and tumor formation in prospective studies.13  One prospective study suggested an association between higher red meat intake in early adulthood and increased risk for breast cancer later in life 14.  This same study showed that by replacing red meat with a combination of other proteins (legumes, poultry, nuts, fish) one could reduce their risk for breast cancer.

The heme iron content of red meat may also be implicated for an increased risk of stomach cancer as heme iron is a growth factor for H. Pylori 15.  The by-products of high-heat cooking have shown an association with an increased risk of pancreatic cancer 16.  Reviewing data from a large prospective cohort study (over 12,000 people) showed that total dietary fat intake, specifically saturated fat from red meat and dairy products, was associated with increased risk for pancreatic cancer 17. Furthermore, a meta-analysis of pancreatic cancer risk factors showed a positive association with red meat consumption, but only in men 18.  Consuming meat 3 or more times per week may also increase the risk of lung cancer by as much as 35%, according to evidence from the meta-analysis of 23 case-control and 11 cohort studies 19.

Processed Meat:

The link between cancer risk and consumption of processed meats is well supported in the literature.  This association is due not only to the mechanistic concerns with red meat, heme iron content, HCAs and PAHs, but also from the processing methods that add an additional layer of toxic compounds. 

As with red meat, the association with processed meat and esophageal cancer is relatively robust.  A 2014 meta-analysis showed high processed meat intake was likely to increase the risk of esophageal cancer 10.  Two other meta-analysis showed associations that individuals with the highest consumption of processed meat, measured between 50-100 grams per day (1.75-3.5 oz), were at highest risk for developing esophageal cancer; up to a 40% higher risk 9 7.  In other observational studies, the risk ranged from 41-55% of developing esophageal cancer for those with a high intake of processed meat 8 20.  With many of these studies the primary concern relates to the heme iron content.  Salted meat consumption in particular seems to have a strong association with esophageal cancer, particularly when it’s combined with alcohol and/or smoking 21.

Salted meat also seems to increase the risk of gastric cancer, according to another meta-analysis 22.  The high salt intake damages mucosa within the stomach and increases the risk for H. pylori infection; a known risk factor for gastric cancer.

As was the case with red meat, the consumption of processed meat poses a significant risk for colorectal cancers.  According to meta-analysis, processed meat intake is even more closely linked with colorectal cancers than intake of fresh red meat 23.  For every 50 gram (1.75 oz) per day increase in processed meat intake there is a 28% increased risk of colorectal cancer 11.  The risk of colorectal cancers significantly increases linearly with every 50 gram per day intake of processed meat up to 140 grams (5 oz) per day, where the curve approaches its plateau 12.

As with that of red meat, the association between processed meat intake and increased risk of breast cancer seems to be due to a combination of factors including heme iron content, the by-products of cooking, exogenous hormones used to treat commercially raised cattle, and an animal sugar known as NEUR5gc, which can be absorbed by human tissue and leads to inflammation and tumor formation 13.  Furthermore, smoked meat seems to be a particular problem.  In a case-controlled study of 400 women, daily intake of smoked meat significantly increased risk for breast cancer in both pre and post-menopausal women 24.

The by-products of processing have also shown associations with several other cancers.  There is an increased risk of oral cancer with increasing processed meat intake 25.  N-nitroso compounds (NOCs) also seem to be particularly problematic as they have been shown to reach the pancreas via the blood stream.  They are potent carcinogens in pre-clinical animal studies thereby increasing the risk for pancreatic cancer 18.  They have also been shown to cross the blood brain barrier, which may increase risk for glioma 26. 

Poultry:

Poultry, on the other hand, may lead to a decreased risk of cancer.    A meta-analysis looking at poultry intake and colorectal cancer showed a significant inverse association with at least a 50 gram (1.75 oz) per day poultry intake 27.  The incidence of esophageal cancer and lung cancer were also decreased with poultry intake 10 19.  The mechanisms responsible for this inverse association may be explained by an overall healthier lifestyle or because of the lower heme iron content in poultry.  There was one study looking at the post-diagnosis diet of over 27,000 men diagnosed with prostate cancer which found an increased risk of cancer from those eating chicken with the skin on 28.   This increased risk is likely secondary to the formations of HCAs and PAHs from high-heat cooking.

Dairy:

Dairy is an interesting category of animal food as it seems to show some negative and some positive associations with risk for cancer.  Certain components of dairy foods such as probiotic content, calcium, vitamin D, and CLA may all have protective effects.  Where-as other compounds such as IGF-1 and phosphorous may have negative effects.

The research is mixed around gastric cancer.  A 2015 meta-analysis of 17 case-control and 6 cohort studies showed protective effects with the highest total dairy intake and hypothesized that it may be due to the anti-cancer properties of vitamin D and calcium or probiotic-rich dairy foods (like yogurt and kefir), which interfere with H. pylori colonization, or even the phospholipid content increasing cellular renewal 29.  Another meta-analysis showed that dairy products were unlikely to be strongly protective against gastric cancer but didn’t increase risk 30.  And yet another meta-analysis showed a non-significant increased risk of gastric cancer with dairy intake 31. As you can see, the research is somewhat conflicting.

The CLA in dairy may be protective against breast cancer due to its anti-inflammatory properties 32.  The authors hypothesize that calcium and vitamin D may also offer anti-carcinogenic benefits on breast cancer cells.  A meta-analysis of colorectal cancer showed a statistically significant reduction in risk with high intake of milk (7 oz per day) and total dairy, excluding cheese (14 oz per day) 33.  The authors hypothesize that the protection is likely due to high calcium intake as trials have found reduced risk of colorectal recurrence with calcium supplementation (1200-2000 mg/day).

There was no association between dairy intake and pancreatic cancer risk observed in a pooled analysis of 14 cohort studies 34.  Although as mentioned above under “Red Meats”, the dietary fat content of dairy foods has shown some association of increased risk for pancreatic cancer 17.  The results of a meta-analysis showed increased prostate cancer risk with high intake of dairy foods; 14 oz/day total dairy, 7 oz/day milk, 3.5 oz/day yogurt, 1.75 oz cheese. One set of authors hypothesize that this is because of dairy’s IGF-1 content, which promotes proliferation of cancer cells and inhibits apoptosis of prostate cells 35.  They also speculate that it could be due to dairy’s high calcium intake interfering with vitamin D levels.  However, another set of authors believe that it’s dairy’s high phosphate content, rather than the calcium content, that reduces systemic serum vitamin D levels 36. Results from epidemiological studies examining the association between the use of dairy products and risk for ovarian cancer have been conflicting.  According to one study, evaluating data from a large Danish population-based case-control study (with over 2000 women), an association between intake of dairy foods and increased risk of ovarian cancer was noted 37.  The association however was strongest with milk and lowest for cheese.

Eggs:

Although eggs are lumped into an overall healthier dietary pattern in some prospective studies (chicken, eggs, legumes, nuts), there are other studies suggesting concern around egg consumption and increased cancer risk.  For prostate cancer in particular, there are several studies which found an increased risk.  According to a prospective study published in Cancer Prevention Research, healthy men who consumed 2.5 or more eggs per week had an 81% increased risk of lethal prostate cancer compared with men who consumed less than 0.5 egg per week 28.  The authors hypothesized that this was due to a high level of cholesterol and choline in the eggs, which are both highly concentrated in prostate cancer cells.  In 2012, the authors looked specifically at choline content and found that men in the highest quintile of choline intake (471 mg/day) had a 70% increased risk of lethal prostate cancer 38.  Although choline is found in all animal foods it is more highly concentrated in eggs at 250 mg per egg.

A meta-analysis published in 2015 showed a modestly elevated risk for not only prostate cancer but also breast and ovarian cancers for those with the highest intake of eggs, more than 5 per week, as compared to those with no egg consumption at all 39.  These authors also suggest that the choline and cholesterol were causative.  Choline may increase the proliferation and progression of prostate cancer 1, while cholesterol serves as a pre-cursor for the biosynthesis of sex hormones and a primary metabolite of cholesterol acts as an estrogen receptor agonist in breast cancer cells 39.  Another 2015 meta-analysis showed an association between egg consumption and increased risk for ovarian cancer 40.  Cohort studies have also shown an association between egg consumption and increased breast cancer risk; especially for post-menopausal women 41.  This seemed to be true for those consuming 2-5 eggs/week.

A 2014 meta-analysis published findings from 37 case-control studies and 7 cohort studies supporting the association between egg consumption and increased risk for GI cancers; with a stronger correlation for colon cancer and in Western populations 42.  Another recent case control study showed a connection between dietary cholesterol intake and risk of bladder cancer 43.  Cholesterol is converted in the liver to primary bile acids, which are then converted in the gut by bacterial flora into secondary bile acids.  The concern is that these bile acids produce reactive oxygen species, NF-kB activation, DNA damage, and increase cellular resistance to apoptosis. 44 The overall risk for bladder cancer increased two-fold with an intake of 6 fried eggs per week versus those who ate 0 eggs.  No association was found with boiled eggs, begging the question that the culprit may again lie with the by-products of high-heat cooking rather than the eggs themselves.43

Fish:

Fish on the other hand seems protective against cancer.  The only exception might be fried fish.  Frying fish decreases the omega-3 content of the fish and generates HCAs, oxidized lipids, and trans-fatty acids, all of which then increase cancer risk.  A case control study found that frying fish increased the risk for pancreatic cancer. 16

Higher consumption of fish has been associated with decreased risks for all of the following cancers: esophageal, breast, ovarian, colorectal, and liver.  The primary mechanism is thought to be related to omega-3 intake.  Omega-3s inhibit eicosanoid release, which reduces inflammation.  Omega-3s can also regulate transcription factor activity, regulate gene expression and signal transduction for cell growth, apoptosis, angiogenesis, and metastasis 45.  Pre-clinical animal studies have shown that supplemental omega-3 has the ability to slow the growth of various cancers and fish consumption has been advised to reduce cancer occurrence 46.

A meta-analysis showed that a daily increase of 50 grams (1.75 oz) per day of fish lowers the risk of esophageal cancer 9.  Another meta-analysis also showed an association between fish intake and lowered risk for esophageal cancer 8.

A meta-analysis of prospective cohort studies supported a higher consumption of fish (dietary marine omega-3) to decrease the risk of breast cancer 45.  A 50% decreased risk was observed for every 1 gram per day intake of marine omega-3.  However, no significant association was seen with plant based dietary omega-3.  The fattier the fish the higher amount of omega-3 it contains.  This association was demonstrated in a meta-analysis, which initially found no association between fish intake and risk for ovarian cancer but did find a significant decrease in risk associated with fatty fish intake 47.

Another meta-analysis found an association between fish consumption and prevention of colorectal cancer 48.  Another study also showed that eating fish reduced the prevalence of colorectal cancers, which the authors felt was due to the high selenium content found in fish 49.

The anti-inflammatory properties of omega-3s were also found to be beneficial in reducing the risk of liver cancer 50.  Liver cancer risk was reduced by 6-18% with a 1-2 servings per week intake of fish respectively 51.

 

What Is Our Recommendation?

The over-arching claim to avoid all meat and animal products is not warranted. You do not need to be vegan to lower your risk for cancer but you do need to consume a lot of plant foods.  After reviewing recent research the risk versus benefit of animal foods varies quite a bit depending on the type of animal protein and the way it is prepared.  Overall the cooking method (e.g., high temperature cooking) and processing of meat (e.g., smoking and curing) provide the strongest concern for cancer risk whereas certain animal proteins, especially fish, may actually be protective.

Here are our guidelines:

  • First and foremost include a large variety of colorful fruits and vegetables in your diet.
  • Next, vary your protein sources, including plant based sources of protein (beans, lentils, nuts, seeds) on a regular basis.
    • Aim to have fish 2 or more times per week; the fattier the better.
    • Poultry, without the skin, would be your next best option.
    • Limit eggs to 1 or 2 times per week; soft and hard boiled will be a better option than scrambled or fried.  Consider avoiding eggs completely if you have had a reproductive cancer or are at high-risk for breast, ovarian, and prostate cancer.
    • Limit red meat to once a week or less and avoid cooking at high temperatures.
  • Buy the best quality animal protein that you can to decrease your exposure to environmental carcinogens, antibiotics, and hormones as well as to improve the fat profile; 100% grass-fed or pasture raised is best.
    • Grass fed and pasture raised animals produce meat, dairy, and eggs that are naturally lower in cholesterol and higher in omega-3s, thus reducing the inflammatory potential of the food.  In the research we reviewed, there have been no studies comparing the cancer causing risk of grass-fed versus conventionally fed meats.
  • Avoid cooking your animal proteins at high temperature.
    • Keep the heat below 400 degrees Fahrenheit (200 degrees Centigrade).
    • HCA formation can also be substantially decreased by increasing the frequency of flipping during cooking; limit the amount of time you leave the meat on the heat surface. 16
    • Choose cooking methods that avoid direct heat: steaming, poaching, stewing
  • Add marinades and other sources of antioxidants.
    • Formation of HCA during cooking of beef, fish, or chicken can be decreased by marinating or brief microwave cooking before frying, broiling, or grilling. 16
    • Combining colorful vegetables with your meats provides good sources of calcium, chlorophyll, vitamin C, and polyphenols, all of which may reduce the negative effects of heme iron found in the meat. 23
  • Avoid the following:
    • Charred, Fried, and Processed meats.
    • Gravy made from meat drippings

 

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