A new observational study, published in The BMJ, suggests an association between eating more red meat and an increased risk of heart disease. It also found links between eating plants - instead of animals - and better health.
Unfortunately, this study is - literally - a repeat of many low-quality studies that have come before it. It uses the same dataset to replicate earlier observational findings.
For this study, the authors enrolled mostly male health professionals in 1986 and followed them until 2016. They accumulated an impressive 1,023,872 person-years of follow-up, which can be defined as the number of people multiplied by the number of years they were followed.
Also, subjects filled out food frequency questionnaires every four years to document their food intake.
The authors found an increased risk ratio for heart disease of 1.28 for those who ate the most red meat compared to those who ate the least, and an increased risk ratio of 1.12 for every extra serving of red meat.
Once again, I'd like to point out how minimal this difference is. As we've described in our guide on observational studies, anything with an odds ratio of less than 2.0 has a high risk of being statistical noise and not a real effect. A risk of 1.28 may sound impressive as "a 28% increased risk," but statistically speaking, it is unimpressive.
More concerning than the minimal hazard ratio, however, is the inherent healthy user bias. In a perfect world, all habits and characteristics would be the same between those who ate the most meat and those who ate the least. That way, any difference in heart disease risk could be assumed to be due to the difference in meat consumption rather than differences in other habits and choices.
Unfortunately, that's not what we find in this study. Those who ate less red meat were less likely to smoke (5% vs. 14%), consume alcohol, and have diabetes. They were more likely to exercise and take a multi-vitamin. These behaviors are well-known markers of individuals who are interested in being healthy. These variables alone should be enough to call the results into question.
But there is an even more significant potential variable.
Those who ate the most meat also ate almost 1,000 calories more per day than those who ate the least. Almost 1,000 calories! If there is one thing nutritional science has taught us, those who over-consume calories with a mixed high-carb, high-fat Western diet tend to have worse health outcomes.
So when we look at this study, we must ask ourselves: How can we be sure it was the red meat contributing to the increased risk of heart disease?
Once again, based on the evidence, we can't draw this conclusion. This study is not designed to credibly prove anything about the harms of red meat or the benefits of eating more plants in place of meat. In contrast, studies that strictly grade the quality of evidence behind nutrition claims find that links between red meat and chronic disease are unreliable.
We can only hope that the media headlines don't overplay this study's significance, which tends to confuse rather than educate the public.
Instead of relying on headlines, we encourage you to read more about the evidence surrounding red meat in our evidence-based guide: Guide to red meat - is it healthy?
Thanks for reading,
Bret Scher MD FACC
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