I received recently a brochure from my Healthcare provider, Kaiser Permanente, that extolled the virtues of a plant-based diet. Full disclosure: My own diet is rich in veggies and fruits, and hardly any meats, except the occasional social barbecue. I promptly relegated it to the recycling basket, until some friends asked me to take a critical look at it.

The first question I asked myself was, WHY haven’t I done it until now. The honest answer is that I didn’t want to do it. Freud would have had a field day with this answer. But the reason is more simple and well-known to social psychologists: selective bias. The same reason conservatives avoid listening to MSNBC and liberals disdain Fox “news”. So now that I scoured the literature looking for solid evidence, where do I stand?


Primum non nocere

First, do no harm,” thus spake Hippocrates 2500 years ago, and every budding physician declaims on graduation day. And in this respect, the plant-based diet shines: Nobody has died from eating it—if you exclude patients with food hypersensitivities (peanuts, tomatoes) or celiac disease—and adverse reactions are practically nil. No wonder that even in the difficult and contentious field of nutrition science, a wide consensus has formed: The plant-based diet is “good for you”. But lack of “bad” doesn’t mean “good”. To qualify for this designation, the scientific bar is much higher.


Forks over knives

plant-based diet
Forks Over Knives

As part of my research, last night I watched the documentary “Forks Over Knives.” What does a documentary have to do with serious inquiry into a scientific field of research? I wondered myself. The documentary presents the work of nutrition scientist T. Colin Campbell and cardiologist Dr. Caldwell B. Esselstyn Jr. who proposed that most of our major health issues—including heart disease, cancer, and diabetes—can be prevented (and even in some cases reversed) by following a vegetables, fruits, and whole grains diet. Their work was rich in assumptions, correlations, clinical observations, and anecdotes—which at the time they did their work was all there was—but falls far short of today’s standards.

Curiously, there was no mention of another mostly plant-based diet advocate, Dr. Dean Ornish. His diet focuses on eating beans, legumes, fruits, grains, and vegetables. You are allowed to have low- or nonfat dairy products such as milk, cheese, and yogurt in moderation. Only 10% of calories are supposed to come from fat. Importantly, the Ornish program also includes stress reduction training.

So, in the spirit of paying credit where credit is due, what do we know about the effectiveness of the Ornish approach?

Dr. Ornish and his collaborators published widely and his program (that also includes stress reduction) is covered by Medicare—a high bar indeed. But, their work came under a withering attack in Scientific American, in an article bearing the provocative title “Why Almost Everything Dean Ornish Says is Wrong.” Wow! I braced myself for a devastating analysis of his life work but as I read it, I remembered that purple prose almost always betrays rather pallid evidence.

Let’s examine the substance of Melinda Wenner Moyer’s argument:

“Ornish goes to argue that protein and saturated fat increase the risk of mortality and chronic disease. As evidence for these causal claims, he cites a handful of observational studies. He should know better. These types of studies—which might report that people who eat a lot of animal protein tend to develop higher rates of disease—’only look at association, not causation’, explains Christopher Gardner, a nutrition scientist at the Stanford Prevention Research Center. They should not be used to make claims about cause and effect; doing so is considered by nutrition scientists to be ‘inappropriate’ and ‘misleading’. The reason: People who eat a lot of animal protein often make other lifestyle choices that increase their disease risk, and although researchers try to make statistical adjustments to control for these ‘confounding variables’, as they’re called, it’s a very imperfect science. Other large observational studies have found that diets high in fat and protein are not associated with disease and may even protect against it. The point is, it’s possible to cherry-pick observational studies to support almost any nutritional argument.”

She is absolutely right about observational studies. Regardless of the most sophisticated statistics, one cannot avoid the fact that they are just associations and do not, nay cannot, uncover causation. But, is Moyer setting up a straw man? Does Ornish actually make claims on the basis of observational studies only?

In his rebuttal in an op-ed in the NYT, Ornish indeed recounts many observational studies, all pointing to the benefits of plant-based diets and the harmful effects of animal fat, red meat, and high protein diets. Except that he is cherry-picking a bit.

The recent multicenter PREDIMED trial supports the notion that fat can be good rather than bad. It found that individuals assigned to eat high-fat (41% calories from fat), Mediterranean-style diets for nearly five years were about 30% less likely to experience serious heart-related problems compared with individuals who were told to avoid fat. (All groups consumed about the same amount of protein.) Protein, too, is actually OK; the 2010 trial published in The New England Journal of Medicine that found individuals who had recently lost weight were more likely to keep it off if they ate more protein. Further, the 2005 OmniHeart trial found that individuals who substituted either protein or monounsaturated fat for some of their carbohydrates reduced their cardiovascular risk factors compared with individuals who did not.


End of argument? Not quite

Ornish and his collaborators showed in randomized, controlled trials that his largely, but not exclusively, plant-based diet and lifestyle changes (e.g., stress reduction) can reverse the progression of even severe coronary heart disease. Episodes of chest pain decreased by 91% after only a few weeks. After five years, there were 2.5 times fewer cardiac events. Blood flow to the heart improved by over 300%.


Checkmate? Not so fast

plant-based diet
High Protein Diet
Source: targetwoman.com

The experimental group was put on a plant-based diet. But they also ceased smoking, got stress reduction training, and followed a moderate exercise regimen. And the control group? They continued in their old “bad” ways—eating red meat, high-fat and high protein, smoking (although Ornish states that only one subject was a smoker at baseline), and not exercising. So which is it? Is the plant-based diet responsible for the spectacular results? Or is it the exercise? Or smoking cessation? Stress reduction? Or a combination of these factors?

Want more “confounding factors”? Are all proteins created equal? Is bacon or sausage equivalent to egg white? Is low-fat milk as bad as whole milk? In other words: What about the quality of the macronutrients? Totally unaccounted for in those trials. Confounding indeed.

And what about the effect of age? Gerontologists have published many observational studies showing that slightly plump people over the age of 65 actually enjoy a longer (and happier?) life. How do you account for that?

In a study published in Cell Metabolism, Valter Longo and Morgan Levine at the University of Southern California in Los Angeles focused on data from 6381 adults over 50 years old who were interviewed once about their diet as part of NHANES, a national survey of health and nutrition. Longo’s team used death records to conclude that those under 65 whose self-reported diets they classified as high-protein—the participants said at least 20% of their calories came from protein—were at much higher risk of illness and death than a group who took in 10% or fewer of their calories from protein. The high-protein eaters were more than four times as likely to die from cancer over the 18 years after they were surveyed, and 75% more likely to die of any cause.

But, (there is always a ‘but’ to spoil the fun) in the over-65 crowd, those who ate lots of protein survived longer, on average, than those who ate less. Longo speculates that elderly individuals may be less likely to absorb the protein they take in, so need more of it.

Perplexed? You are not alone. And I won’t bore you with the molecular science, showing that high protein intake could increase the level of IGF1, a growth factor associated with obesity, type 2 diabetes, and cancer.


So what’s the bottom line

As I said at the beginning, nothing’s wrong with plant-based diets. But there is no reason to be a fanatic about it. In fact, five years ago, Ornish put Bill Clinton on a diet that included salmon once a week; and featured on the cover of his most recent book, The Spectrum, is a slab of salmon fillet! So what do we get if we add salmon and extra virgin olive oil to the plant-based diet? The Mediterranean Diet! And it’s backed up by excellent science. So what’s not to like?

And that, dear reader, is my take. Follow this diet, exercise, mellow out, and most important, don’t smoke—and you’ll be OK.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. Great review – have to say olive oil is plant based the last time I checked. Cleveland Clinic has come our and said plant based diet better than Heart Association Diet. The benefits out weigh any problems.


    • Thanks for the kind comment. There is really no downside to the plant-based diet. But there is no reason to adhere to it religiously. You could safely add olive oil, nuts and fish. This is the Mediterranean diet; it received strong scientific evidence ( see link in the post for NEJM, 2010) and it is a lot easier to maintain long term.

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