When my vegan friends and colleagues say that a vegan diet is categorically the best for human health, it is an excess of zeal. We simply do not have evidence to substantiate the claim. I know; I've looked, hard.
If you are inclined to doubt my assertion, just consider what such evidence would look like. I don't think, when we say “best for human health,” that any of us really means: produces the lowest LDL over a 6-month period. I don't think we mean produces the highest antioxidant levels in the blood, or the lowest blood pressure over a span of months either. I think what we really mean is: leads, on average, to the most years in life, and the most life in years over an entire lifetime. The “best health” is enjoyed by people who live long, and prosper from their enduring vitality.
Given that, the trial to show that any given diet is truly “the best” has not been done, and is unlikely ever to be done. First, the outcome measure would need to be a composite of years in life and life in years over a lifetime, meaning something like quality-adjusted life years measured over many decades. The trial would need to run for 100 years.
Second, if we are to measure the effects of diet on health, we have to consider that those effects may be exerted early, late, or in between. For instance, we have some reason to believe that it is the diet and health of young girls that most influences the likelihood of breast cancer in the women they will become. With such considerations in mind, our intervention would need to cover all phases of the life cycle to be robust.
I could keep going, but the conclusion is already clear, so let's just stipulate it. To show, decisively, that any given diet is “best” for human health would require randomly assigning a large number of women soon to become pregnant to each of the competing dietary assignments, since we know that dietary effects begin in utero. Then, each of the neonates would need to be breast fed by a mother adhering to their dietary assignment, and then after all being weaned at the same time, the babies would need to adopt their diet: vegan; Mediterranean; Paleo; etc. They would then need to be followed for the rest of their lives to determine which group “wins.” The investigators planning and running the trial would not live to see it concluded, and the cost would be stupefying.
So, no, it has not been done, and won't be. The result is we have a vast amount of evidence about the fundamentals of healthy eating, and no definitive evidence to say which variant on the well-established theme is truly the best. Claims to the contrary are, as noted, born of zeal.
I note in passing that the direct comparisons of “this” dietary pattern to “that” suffer a variety of important limitations. Generally, they are of rather short duration, and none has ever run for the decades required to give us meaningful data about lifelong effects. Generally, they are directed at a primary outcome of only secondary significance, such as weight, or blood pressure. Most importantly, all such comparisons tend to be conducted by investigators with a particular interest in a particular diet, and that diet inevitably gets favorable treatment. Those, for instance, seeking to show the benefits of a low-carb diet will tend to compare it to a straw man version of a low-fat diet; and vice versa, just as surely. What we really need, and never get, if we are to say what specific diet is truly “best” for health, is a comparison of the optimal version of each contestant to its comparably optimized counterparts. Good luck finding that.
Because of this, claims that any given diet is the one best choice for human health represent the triumph of rhetoric over research. The evidence is not there; not for those in the vegan vanguard, not for the adamant Paleo proponents. Just as surely, the evidence is there- abundantly, consistently, and transculturally -- for the fundamentals of healthy eating that transcend the claims for any specific variant on the theme.
I am not a vegan myself. I eat a mostly, but not exclusively, plant-based diet. But my sympathies angle toward the vegans, for various reasons. Consequently, I generally regret the excessive zeal and unverifiable claims about health made to advance the vegetarian cause; they are unnecessary. There are important considerations in the mix other than human health.
Human beings are, irrefutably, omnivorous. There are specific aspects of our physiology particular to meat consumption, and perhaps even the consumption of cooked meat per se. There are adaptations even since the advent of civilization that are particular to dairy consumption. Valid arguments over the place of dietary meat and dairy in human health are far more challenging and nuanced than the opposing clamors would suggest, and might well come down to: what meat, and which kind of dairy?
But there are considerations other than human health we simply cannot ignore. I cannot improve on a recent headline in IFLScience: “meat is a complex health issue but a simple climate one.” Amen. Whatever the arguments for the merits of game in the diet, they just don't pertain to a population of 7 billion Homo sapiens busily cooking their planet.
I'm mostly with the vegans as well on the issue of ethics. I don't, by any means, think it is intrinsically unethical to eat meat; many of our fellow species do so, and some have no choice. But we humans, obviously, don't just hunt, kill, and eat animals anymore. We at times torture and abuse them. We force feed and fatten them.
Our capacity for double-bookkeeping on the subject is impressive. I imagine, for instance, that some who enjoy a visit to their neighborhood zoo in the spring to see the delightful antics of newly born animals might also enjoy veal parmigiana. Veal, as you no doubt know, is the flesh of calves; animals raised to be slaughtered, and eaten, while in that very phase of exuberant youth that makes closely related animals at the zoo so captivating. While standards of treatment have improved, veal historically comes from calves that are fattened and tenderized by strict confinement all the days of their short lives. There is the comparably incongruous tendency to adopt into our very families some beloved, four-legged mammals; while putting others of equal or greater intelligence and sensitivity on our dinner plates. That's not cannibalism exactly, but it's not entirely rational, either.
Among my many detractors out here in cyberspace are some inclined to beat me over the head with a figurative leg of lamb as a matter of routine. They seem to imply they have personally discovered the tonifying wonders of meat, to which I am oblivious. They state explicitly that my arguments for mostly plant-based eating by the human population at large make me benighted, unenlightened, ignorant, and/or conflicted. But they are wrong.
I am a public health pragmatist, not an ideologue. I know that debate over the health benefits of grass-fed beef will be moot when we've run out of water to grow grass. I know that the relative contributions of salmon to our health will be impertinent after we've eaten the last one.
The vast preponderance of real-world evidence of health outcomes in actual populations argues for diets of food, not too much, mostly plants. That could be enough. If it's not, we might constructively consider the plight of the planet, or the sorry fate of those veal calves.
There simply is no need for overzealous argument to make the case that we are in serious trouble if we keep eating as if the future does not depend on our choices. It does.
David L. Katz, MD, FACP, MPH, FACPM, is an internationally renowned authority on nutrition, weight management, and the prevention of chronic disease, and an internationally recognized leader in integrative medicine and patient-centered care. He is a board certified specialist in both Internal Medicine, and Preventive Medicine/Public Health, and Associate Professor (adjunct) in Public Health Practice at the Yale University School of Medicine. He is the Director and founder (1998) of Yale University's Prevention Research Center; Director and founder of the Integrative Medicine Center at Griffin Hospital (2000) in Derby, Conn.; founder and president of the non-profit Turn the Tide Foundation; and formerly the Director of Medical Studies in Public Health at the Yale School of Medicine for eight years. This post originally appeared on his blog at The Huffington Post.