Animal Protein and Cancer


This video summarizes medical literature addressing the links between animal protein consumption and increased cancer risk.

In the previous chapter of our health section, we discussed the basics of protein and diet. In this chapter, we’ll be discussing the relationship between animal protein and cancer.

Dr. T. Colin Campbell, from Cornell University, one of the most prominent figures of nutritional biochemistry, noted that “an elevated intake of protein (particularly animal protein) is a risk factor for certain cancers” and that it “promotes carcinogenesis.”

In this particular study, researchers from Berkeley and Cornell, including Dr. Campbell, delivered a liver carcinogen to rats and then fed those rats different amounts of animal protein. One group of the rats was fed a high protein diet containing 22% animal protein. The other group was fed a diet with only 6% protein. Again, all of the rats were first given the same liver carcinogen. It was observed, at 100 weeks, that most of the lower protein fed rats were still alive and “were healthier and thriftier in appearance”, whereas many of the higher protein fed rats were “already dead or were severely burdened with tumors”. The group with the highest animal protein intake — 22% — developed the highest incidents of tumors (90% developed tumors), had the largest overall tumor mass and had three times higher prevalence of tumor metastases as compared to the group fed the lowest animal protein diet.

In another experiment, rats were similarly administered a carcinogen and then fed differing levels of animal protein. Specifically, the rats were given a liver carcinogen called aflatoxin, then half were fed a diet containing 20% animal protein. As shown on the upper curve of this graph, that group exhibited the expected increased cancer development corresponding with the higher doses of the carcinogen they received. This is not surprising. The more carcinogen they were given, the more cancer they developed. But, look what happened to the other half of the rats who were instead fed a lower protein diet after being given the carcinogen. Cancer development did not increase for these rats, as would have been expected with the increasing doses of the carcinogen they received. Decreasing the amount of animal protein in their diet almost made the cancer producing substance stop having its cancer producing effect. As the authors of the study put it, “These data rather convincingly show that the growth and development of preneoplastic foci” (…the precursor clusters of cancer cells that grow into tumors…) “primarily occur in response to the level of dietary casein[,]” providing evidence that, “nutrient intake…is more rate limiting towards the development of these preneoplastic lesions than is the carcinogen dose.” In other words, this study indicated that some cancer development may be controlled more by dietary protein levels than by exposure to the underlying carcinogen.

These two studies that we just reviewed examined animal protein. But what about plant protein? Would it have the same effect on cancer development that animal protein has? The answer is, “No” it does not. When rats under the same conditions were similarly dosed with the same liver carcinogen (aflatoxin), but then fed diets with plant protein instead of animal protein, they did not exhibit the same increased cancer development — even when they were given diets with high levels of plant protein after being dosed with the carcinogen. The study found that the rats fed a diet of 20% plant protein after being dosed with the carcinogen exhibited far less cancer development than rats fed diets with 20% animal protein. Again, all of the rats were dosed with the same liver carcinogen, but the rats fed a high animal protein diet exhibited the high levels of cancer development, while the rats fed a high plant protein diet did not. The conclusion of this and similar studies is that large amounts of animal protein greatly enhance tumor growth, while large amounts of plant protein do the opposite.

The studies we’ve reviewed so far are based on experiments conducted on rats. However, it is important to note that extrapolating results from animal experiments can be problematic for a number of reasons, not the least of which is the underlying biological differences between humans and nonhumans. As a general matter, we do not support the use of animals (or non-consenting humans) in biomedical experiments, which we’ll further discuss in a separate chapter under our “Ethics” section. However, we still believe it is important to include these studies here, in our Health Section, since they have nonetheless contributed to the medical literature examining the links between animal protein and cancer. While these animal experiments are compelling, they may not have been altogether necessary given the extensive number of human studies that have been conducted as well.

As we begin to review those human studies on this topic, we need to start with one factor that, in particular, appears to contribute to the association between animal protein and cancer. It’s a hormone called insulin-like growth factor-I, or IGF-I. IGF-I promotes cell growth in both normal and cancer cells. For this reason, high levels of IGF-I are associated with higher levels of carcinogenesis. This hormone appears to be nutritionally regulated and having animal protein available in our body from our diet results in higher levels of this cancer promoting hormone.

This study, published in the journal of Cancer Epidemiology, Biomarkers & Prevention, noted that consumption of animal protein was associated with increased levels of IGF-I. As mentioned earlier, IGF-I is a “hormone that stimulates cell proliferation and inhibits cell death”. Because cell proliferation is what cancer is all about, higher levels of IGF-I have been associated with various types of cancer. This study further notes that, “In particular, restriction of protein rich in essential amino acids substantially reduces IGF-I production in vitro and in vivo.” The protein that is rich in these amino acids is animal protein — or meat, fish, poultry, eggs and dairy. Although plant protein also contains all of the essential amino acids, it is animal protein that has a higher proportion of these amino acids, and this is what has been shown to increase levels of IGF-I and, as previously reviewed in the earlier cited studies, is associated with increased cancer development. “The aim of this cross-sectional study was to determine whether a plant based vegan diet is associated with lower circulating levels of IGF-I compared with a meat-eating or lacto-ovo-vegetarian diet.” This study found that IGF-I levels were 13% lower in vegans than in meat eaters or lacto-ovo-vegetarians. So, dropping meat, poultry and fish, while continuing to eat eggs and dairy, which is what lacto-ovo-vegetarians do, did not produce the drop in this cancer promoting hormone as we see in vegans. The study concludes that, “These data suggest that a plant-based diet is associated with lower circulating levels of total IGF-I”, which may explain the correlation between animal protein and cancer risk.

In another study published in the British Journal of Cancer, IGF-I was found to be 9% lower in vegan men compared to meat-eaters, and men who developed prostate cancer were found to have an 8% higher level of IGF-I than those who remained healthy. This suggests that “the 9% difference…observed in vegan men was large enough to significantly alter prostate cancer risk.”

Perhaps that’s why other medical reviews, like this one in this cancer journal, suggest that, “Diets high in dairy and meats are related to higher risk of prostate cancer incidence or mortality in most ecologic, case-control, and prospective studies.”

This study was published in The Journal of Urology, and involved a number of institutions, including UCSF, UCLA and Memorial Sloan-Kettering Cancer Center, which is one of the leading cancer centers in the U.S. It explored the effects of diet changes on the progression of prostate cancer with men who had early, biopsy-proven prostate cancer. Some of these men were randomly assigned to an experimental group that was put on a vegan diet. The remaining men comprised the “control group” and continued their traditional Western diet. After one year, they drew blood from both the vegan group and the control group and found that the blood from the vegan group was almost eight times more potent at inhibiting the growth of cancer cells compared to the blood of the non-vegan control group. As shown in this graph, the blood from the vegan group inhibited cancer cell growth by 70%, compared to only 9% for the non-vegan control group. One additional footnote to this study is that six of the men from the non-vegan control group withdrew from the study to undergo conventional treatment due to an increase in prostate-specific antigen (PSA) levels or progression of their prostate cancer. “In contrast, no experimental group patients” who were put on a vegan diet “underwent conventional treatment during the study.”

Quite similarly, with respect to breast cancer, this study in the Journal Endocrine Reviews describes how systemic administration of IGF-I causes mammary hyperplasia, or proliferation of the cells in the breast glands, and that the overproduction of IGF-I can cause the development of breast cancer.

Even small changes in diet can have a profound impact on health. As published in the International Journal of Cancer, this large multi-centered study on diet and bladder cancer found, “a 3% increase in the consumption of energy intake from animal protein was associated with a 15% higher risk,” whereas “a 2% increase in energy from plant protein intake was associated with a 23% lower risk.” It is quite remarkable that such a small amount of animal protein consumption is associated with such a correspondingly large increase in cancer risk, while the opposite is shown with regard to consumption of plant protein.

It has been known for some time that plant proteins may reduce the risk of cancer, obesity and cardiovascular disease. With regard to cancer, this review notes that, “high IGF-I activity associated with heavy ingestion of animal products may be largely responsible for the epidemic of ‘Western’ cancers in wealthy societies. …An unnecessarily high intake of essential amino acids…may prove to be as grave a risk factor for ‘Western’ degenerative diseases as is excessive fat intake. …In general, vegan proteins tend to contain a higher fraction of the non-essential amino acids than the main animal-derived dietary proteins do.” The basic thesis of this review is that “animal protein – precisely because it is [so-called] ‘high-quality’ protein, rich in essential amino acids – will up-regulate IGF-I activity, and thereby act as a cancer promoter;” whereas so-called “‘low-quality’ vegan proteins can be expected to have the opposite effect.” So, basically what is referred to as “high-quality” protein, which simply refers to the higher concentration of essential amino acids in animal food, would be more appropriately labeled “high-risk” instead.

This large multi centered cohort study published in the British Journal of Cancer compared the incidence of cancer in people following different diets. Noting that, “Meat has been suspected of influencing the risk for several types of cancer”, the study included over 60 thousand men and women on various types of diets, and followed them for an average of over 12 years. They found that the vegetarians had a decreased risk of several cancers, including a statistically significant 64% risk reduction for stomach cancer, over 40% risk reduction for lymphatic and blood cancers, 43% less risk for non-Hodgkin lymphoma, and a 75% risk reduction for multiple-myeloma. The authors of the study noted that, “The most striking finding was the relatively low risk for cancers of the lymphatic and hematopoietic tissues among vegetarians”.

Another large prospective cohort study was conducted across 10 European countries and published in the International Journal of Cancer. Called “EPIC” (for the “European Prospective Investigation into Cancer and Nutrition”), the study found that “A high intake of poultry was related to an increased risk of B cell lymphomas.” While the study refers to “high intake”, it’s really not talking about that much poultry at all, considering that it found a 22% increased risk of this cancer per just 10 grams of poultry intake. Keep in mind that a single chicken breast is about 85 grams or more.

That was poultry. Now this is about red meat. Yet another review of two large-scale cohort studies involving almost 40,000 men and over 80,000 women examined links between red meat and mortality. It concluded that, “Red meat consumption is associated with an increased risk of total, [cardiovascular disease], and cancer mortality. Substitution of other healthy protein sources for red meat is associated with lower mortality risk.”

So, overall, what general conclusions can we make? Well, while diet is not the only risk factor for cancer, the strong association between animal protein and cancer development should not be ignored. And, it is a modifiable risk factor that can easily be avoided by simply changing one’s diet. As this 2013 peer-reviewed cancer research article puts it, “vegan diets showed statistically significant protection for overall cancer incidence.”

This transcript is an approximation of the audio in above video. To hear the audio and see the accompanying visuals, please play the video.

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This presentation was written and narrated by Sofia Pineda Ochoa, MD, and edited by Bob Rapfogel.

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