There is a vociferous choir of critics that makes its living, or some portion thereof, implying we know nothing reliably about dietary intake patterns and human health outcomes.
We absolutely, confidently, and irrefragably know the fundamentals of feeding Homo sapiens well- as we know the same for lions and tigers and bears; chicken, sheep, and horses. I will debate anybody, any time, on this topic- and if I lose, I will become a hula dancer.
What, then, is the basis for the critical claims? One is a misguided sanctimony about righteous research methods, as if one kind of study is the correct way to answer every question. That is no more true than the notion that one tool is best for every job. A hammer, for instance, makes an excellent hammer and a lousy saw.
Another is the wayward idea that if we don’t know everything, we can’t know anything. There are, of course, many details of nutrition we do not know, and in some cases never will, even while knowing the fundamentals. Somehow, in nutrition rather uniquely, the details we do not know are invited to obviate all we do know. Consider if the same rationale were applied to exercise.
We do not know if step 326 or step 623 is the “active ingredient” in the benefits of walking a mile; therefore, we must not know the benefits of walking. We do not know if blood flow to the right earlobe is better enhanced by a step with the left foot, or a step with the right- so again, we cannot know the benefit of walking. We cannot say with confidence that hiking is “better” than biking, or vice versa, to say nothing of swimming or dancing- and thus, we must not know anything about the benefits of any of these.
Such thinking is not applied to physical activity, presumably for the obvious reason that it is insipid to the point of idiocy. Yet such thinking passes for erudition in nutrition. It should not- but likely will so long as we, the people, are willing to stomach it.
And finally, a third is the ignominious treatment of nutrition research as the ball in a game of Ping-Pong.
The immediate provocation for this rhetorical hurl is a paper just out in the Lancet, entitled “Long-term secondary prevention of cardiovascular disease with a Mediterranean diet and a low-fat diet (CORDIOPREV): a randomised controlled trial.” To be clear, I am a fan of the Mediterranean diet for many reasons, and my vote, as a member of the US News & World Report judging panel, is among those that lifts it to the top of the “best diets” rankings year after year.
But what, really, does this title mean? There is latitude for the “Mediterranean diet,” to be sure, but there is also an established anatomy, and a validated measure of conformity to it. We can have some confidence what the designation denotes.
Not so at all for the “low-fat” diet. At the extremes, that could mean a stellar diet made up of diverse, whole plant foods that just happen to be low in fat: vegetables, most fruits, beans, lentils, whole grains- with some allowance for the higher fat content of certain fruits (e.g., olives, avocados), nuts, and seeds. At the other end of the diet quality spectrum, ingestion of- and only of- Coca Cola and Snackwell cookies would qualify as “low-fat.” For that matter, 2000kcal daily of sugar would presumably qualify, too.
In other words, in characterizing diet quality- something that can and should be measured objectively, and should be matched between diets if the research question is about the differential effects of diet composition- the stipulation of “low-fat” is of no use whatsoever. Diet quality cannot be inferred from it, and the field of nutrition has long abandoned the idea that summary avoidance of fat is beneficial or sensible. The concept derived from good intentions, but these were coopted and corrupted early, and has proven in practice to be misguided and now obsolete.
The particulars of the dietary assignments in this study- which enrolled roughly a thousand Spanish adults, mostly men- reveal that the Mediterranean diet was intended to be of reliably high-quality, while the low-fat diet was mediocre at best. Even more importantly- for those concerned with truth in advertising- it was not meaningfully low in fat either. The Mediterranean diet assignment aimed at 35% of total calories from fat, while the “low-fat” diet assignment aimed at 30%- and achieved a mean of just over 32%. This difference is inconsequential, and none of my colleagues devoted to studying the health effects of immanently low-fat whole food, plant-based diets would agree that this qualifies as low in fat. Nor do I.
So, the low-fat diet was not actually low in fat. But, it did exclude fatty fish (known to be the most health-promoting) and required lean fish (less healthful) in their place. It limited nut intake (known to be healthful); limited extra virgin olive oil (EVOO; known to be healthful) while allowing only for other, less healthful oils; and required consumption of low-fat dairy products 2 to 3 times daily. There are low-fat dietary patterns I could recommend with the same enthusiasm I have for the Mediterranean diet; to put it bluntly, this is not among them. Of note, the drop-out rate in the study was significantly higher, and adherence to the assignment considerably lower, for the so-called low-fat than for the Mediterranean group. Interpret that as the spirit moves you.
I trust you see the problem here. The next set of comparably qualified researchers may happen to favor some version of low-fat eating, and might use these exact methods to compare a high-quality, low-fat diet with a marginal Mediterranean diet, and “prove” the opposing conclusion. The nihilists, then, will be quick to chime in with: “see, we told you that nobody knows anything about nutrition.”
I hasten to note that I am not impugning the efforts of the researchers responsible for this particular paper; their work conforms to the prevailing standard, as affirmed by securing a rarefied perch on the prestigious pages of the Lancet. I am impugning the prevailing standard in which medicine, media, and we are all complicit.
Imagine a headline like this: “In Stunning Upset, Female Boxer Defeats Male Opponent.” The invitation here is clearly to picture two comparably qualified contestants, which, if true, would justify the titillating headline. But now imagine further that we learn only in paragraph 7 that the woman in question is an Olympian in her prime, and the “male” in question is a 9-year-old taking boxing lessons at the local gym. We would feel cheated by the divide between implication and reality, and the journalists, editors, and periodical involved would likely pay the price of our disdain.
But not so for nutrition. Peddle us just this sort of overcooked, ultraprocessed misrepresentation, and we eat it up again and again. A whole cottage industry of nutritional contrarianism runs on it, in fact, spawning professional notoriety, morning show segments, fad-diet best-sellers, and disheartening trends in the health and weight trajectories of a gullible public.
I am a fan of the Mediterranean diet. I am neither friend nor foe to “low-fat” diets because the term is meaningless with regard to what actually matters: wholesome foods in a balanced, sensible assembly and overall diet quality. When that balance is achieved, such diets can be among the best; when not, the rubric is a convenient straw-man in research that has determined the answer before asking the question. From my perspective, that is not genuine research; it is theater.
We absolutely, irrefutably, and decisively know the fundamentals of feeding our kind of animal well. They allow for diets both high and low in total fat.
Until or unless we can affirm that we know what we know, while allowing for all the particulars we as yet do not- the appetite for ultraprocessed, uninformative diet science shared among peer-reviewed journals, the media, and us- will go perpetually fed, and forever unsated. The contrarians will profit, and public health will suffer.
David L. Katz, MD, MPH, FACPM, FACP, FACLM, is the Founding Director (1998) of Yale University’s Yale-Griffin Prevention Research Center, and current President of the American College of Lifestyle Medicine. He has published roughly 200 scientific articles and textbook chapters, and 15 books to date, including multiple editions of leading textbooks in both preventive medicine, and nutrition. He has made important contributions in the areas of lifestyle interventions for health promotion; nutrient profiling; behavior modification; holistic care; and evidence-based medicine. David earned his BA degree from Dartmouth College (1984); his MD from the Albert Einstein College of Medicine (1988); and his MPH from the Yale University School of Public Health (1993). He completed sequential residency training in Internal Medicine, and Preventive Medicine/Public Health. He is a two-time diplomate of the American Board of Internal Medicine, and a board-certified specialist in Preventive Medicine/Public Health. He has received two Honorary Doctorates.