… strong anti-meat dietary recommendations are not justified by the best currently available evidence...
I’m A Physician, And I’ll Continue Eating Red Meat
Paul Hsieh, Contributor, Forbes
Nov 24, 2019
Last month, a group of 14 health researchers led by epidemiologist Dr. Bradley Johnston of Dalhousie University sparked enormous controversy by publishing a series of articles challenging the conventional wisdom that people should reduce their consumption of red meat. Their recommendations in the prestigious Annals of Internal Medicine basically stated that adults could “continue current levels of red meat and processed meat consumption.”
These conclusions were a direct challenge to current recommendations urging that people significantly reduce red meat consumption. These new recommendations have met with “fierce criticism“ from the American Heart Association, the American Cancer Society, and the Harvard T.H. Chan School of Public Health. In particular, the Harvard group called these new recommendations “irresponsible and unethical” and warned that this could “harm the credibility of nutrition science and erode public trust in scientific research.”
So how did Dr. Johnston’s group arrive at their conclusions, and what should ordinary people make of this controversy?
Johnston and colleagues used a well-established technique called meta-analysis, to analyze 61 prior major studies covering 4 million participants investigating relationship between eating red meat and subsequent risks of cardiac disease, stroke, or diabetes. They also evaluated the certainty level of each study’s conclusions using a method called GRADE (Grading of Recommendations Assessment, Development and Evaluation). GRADE is commonly used to evaluate the reliability of results in pharmaceutical trials studying whether a drug has a beneficial (or harmful effect).
Most of the available data came from observational studies, and those recommendations were deemed “weak.” Johnston’s analysis showed that these studies showed only very small health benefits from reducing meat consumption.
Observational studies are notoriously unreliable, in part because they are based on people’s reported recollections of what they ate — sometimes weeks in the past. I sometimes have a hard time remembering what I had for dinner the night before, let alone 3 weeks ago!
Another problem with observational studies is the issue of confounding factors. We must be careful not to confuse correlation with causation. As Washington Post science journalist Tamar Haspel notes:
Heavy plant-eaters are different from, say, heavy meat-eaters in all kinds of ways (income, education, physical activity, BMI). Red meat consumption correlates with increased risk of dying in an accident as much as dying from heart disease. The amount of faith we put in observational studies is a judgment call.
Tamar Haspel, Washington Post
In other words, people who consume more red meat might have slightly higher rates of heart disease, but because both might be correlated with some third factors, rather than meat consumption directly causing disease. (For more on the problems of bad nutritional science, see this excellent interview with Stanford professor Dr. John Ioannidis.)
Johnston’s analysis also included data from one major available randomized control study, the Women’s Health Study. Such studies are considered more scientifically rigorous than observational studies. Because study participants are randomized ahead of time into the two groups, this addresses the issue of confounding factors. The Women’s Health Study data showed no decrease in cardiovascular disease or cancer mortality from lowering meat consumption.
Some critics have argued that it is unfair to apply the same standards to nutritional studies as for drug trials. They correctly note that rigorous and reliable long-term nutritional studies are difficult to perform, as opposed to more straightforward drug studies to determine, say, whether or not a new drug reduces the risk of stroke or cancer.
But this argument merely confirms Johnston’s main point — namely, that strong anti-meat dietary recommendations are not justified by the best currently available evidence...
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