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 a 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 and tumors”.
The group with the highest
animal protein intake — the 22% —
developed the highest incidence of tumors
(90% of them developed tumors), they had the largest overall tumor mass
and they had three times higher prevalence of tumor metastases as
compared with the group fed the lowest amount of protein in their 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 in the upper curve of this graph,
that group exhibited the expected increased cancer development
corresponding with a 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 increasing doses of the carcinogen they received.
Decreasing the amount of animal protein in their diets
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 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 then carcinogen dose.” In other words, the 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,
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. This study found that the rats on a 20%
plant protein diet, after being dosed with a carcinogen,
exhibited far less cancer development
than rats fed diets with 20% animal protein.
Again, all the rats were dosed with the same liver carcinogen,
but the rats fed a high animal protein diet
ehibited 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 enhance
cancer growth, 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 non-humans.
As a general matter, we do not support the use of animals
(or non-consenting humans) in biomedical experiments, which will discuss further
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 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 one, or IGF-1.
IGF-1 promotes cell growth
in both normal and cancer cells.
For this reason,
high levels of IGF-1 are associated with higher levels of carcinogenesis.
This hormone appears to be nutritionally regulated
and having animal protein 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-1.
As mentioned earlier, IGF-1 is a "hormone that stimulates
cell proliferation and inhibits cell death."
Because cell proliferation is what cancer
is all about, higher levels of IGF-1 have been
associated with various types of cancer.
This study further notes that,
“In particular, restriction
of protein rich in essential amino acid substantially reduces
IGF-1 production in vitro and in vivo.” The protein that is rich in these
animo 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 the higher proportion of these amino acids,
and that is what has been shown to increase levels of IGF-1
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-1 compared with a meat-eating or lacto-ovo-vegetarian diet.”
This study found that IGF-1
levels were 13% lower in vegans than most 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. This study concludes that,
“These data suggest that a plant-based diet
is associated with lower circulating levels of total IFG-1”,
which may explain the correlation between animal protein and cancer risk.
In another study published in the British Journal of Cancer,
IGF-1 was found to be 9% lower in vegan men
compared to meat-eaters, and
men who develop prostate cancer were found to have an
8% level of IGF-1, higher level of IGF-1
than those who remained healthy.
This suggests that “the 9% difference…
observed in vegan man 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 a higher risk of prostate cancer
incidents or mortality in most
ecologic, case-control and prospective studies.”
The study was published in the Journal of Urology, and involved institutions
including UCSF, UCLA, and Memorial Sloan-Kettering Cancer Center, which is
one of the leading cancer centers in the United States.
It explored the effect of diet changes
on the progression of prostate cancer with men
who had early biopsy proven prostate cancer.
Some 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 tradition
Western diet. After one year, they drew blood from
both the vegan group and the control group
and found that that 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
it 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 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 of Endocrine Reviews describes how
systemic administration of IGF-1
causes mamary hyperplasia, or proliferation of the cells in the
breast glands, and that the overproduction IGF-1
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 multi-center study on diet and bladder cancer
found that, “a 3% increase
in the consumption of energy from animal protein
was associated with a 15% higher risk” of cancer,
whereas "a 2% increase in energy
from plant protein intake was associated with
a 23% lower risk” of cancer.
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 sometime that plant proteins may
reduce the risk of cancer, obesity and cardiovascular risk.
With regard to cancer, this review notes that,
“high IGF-1 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 theses for this review is that
“animal protein — precisely because it is [so-called] ‘high quality’ protein,
rich in essential amino acids — will up-regulate IGF-1
activity, and thereby act as a cancern 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”,
this study included over 60,000 men and women on various types of diets and followed
them for an average of 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, cohort study
was conducted across ten European countries
and published in the International Journal of Cancer.
Called “EPIC” (for the European Prospective Investigation into Cancer and Nutrition),
this study found that “A high intake
of poultry was related to an increased risk of
B-cell lymphomas.” While the study refers to a
“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 of 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 woman examined the 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 show statistically significant protection for overall cancer incidence."