
You may have heard of the new Netflix docuseries suggesting that a vegan diet is superior to an omnivorous one for heart health. But is there solid evidence to back this claim?
In this article, I expose the flaws and conflicts of interest behind the study that inspired the documentary. I also explain why you shouldn't trust everything you see on Netflix when it comes to nutrition and health.

Is a vegan diet better for heart health than an omnivorous diet? If you watched the new Netflix docuseries “You Are What You Eat: A Twin Experiment,” you may have come away with that conclusion. But before you ditch your eggs and meat, you may want to take a closer look at the study behind the documentary.
The study, published in JAMA Network Open in November 2023, compared the effects of a vegan diet versus an omnivorous diet on various health outcomes in identical twins. The main finding was that the vegan diet group had a significant reduction in LDL cholesterol (the so-called “bad cholesterol”) compared to the omnivorous diet group. They also had significant improvements in two other health outcomes. (Note: I use the word “significant” only when I refer to statistically significant differences. However, some differences may be clinically relevant even if they are not statistically significant.)
Certain features of this study make it stand out—almost made for prime time:
It is a study done in identical twins. The twins act as their own controls, eliminating genetic influence as a confounding factor. However, unless twins have shared the same environment their entire life, this does not account for factors that may affect the twins differently, such as lifestyle, stress, or exposure to toxins.
It is a randomized clinical trial (RCT). This is considered the gold standard of evidence in medicine. However, this is a population-based RCT, which means that the researchers have no control over how well the participants followed the diet protocols. Additionally, there is no blinding, which means that both the researchers and the participants know which diet they are assigned to. This can introduce the Pygmalion effect and expectation bias, where the expectations of the researchers or the participants can influence the results.
It shows that a healthy vegan diet may be better than a healthy omnivorous diet for cardiometabolic outcomes. This is the authors conclusion, so much so that they even suggest that “clinicians can consider this dietary approach as a healthy alternative for their patients.” To fully elucidate this point, we have to dig a little deeper and follow the money, as they say.
Some people may already be thinking the obvious: “How can you conclude a certain diet is good for heart health after only 8 weeks?” The answer is, you can't. Weight and health outcomes are a result of consistent dietary habits for months or years. Therefore, the lab differences seen in these patients don't necessarily reflect the long-term effects of the diets. A longer follow-up period of at least several months would be required for that. That's one of several serious limitations in this study:
The sample size is only 44 people (22 pairs of identical twins). This means the study doesn’t have enough power to show any meaningful results. The authors acknowledge this: “a sample size determined by resource availability rather than a formal power calculation.”
Both groups were assigned to a diet that was healthier than their pre-study diet. Importantly, the diet was not “isocaloric.” This means the number of calories consumed each day was different. It is possible, therefore, that some individuals had a lower calorie intake than their requirements and as a result, lost weight. This weight loss may have contributed to the LDL reductions—which, to be fair, happened in both groups, but the reduction was greater in the vegan diet group.
Conveniently, the authors didn't emphasize the biggest point difference in the study: the large decrease in vitamin B12 in the vegan group (the mean difference was −103.0 pg/mL). Vitamin B12 is essential for the proper functioning of the nervous system, the production of red blood cells, and for DNA synthesis. At least the authors recognize that “long-term vegans are typically encouraged to take a cyanocobalamin (vitamin B12) supplement.”
Those in the vegan diet had a “lower dietary satisfaction.” In other words, they weren't too happy with their diet. This may affect their adherence and motivation to continue the diet after the study.
The chosen population was generally healthy and as such, the results may not be generalizable (the mean baseline LDL cholesterol was 114 mg/dL; not bad!) Therefore, the results may not apply to people with higher risk of heart disease or other conditions.
Since the participants' diets during the study were healthier than their pre-study diets, some of the positive effects could be attributed to what's called the “honeymoon period.” In fact, I argue that the “honeymoon period” effect is the most you can conclude from a study that lasted for only 8 weeks.
So, how can the authors draw such a strong conclusion that “a healthy plant-based diet offers a significant protective cardiometabolic advantage compared with a healthy omnivorous diet”? To answer that, we need to look at the incentives behind the study.
First, let's look at the conflicts of interest. One of the coauthors, Dr. Gardner, reported receiving funding from Beyond Meat (outside the submitted work). For those who don't know, that's a company that produces plant-based meat alternatives, which may benefit from the promotion of veganism. I wonder if the hype around this study has anything to do with the Beyond Meat stock price falling from a high of $235 to $6 today.
Second, let's look at funding. After all, clinical trials are very expensive to run. This study was funded in part by the Vogt Foundation. I did some digging. The Vogt Foundation is a private foundation that provides grants to various causes, such as education, health, and arts. But there's more to the story. The Vogt Foundation supports the Oceanic Preservation Society (OPS), another non-profit that promotes marine conservation and environmental protection. Its founder and executive director—Louie Psihoyos—is vegan (he has an interesting story on what brought him to veganism, which you can listen to here: The Gruesome Experience That Made Louie Psihoyos a Vegan). But guess what? He is also the director of the Netflix documentary, which is based on the study.
In conclusion, this is a biased study with serious conflicts of interest. It does not provide solid clinical evidence that a vegan diet is better for heart health than an omnivorous diet. Be your own judge.