Several days ago I came across a "heated debate" between Dr.Greger, an expert in vegetarian diet (at least he was positioned like that in the video), and an unnamed keto promoter. During this debate Dr.Greger stated that "there is only one diet that had ever been proven to reverse heart disease in the majority of patients and that's all-plant-based diet". I was very curious about what he was referring to - I'm not a cardiologist, but during my years in medical school I should have been informed about such benefits of plant-based nutrition, don't you think?
He referred to it a couple times more and there was a reference to the actual scientific article1 in the video, so I felt compelled to dive deeper and to read this study. It seemed important if not for my psychiatric practice, but for my overall understanding of nutrition - a real study, a randomized controlled clinical trial (and you know how much I love them), on nutrition, published in the Lancet, the most reputable medical journal, by Dr.Dean Ornish, with what looks like phenomenal outcomes! How come I've never heard of it? If it was so great, people should be talking about it all day long.
Anyways, I decided that it's time to close the gap in my knowledge, so, with a mixed feeling of excitement and skepticism, I've downloaded the article, full-text, and after reading it I decided to start a new series of blogs - Critical Appraisal (yes, I was so impressed). So, let's do a critical appraisal of this remarkable study and see if the claim that "the all-plant-based diet reverses heart disease" voiced during the debate actually reflects the real findings of the study and what we can learn from it.
I decided to split this review into two parts as effectively we have two questions to answer - I will do an actual critical appraisal of the article in the second part of this review, whereas in this blog, I'd like to see if Dr.Greger's claim is really substantiated by the study.
All we need for that is the abstract - it's in the public domain, anyone can access it freely and, quite honestly, it has enough information to draw conclusions related to the claim. I'd like to start with good things - obviously the fact that it's a randomized controlled clinical trial is already amazing as it implies that people were randomly assigned to one of two interventions and thus, the design has eliminated lots of confounders. Then the fact that it was published in the most reputable medical journal, the Lancet, makes me feel that the peer-review process was very rigorous (more on that later, when we get to the full article review in Part II). At the same time there are several items in the abstract that confused me a bit. Let's tackle them one by one:
1. Randomization: The researchers have randomly assigned 48 subjects to the two groups and somehow managed to have the groups of n=28 and n=20 subjects. It's actually quite unlikely to end up with having two groups of such uneven sizes when you assign subjects randomly (check out the binomial distribution statistics, I'll make a post on that as part of my statistics and probability theory review). So, from the get go it looks like there is something odd happening with the randomization.
2. The authors don't talk about heart disease (it's a group of disorders), but they are rather focused on the coronary artery disease, a very specific condition, so the claim that the all-plant-based diet can reverse heart disease cannot be substantiated in principle.
3. The main problem is that the experimental group did not receive just a dietary intervention. No, it was a comprehensive intervention including low-fat vegetarian diet, smoking cessation, stress management training, and moderate exercise. Each of these components is well-known to reduce cardiovascular mortality and morbidity, so we could never ever be able to tell whether it was the diet or something else that contributed to the positive outcomes.
4. Finally, the outcomes: the changes in stenosis diameter are very slim - they observed a 2.2% reduction in the mean stenosis diameter (from 40.0% to 37.8%) in the experimental group and an increase of 3.4% (from 42.7% to 46.1%) in the control group. Notably, no mentioning of statistical significance in the abstract, though they did show the standard deviation (SD) values. Again, they did it for "before" and "after" means in each group, instead of the SD for the before-after difference in each group. These SD are rather large compared to the differences, but it would be statistically irresponsible for me to draw any conclusions based on that (more on that in the full review part). I'll just state what I see in the abstract: no statistical significance reported and the changes are very small, likely comparable to the measurement error, which cannot be important clinically.
Before I get to my conclusion and the summary of my analysis, I'd like to give credit to the authors who cautiously and responsibly stated the following as their conclusion: "Comprehensive lifestyle changes may be able to bring about regression of even severe coronary atherosclerosis after only 1 year, without use of lipid-lowering drugs." I believe that this statement is quite far from the one that we have heard in the video as the study was focused only on one specific type of heart disease, the intervention was complex and along with the diet it included number of other interventions, it had a very small effect size and, finally, it did not really "prove" anything - it simply indicated some positive tendency that must have been shown in multiple other independent trials in order for it to be considered a "proof". Also, in my humble opinion, the fact that someone quotes the study published in 1990 (effectively conducted in 1980s) as the ultimate proof of whatever they want to show is already a huge red flag - it seems that there are no other studies to support their theory.
In any case, I hope that this short digest helped you to see the difference between an exaggerated and sensationalist claim and an actual clinical trial. I will elaborate more on that in the second installment of this series. It will be fun, I promise - stay tuned! As always, feel free to ask questions, to make comments, to explore my website and to subscribe to my YouTube channel for updates.
1. Ornish D, Brown SE, Scherwitz LW, et al. Can lifestyle changes reverse coronary heart disease? The Lifestyle Heart Trial. Lancet. 1990;336(8708):129-133.