“Try and avoid red meat. It’s not good for you”
This is advice most non-vegetarians have received – and turns out the scientific data we have available doesn’t support it. Researchers who reviewed various studies in scientific journals have concluded that there is no strong evidence linking red meat with cancers, heart disease and diabetes. A third review, in which scientists examined dietary guidelines for processed and unprocessed red meat, suggests adults can continue to consume them.
Annals of Internal Medicine published the three reviews, along with two more, all of which addressed health outcomes, including cancer, and dietary guidelines concerning the consumption of red meat. An independent group of clinical, nutritional and public health experts called NutriRECS, which specialises in conducting systematic reviews and framing practice guidelines, undertook the exercise.
The first one was concerned with the risk of developing heart disease, lifestyle-related diabetes, high blood pressure and cancer – all conditions that experts have increasingly collected under the umbrella term of ‘cardiometabolic’ illnesses. The researchers looked for clinical trials (published in multiple languages) to compare diets with different quantities of red meat, each differing by a gradient of at least one serving per week for six months or more. There were only a few such trials, and the reviewers found that most of them largely addressed only surrogate outcomes and with small differences in red meat consumption.
So the team concluded that there is “low- to very-low-certainty evidence” that suggests diets with smaller quantities of red meat may have little or no effect on heart or diabetes risks as well as on the incidence of cancer.
“Our results from the evaluation of randomised trials do not support the recommendations in the United Kingdom, the United States or World Cancer Research Fund guidelines on red-meat intake,” their paper reads. “One could argue, however, that neither do they seriously challenge those recommendations.” In other words, the advice to avoid red meat isn’t founded on what we know.
As the paper added, “Our results highlight the uncertainty regarding causal relationships between red meat consumption and major cardiometabolic and cancer outcomes.”
For the second review, researchers scrutinised observational studies on links between reduced intake and processed meat intake – and cancer mortality and incidence. They concluded that the studies suffered from biases thanks to limitations in the way they assessed people’s diets (including recall bias), how they adjusted for confounding factors and didn’t have enough data for subgroup analyses.
“The possible absolute effects of red and processed meat consumption on cancer mortality and incidence are very small, and the certainty of evidence is low to very low,” the second paper said, much like the first.
The third review evaluated people’s health-related values and preferences, since they determine how much red meat they consume. Its corresponding paper said that – again – the evidence that non-vegetarians “are attached to meat and are unwilling to change this behaviour when faced with potentially undesirable health effects” is of low-certainty.
The fourth review found low-certainty evidence to suggests that omnivores are attached to meat and are more reluctant to change their eating habits, even in the face of well-grounded advice. The fifth offered a set of recommendations on eating red meat and processed meat based on its analysis, including suggesting that adults continue their extant diets containing unprocessed and processed red meat.
Several experts The Wire contacted agreed with the conclusions but not entirely with how the reviewers are interpreting them.
Anura Kurpad, a professor of physiology and nutrition at St John’s Medical College, Bengaluru, and a former president of the Nutrition Society of India, explained that the findings of the new reviews don’t represent reversal of what we know but a “reversal of interpretation and translation”. The point was that the observed negative effects were small enough to translate into “don’t [worry] about this”.
“The finding is still that eating meat has a small risk. The interpretation is that the risk is small enough to ignore.”
According to him, the reviews selected a few studies based on a criterion that is really very strict and more suited to drug trials. “So a lot of observation studies that could also be informative fell through.” They also didn’t compare ‘non-meat eaters’ – or vegetarians – to meat eaters, he said, indicating that the reviews would have been if the they’d looked at lower intakes or even zero intake as the base.
In other words, the reviews’s conclusions are restricted by how the reviewers analysed them, and shouldn’t change the way we think about modifying the amount of red meat we eat. Reducing dietary risk has always been an important nutritional goal, and it would be prudent to continue with what experts have inferred from larger evaluations.
John Ioannidis, a professor of medicine at Stanford University, said, “The numerical results practically are almost identical to what previous meta-analyses have shown: randomised trials show no health benefits and observational studies show no or small health benefits, and the evidence is of low quality.”
The only difference, he continued, is the authors’ conclusion: that the evidence is of low quality and so one needs to be cautious, although he added they “cannot make any strong recommendation”. In contrast, the authors of previous studies have stressed the benefits of cutting back on red meat even if the evidence has been weak.
The methods of the Annals papers “are very rigorous and clearly superior to what was done previously,” according to Ioannidis, and that “their conclusions are appropriate.” Frederic Leroy, a professor at the department of industrial microbiology and food biotechnology at the Vrije Universiteit Brussel, agreed, saying the authors “have performed a very rigorous quality check of the evidence based on the highest scientific standards”.
And while the intentions of previous authors who recommended cutting down red meat may be good, “it would be irresponsible for us to tell people that we know for sure that cutting back on red meat will save their lives.”
In September this year, Leroy had reported that science doesn’t yet support decisions about whether you should reduce your meat intake as well as that doing so could potentially be detrimental to public health, especially in certain vulnerable groups. “Restricting the diet by taking away a valuable nutrient source needs careful planning, nutritional knowledge, and may not be for everyone,” he explained, “especially not when special needs are present, such as in the young and the elderly.”
Indeed, his study as well as the reviews relate only to the health aspects. “Other issues, such as environmental effects, ideological or religious concerns, and animal welfare aspects require another debate, where context and nuance are also often neglected, unfortunately,” he said.
Most of the evidence that nutritionists have used to argue that people should eat less red meat and processed meat has come from observational studies. In these studies, scientists make note of how much meat the studies’ participants eat and then observe them for many years, recording the incidence of certain chronic diseases, like type-2 diabetes, and the mortality.
But this observation isn’t direct, Most of it comes form participants through self-reported questionnaires, these questionnaires have been known to struggle to capture actual eating behaviour, Leroy said. Plus, health-conscious people in the West who report in these questionnaires are also more physically active, avoid cigarettes and alcohol; and have better access to healthcare. “The observed associations tend to disappear or even reverse when research is done in non-Western settings.”
“This debate offers a nice example of the difference between what we want to believe and what we can really know,” Ioannidis finished.