Red Meat
Atkins, Keto, Paleo, 5:2… Grapefruit, kale, acai, kombucha…
For those who want to eat healthy, there's a seemingly endless buffet of advice, and every year something new (or not so new) generates buzz.
In this newsletter I'll be talking about the red meat controversy (i.e., the recent debate about whether typical levels of red meat consumption are healthy or not). Although I conclude with suggestions for how much red meat one can safely eat each week, this controversy shows why you shouldn't accept every dietary recommendation you hear.
Why the need for caution?
1. Not all dietary advice is scientifically based. For example, the mono diet (eat just one food, or one food group for an extended period) has never been evaluated in peer-reviewed studies, because it's clearly a terrible idea. If you happen to be eating a lot of unhealthy foods, replacing them with one healthy food may be a good start, but no single food or food group can meet all of your nutritional needs. For this reason, you won't find studies lauding the so-called Carnivore Diet (animal products only). Perhaps all one needs to know about this diet is that its main proponent, Shawn Baker, was recently stripped of his medical license. Here the problem isn't meat per se. Eat nothing but bananas and you may lose weight, but you'll also develop an irregular heartbeat (too much potassium) and eventually die (lack of essential nutrients). Death is an effective weight loss strategy, but I don't recommend it.
2. Most dietary research is methodologically limited. Blame the complexity of the topic rather than the researchers. What we're most interested in is how food choices affect our health over lengthy periods of time, and that's inherently difficult to study. Multi-year studies are expensive. Researchers must rely on participants' imprecisely reported eating habits. And, even if the self-reports were accurate, individuals differ in their metabolisms, their food choices, and their health-related behaviors, all of which make identifying cause and effect over time exceptionally challenging.
Here's the kind of study we need but, for ethical as well as practical reasons, we will never have: 1,000 healthy, physiologically similar 18-year-olds are asked to maintain identical diets and activity levels for 30 years. They're also asked to abstain from tobacco, alcohol, and other drugs. 500 of them are asked to eat red meat four times per week, while the other 500 are asked to abstain. This kind of long-term experimental study is an unattainable ideal in dietary research – a point I'll return to later.
3. Dietary research is often funded by industries with financial interests in the outcomes. One of my favorite examples is a review of findings entitled "Recent research on the health benefits of blueberries and their anthocyanins", published in the journal Advances in Nutrition. Sounds like a credible, somewhat technical paper published in a reputable journal, right? The first hint of something amiss is that the abstract (which is indeed technical) closes with the delightful but unscholarly remark that "it is widely agreed that the regular consumption of tasty, ripe blueberries can be unconditionally recommended." I love the sudden appearance of the term "tasty" – it's so deliciously unscientific. The article itself is relentlessly positive, to a far greater extent than the data warrant. At the very end, the authors disclose that "The United States Highbush Blueberry Council (USHBC) offered support for this article by providing an honorarium to each author..."
(I'm for blueberries too. Especially when they're tasty. But nobody paid me to say this.)
4. Nutrition advice is sometimes contradictory. It's almost a meme now that every few months or so a new study comes out with nutrition advice that contradicts the last thing you read about it. The red meat controversy is a great illustration of how and why this happens.
Actually, up until recently there wasn't much controversy. Considerable evidence linked high consumption of red meat to cardiovascular disease, type 2 diabetes, and various cancers. Although it's unclear exactly what constitutes "high", experts agree that some Americans consume too much red meat, that the health consequences of overconsumption are evident, and that some of us should therefore eat less of it.
This near-consensus was rocked in 2019 by the appearance of five reviews in a single issue of the prestigious Annals of Internal Medicine. The panel that authored these reviews concluded with the following, widely-quoted recommendations:
"The panel suggests that adults continue current unprocessed red meat consumption... Similarly, the panel suggests adults continue current processed meat consumption."
Scientific response was swift and unusually heated. Before I get to the methodological and statistical issues, it's worth mentioning three cultural objections that experts have raised. (I call these objections "cultural" because they point to violations of accepted practice in the culture of science.)
(i) The objectivity of the authors is questionable. The "panel" was self-appointed, only 2 of the 14 panel members had topical expertise, and Bradley Johnston, the lead author, failed to disclose industry-based funding and employment. As a result, Annals of Internal Medicine published a correction later noting the potential conflict of interest. (Attention to Dr. Johnston's industry ties led the journal to then publish a correction to one of his 2016 studies in defense of sugar, in which he also failed to note that he was paid to write the paper by a relevant industry group, in this case the International Life Sciences Institute, which is funded by Pepsi, McDonald's, Nestlé, etc.).
(ii) The editorial approach was problematic. Although Annals of Internal Medicine is one of the most prominent journals its field, concerns have been raised in this case about its internal process (several reviewers had recommended that the papers be rejected) as well as its public outreach (the journal circulated a press release entitled "New guidelines: No need to reduce red or processed meat consumption for good health" which is clearly not an impartial summary of findings).
(iii) Public health conventions were violated. Experts at Harvard's School of Public Health and elsewhere have noted that the recommendation to continue current levels of red meat (and processed meat) consumption is not sufficiently precautionary in light of available evidence. Experts have also pointed out that the panel chose not to consider environmental impacts, even though the beef industry is known to affect the environment in ways that undermine public health (for an excellent summary see here).
Now for the concerns related to methodology and statistics…
The panel's approach to the literature reviews doesn't seem problematic at first glance. The three main reviews focused on studies published during the past two decades that were either experimental or relied on longitudinal methods with at least 1,000 adults per cohort. A variety of health outcomes were considered (type 2 diabetes, cardiovascular disease, cancer, etc). Across all studies combined, data were obtained from over 6 million people. The main finding was that less red meat consumption was consistently associated with significantly reduced rates of type 2 diabetes, cardiovascular disease/mortality, and cancer mortality.
You read that right. The panel found exactly the sorts of associations identified in prior research. Why then was there any controversy? Why did the panelists end up recommending no changes to how much red meat we consume? What was their rationale? Here’s where we can see the root of some bad advice (which got a lot of press because it contradicted what we thought we knew).
First, the panelists downplayed the risks of red meat consumption by noting that effect sizes in the literature were small. "Small," however, is in the eye of the beholder. In a typical example, cross-study calculations showed that reducing red meat consumption by 3 servings per week would’ve resulted in 7 out of 1,000 fewer people dying of cancer within a 5 to 28 year period. Not just getting cancer, but dying from it. Is that "small?" Well…given the combined sample size (875,291), that's as many as 6,127 people during the time period in question. (I say "as many as", because some people were already eating fewer than 3 servings of red meat per week and therefore couldn’t have reduced their consumption by 3 servings. My rough extrapolation from the data predicts at least 4,000 fewer cancer mortalities if those who could've reduced their red meat consumption had actually done so.)
Other "small" but significant effects emerged when comparing the top third of red meat consumers to the lowest third. For example, those who consumed the least red meat averaged 3 in 10,000 fewer cardiovascular mortalities in a 13.8 year period (or less), and 5 in 10,000 fewer breast cancer mortalities in a 16.1 year period (or less). Are 3 or 5 in 10,000 small numbers in this context? I personally wouldn’t say that. Perhaps it's not even helpful to use terms like "small" to begin with...
In any case, my point is that the reviews consistently showed significant associations between red meat consumption and negative health outcomes. Small but significant. This should’ve been one of the bottom-line conclusions of the reviews, but it wasn't.
Along with references to small effect sizes, the panelists claimed that prior research was methodologically weak. During the review process, panel members evaluated the strength of the evidence provided by each study. To do this, they used a system called Grading of Recommendations Assessment, Development and Evaluation (GRADE), which was developed for use in evaluating drug trials. The problem with using GRADE to evaluate dietary studies is that it values experimental designs most highly, while downgrading observational studies. As mentioned earlier, both ethical and practical considerations make it almost impossible to conduct experimental studies on the long-term effects of diet. Long story short, the panelists judged a number of longitudinal studies as "low- or very-low" with respect to quality of evidence, even though those studies are widely considered rigorous, and the results are consistent with research, theory - and the panelists' own findings!
So, what's the bottom line? I'm going to be simpler and more forceful than usual here:
1. There's no controversy. Neither the 2019 reviews nor the panelists who authored them are credible. (And, since 2019, studies have continued to yield findings consistent with older ones (see here and here.)
2. Regarding your own dietary choices, seriously consider the evidence-based guidelines given by health organizations and experts, even if they’re not perfectly consistent. For example, in the UK, the National Health Service recommends no more than 630 grams of red meat per week. In the U.S., one widely referenced guideline is a cap of about 350 to 500 grams per week (about 12 to 18 ounces, cooked. The uncooked values are about 18 to 24 ounces). Although these recommendations aren’t identical, they do provide guidance.
3. Be cautious about any guidelines, if possible check out the research behind them (see my earlier comments), and expect some guidelines to change over time as the research evolves.
4. Read The Omnivore's Dilemma, by Michael Pollan, if you haven't already. You may reconsider meat.
Thanks for reading!