“Given the weak associations in human studies and lack of evidence in animal studies, it is hard to reconcile the committee’s vote,” says nutritional toxicologist James Coughlin, Ph.D., CFS. “Of more than 900 items IARC has reviewed, including coffee, sunlight and night shift work, they have found only one ‘probably’ does not cause cancer, according to their classification system.”
Coughlin is referring to Monday’s widely publicized report from the World Health Organization’s International Agency for Research on Cancer (IARC) which suggests that eating processed red meats is akin to smoking cigarettes when it comes to cancer risk. The same report also labels red meat consumption with the second-highest carcinogenic risk classification.
Specifically, the IARC Monographs Programme classified consumption of processed red meat as, “carcinogenic to humans (Group 1), based on evidence that it causes colorectal cancer.”
The IARC Monographs Programme classified the consumption of red meat as, “probably carcinogenic to humans (Group 2A), based on limited evidence that the consumption of red meat causes cancer in humans and strong mechanistic evidence supporting a carcinogenic effect.”
These are the IARC risk classifications: Group 1 (carcinogenic to humans); Group 2A (probably carcinogenic to humans) Group 2B (possibly carcinogenic to humans); Group 3 (not classified as to its carcinogenicity to humans); Group 4 (probably not carcinogenic to humans).
Scientific community critical of IARC process, findings
Coughlin, a toxicologist with more than 40 years of experience in meat and cancer, is critical of the IARC review process due to the lack of transparency, selective inclusion or exclusion of studies and broad interpretation of study results that are inconsistent with the conclusions of the study authors.
“In my experience as an observer to an IARC working group, the process typically involves scientists who have previously published research on the substance being reviewed and may have a vested interest in defending their own research,” Coughlin says. “In the case of red and processed meat, the overall scientific evidence simply does not support their conclusion.”
“It was clear sitting in the IARC meeting that many of the panelists were aiming for a specific result despite old, weak, inconsistent, self-reported intake data,” says Betsy Booren, Ph.D., vice president of scientific affairs for the North American Meat Institute. “They tortured the data to ensure a specific outcome.”
According to Booren, red and processed meat are among 940 agents reviewed by IARC and found to pose some level of theoretical ‘hazard.’
“Only one substance, a chemical in yoga pants, has been declared by IARC not to cause cancer,” Booren explains. “IARC says you can enjoy your yoga class, but don’t breathe air (Group 1 carcinogen), sit near a sun-filled window (Group 1), apply aloe vera (Group 2B) if you get a sunburn, drink wine or coffee (Group 1 and Group 2B), or eat grilled food (Group 2A). And if you are a hairdresser or do shiftwork (both Group 2A), you should seek a new career.”
IARC's findings run counter to wide-ranging scientific opinion
A large meta-analysis, published online in May in the Journal of the American College of Nutrition, analyzed the relationship between red meat intake and risk for colorectal cancer and concluded, “red meat does not appear to be an independent predictor of CRC risk.” That’s according to Dominik Alexander, Ph.D., MSPH, the epidemiologist who conducted the research on behalf of the Beef Checkoff.
According to Alexander, studies in nutritional epidemiology can be highly prone to bias due to such things as self-reported dietary intake, for which habits may change over time. Because of this, associations reported in nutritional epidemiology may be surrounded by uncertainty.
For instance, most, if not all, of the observational studies with red meat are limited by confounding factors. For example, studies have shown that people who consume the most red meat are the most likely to smoke, eat fewer fruits and vegetables and be overweight or obese – all of which may confound the relationship between eating red meat and risk of cancer.
As well, more recent studies in large cohorts find no association or non-significant associations between red meat and cancer. For instance, a recent Harvard study using the well known The Nurses’ Health Study (NHS) and The Health Professionals Follow-up Study (HPFS) found unprocessed meat intake had an inverse association with distal colon cancer and a weak, statistically non-significant, positive association with risk of proximal colon cancer.
“There are a constellation of factors that are associated with the probability of getting cancer, which include age, genetics, socioeconomic characteristics, obesity, lack of physical activity, where you grew up, alcohol consumption, smoking and even your profession,” says Alexander. “The bottom line is the epidemiologic science on red meat consumption and cancer is best described as weak associations and an evidence base that has weakened over time. And most importantly, because red meat is consumed in the context of hundreds of other foods and is correlated with other behavioral factors, it is not valid to conclude red meat is an independent cause of cancer.”
In addition, gold-standard nutrition evidence such as the Women’s Health Initiative and the Polyp Prevention Trial, two large, multi-year randomized controlled dietary interventions, found that a 20% reduction in red meat consumption did not reduce the risk of colorectal cancer and/or had no effect on adenoma recurrence in the large bowel. These studies were disregarded from the IARC review.
Shalene McNeill, Ph.D., RD, beef checkoff nutrition scientist and registered dietitian, observed the recent IARC process. After seven days of deliberation, rather than consensus agreement, which it typically seeks, the committee of 22 settled for “majority” agreement.
“Cancer is a complex disease that even the best and brightest minds don’t fully understand,” McNeill says. “Billions of dollars have been spent on studies all over the world and no single food has ever been proven to cause or cure cancer. The opinion by the IARC committee to list red meat as a probable carcinogen does not change that fact. The available scientific evidence simply does not support a causal relationship between red or processed meat and any type of cancer.”
Most scientists agree that it is unrealistic to isolate a single food as a cause of cancer from a complex dietary pattern further confounded by lifestyle and environmental factors.
“As a registered dietitian and mother, my advice hasn’t changed. To improve all aspects of your health, eat a balanced diet, which includes lean meats like beef, maintain a healthy weight, be physically active and, please don’t smoke,” says McNeill.
According to IARC, a summary of the final evaluations is available online in The Lancet Oncology.
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