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Carcinogenicity of consumption of red and processed meat
In October, 2015, 22 scientists from
ten countries met at the International
Agency for Research on Cancer (IARC)
in Lyon, France, to evaluate the
carcinogenicity of the consumption
of red meat and processed meat.
These assessments will be published in
volume 114 of the IARC Monographs. 1
Red meat refers to unprocessed
mammalian muscle meat—for example,
beef, veal, pork, lamb, mutton, horse, or
goat meat—including minced or frozen
meat; it is usually consumed cooked.
Processed meat refers to meat that
has been transformed through salting,
curing, fermentation, smoking, or
other processes to enhance fl avour or
improve preservation. Most processed
meats contain pork or beef, but might
also contain other red meats, poultry,
off al (eg, liver), or meat byproducts such
as blood.
Red meat contains high biological-
value proteins and important
micronutrients such as B vitamins, iron
(both free iron and haem iron), and
zinc. The fat content of red meat varies
depending on animal species, age,
sex, breed, and feed, and 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, but 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. 2,3
Depending on the country, 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 equalling
more than 200 g per person per day. 4
Less information is available on the
consumption of processed meat.
The Working Group assessed more
than 800 epidemiological studies
that investigated the association of
cancer with consumption of red meat
or processed meat in many countries,
from several continents, with diverse
ethnicities and diets. For the evaluation,
the greatest weight was given to
prospective cohort studies done in
the general population. High quality
population-based case-control studies
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 for the
cancer sites concerned.
The largest body of epidemiological
data concerned colorectal cancer.
Data on the association of red meat
consumption with colorectal cancer
were available from 14 cohort studies.
Positive associations were seen with
high versus low consumption of red
meat in half of those studies, including
a cohort from ten European countries
spanning a wide range of meat
consumption and other large cohorts
in Sweden and Australia. 5–7 Of the
15 informative case-control studies
considered, 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 cohort studies that provided
relevant data, including studies
in Europe, Japan, and the USA. 5,8–11
Supporting evidence came from six
of nine informative case-control
studies. A meta-analysis of colorectal
cancer in ten cohort studies reported a
statistically signifi cant dose–response
relationship, with a 17% increased
risk (95% CI 1·05–1·31) per 100 g per
day of red meat and an 18% increase
(95% CI 1·10–1·28) per 50 g per day of
processed meat. 12
Data were also available for more
than 15 other types of cancer. Positive
associations were seen in cohort
studies and population-based case-
control studies between consumption
of red meat and cancers of the
pancreas and the prostate (mainly
advanced prostate cancer), and
between consumption of processed
meat and cancer of the stomach.
On the basis of 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
unlikely as explanations, a majority
of the Working Group concluded
that there is sufficient evidence in
human beings for the carcinogenicity
of the consumption of processed
meat. Chance, bias, and confounding
could not be ruled out with the same
degree of confi dence for the data on
red meat consumption, since 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.
The Working Group concluded that
there is limited evidence in human
beings for the carcinogenicity of the
consumption of red meat.
There is inadequate evidence
in experimental animals for the
carcinogenicity of consumption of red
meat and of processed meat. 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 preneoplastic
lesions was reported in three and four
studies, respectively. 13–15
The mechanistic evidence for
carcinogenicity was assessed as
strong for red meat and moderate
for processed meat. Mechanistic
evidence is mainly 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 (preneoplastic lesions) of
the colorectum that was consistent
across studies. 16 For genotoxicity
and oxidative stress, evidence was
moderate for the consumption of
red or processed meat. In human
beings, observational data showed
slight but statistically significant
associations with APC gene mutation
or promoter methylation that were
identifi ed in 75 (43%) and 41 (23%)
of 185 archival colorectal cancer
samples, respectively. 17 Consuming
well done cooked red meat increases
the bacterial mutagenicity of human
urine. In three intervention studies in
human beings, changes in oxidative
stress markers (either in urine, faeces,
or blood) were associated with
consumption of red meat or processed
meat. 18 Red and processed meat intake
increased lipid oxidation products in
rodent faeces. 13
Substantial supporting mechanistic
evidence was available for multiple
meat components (NOC, haem
iron, and HAA). Consumption of
red meat and processed meat by
man induces NOC formation in the
colon. High red meat consumption
(300 or 420 g/day) increased levels
of DNA adducts putatively derived
from NOC in exfoliated colonocytes
or rectal biopsies in two intervention
studies. 19,20 Few human data, especially
from intervention studies, were
available for processed meat. Haem
iron mediates formation of NOC,
and of lipid oxidation products in
the digestive tract of human beings
and rodents. Haem iron effects can
be experimentally suppressed by
calcium, supporting its contribution
to carcinogenic mechanisms. Meat
heated at a high temperature contains
HAA. HAA are genotoxic, and the
extent of conversion of HAA to
genotoxic metabolites is greater in
man than in rodents. Meat smoked or
cooked over a heated surface or open
fl ame contains PAH. These chemicals
cause DNA damage, but little direct
evidence exists that this occurs
following meat consumption.
Overall, the Working Group classifi ed
consumption of processed meat as
“carcinogenic to humans” (Group 1)
on the basis of sufficient evidence
for colorectal cancer. Additionally,
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 Working
Group took into consideration all
the relevant data, including the
substantial epidemiological data
showing a positive association
between consumption of red meat
and colorectal cancer and the strong
mechanistic evidence. Consumption of
red meat was also positively associated
with pancreatic and with prostate
cancer.
We declare no competing interests.
Véronique Bouvard, Dana Loomis,
Kathryn Z Guyton, Yann Grosse,
Fatiha El Ghissassi,
Lamia Benbrahim-Tallaa, Neela Guha,
Heidi Mattock, Kurt Straif, on behalf of
the International Agency for Research
on Cancer Monograph Working Group
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International Agency for Research on Cancer,
Lyon, France
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