What You Need to Know About Plant-Based Diets

By Dov Michaeli, MD, PhD | Published 5/6/2019 11

consuming more fresh fruits and vegetables

My healthcare provider, Kaiser Permanente, once sent me a glossy brochure extolling the virtues of a plant-based diet. My own diet is already rich in fruits and veggies and I rarely eat meat except for the occasional social barbecue.

So, I promptly relegated the brochure to the recycling basket. I didn’t think about the topic again until some friends asked me to take a critical look at it.

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 film 791 x 1024As part of my research, I watched the documentary “Forks Over Knives.” What does a documentary have to do with a serious inquiry into a scientific field of research you might ask? I wondered myself.

The documentary presents the work of nutrition scientist T. Colin Campbell and cardiologist Dr. Caldwell B. Esselstyn Jr. They proposed that most of our major health issues can be prevented and even reversed, including heart disease, cancer, and diabetes. How? By following a diet of vegetables, fruits, and whole grains.

Their work was rich in assumptions, correlations, clinical observations, and anecdotes. That’s because at the time they did their work was all there was. However, this approach falls far short of today’s scientific standards.

Finally, a randomized controlled study

Then along came Dean Ornish, MD, and his collaborators. They carried out a randomized, controlled trial which is the gold standard in research.

They found that a predominantly (but not exclusively) plant-based diet combined with lifestyle changes, such as 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%

But, that’s not the end of the story.

Let’s look at the study design

There is always a ‘but’ to spoil the fun. A deeper dive into the study design showed that 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,
  • high protein,
  • smoking (although Ornish states that only one subject was a smoker at baseline), and
  • not exercising.

So, what was responsible for the spectacular results? The plant-based diet? The exercise? Or smoking cessation? Stress reduction? Or a combination of these factors?

There are statistical methods to account for such confounding factors but it requires a very large number of participants to increase their validity. There were not enough people in this study to do that.

Another strengthened the conclusions

That being said, a later study strengthened the conclusions of the early studies that plant proteins were superior to animal proteins in terms of health.

Song et al.’s large (131,342 participants) prospective cohort study of U.S. nurses and other health care professionals was published in 2016. It described the association between animal protein intake and cardiovascular, cancer, and all-cause mortality.

The major conclusion of this study was that higher intake of animal protein (including processed red meat, unprocessed red meat, dairy, poultry, and eggs) was associated with higher mortality. Whereas, a high intake of plant protein was associated with lower mortality. 

Are all plant-based diets created equal?

Before going any further, let’s pause for a minute to explore whether all plant-based proteins created equal. We can answer with an (almost) rhetorical question: Are mashed potatoes equal to broccoli in their effect on cardiac health?

In a prospective series of 209,298 participants, Satija and colleagues examined the relationship between cardiovascular disease and healthful and unhealthful plant-based diets.

They used food intake surveys to index dietary patterns into graded plant-based diet indices. The index of the overall study population was called the plant-based diet index or PDI. The other two indices were the healthful (hPDI) and the unhealthful plant-based diets(uPDI). 

They found that higher adherence to PDI was independently associated with a lower incidence of cardiovascular disease (hazard ratio 0.92) compared to the diet of the general population which has a hazard ratio of 1.0.

But when they looked at the ‘healthful’ group (hPDI), the hazard ratio dropped even more to an impressive 0.75. But the ‘unhealthful’ group (uPDI) had an alarming increase in the hazard ratio to 1.32.

These results are unequivocal. Good fruits and veggies showed -0.25% drop in the risk of heart disease, however, bad ones showed a 32% rise in risk.

Does age make a difference?

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. They had all been 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 (at least 20% of 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, (again the pesky ‘but’) 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.

Do you need to go completely vegetarian?

So do you need to go completely vegan or vegetarian to mitigate the risks of cardiovascular disease, stroke, and cancer? The answer to that is no. A more varied and equally healthful alternative is the Mediterranean diet that includes a moderate intake of protein from animal sources, such as certain fish and chicken. The intake of dairy products, red meat, processed meats is infrequent.

An excellent study in the New England Journal of Medicine showed that among persons at high cardiovascular risk, a Mediterranean diet (supplemented with either extra-virgin olive oil or with nuts) reduced the incidence of major cardiovascular events. The hazard ratio was 0.69 for oil supplemented diet and 0.72 for the nuts supplemented diet. These are slightly superior to 0.75 hazard ratio of a strict vegetarian diet.

So what’s the bottom line

As I said at the beginning, plant-based diets are good for you but there is no reason to be a fanatic about it. In fact, five years ago, Ornish put former President Bill Clinton on a diet that included salmon once a week. Further, he featured a slab of salmon filet on the cover of one of his books, The Spectrum: A Scientifically Proven Program to Feel Better, Live Longer, Lose Weight, and Gain Health

So, dear reader, my bottom line is this:

Follow a good diet, be it vegan, vegetarian, or Mediterranean; exercise, and mellow out. Most importantly, don’t smoke. And you’ll be OK.


Related Content:
The Most Advanced Lifestyle of Primitive Hunter-Gatherers
Can You Have Your Carbs and Eat Healthy, Too?

This article was first published on June 7, 2015. It has been reviewed by the author and updated for republication.

Dov Michaeli, MD, PhD

Dov Michaeli, M.D., Ph.D. (now retired) was a professor and basic science researcher at the University of California San Francisco. In addition to his clinical and research responsibilities, he also taught biochemistry to first-year medical students for many years.

During this time he was also the Editor of Lange Medical Publications, a company that developed and produced medical texts that were widely used by health professionals around the world.

He loves to write about the brain and human behavior as well as translate knowledge and complicated basic science concepts into entertainment for the rest of us.

He eventually left academia 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 that developed products to improve post-surgical pain control.

Now that he is retired, he enjoys working out for two hours every day. He also follows the stock market, travels the world, and, of course, writes for TDWI.


  • HI. What do you make of the claims that oils (olive oil, coconut oil etc) are unnecessary and dangerous fats..” the savoury version of refined sugars.”? From my understanding, Ornish, Esselstyn etc. claim that you get enough fats from the plant based foods in themselves and don’t need these extra extracted oils. Some people think after quitting oils their health improved.

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