.Are Plant-Based Vegan Diets Healthier for Humans?

How Stanford used identical siblings to study the benefits of a plant-based diet

In 2021, Bay Area photographer, vegan and docu series producer Louie Psihoyos wanted to take a fresh look at the modern nutrition debate. He reached out to Stanford Medical School to study and film the effects of a plant-based diet on identical twins, assigning one a vegan diet while the other remained omnivore. 

Intrigued by the idea of working with twins as a way to minimize variables like age and genetic factors,  Stanford study senior author Christopher Gardner, Ph.D. explains: “Louie didn’t know how this could/should be done. We brainstormed some ideas and when it became clear that we could likely form a productive collaboration and conduct a legitimate research study, we moved forward.” 

The result is a Netflix series, You Are What You Eat: A Twin Experiment, released on January 1, 2024. The undertaking has generated no small amount of controversy, as might be expected with findings that challenged the American way of life and the large industries that promote consumption of animal protein.

Contrasting the two dietary approaches may seem random, yet vegetarianism in its multiple versions has gained considerable attention in recent years. While vegetarians avoid meat and seafood, vegans eschew all animal products, including eggs and dairy. Meanwhile, omnivores embrace a diet that encompasses a wide range of food categories, most closely resembling the standard American diet. 

According to a 2023 study, public interest in dietary habits is growing in time with the obesity epidemic and related disease rates. Amidst this sea of confusion, one thing is clear: the omnivore approach, as currently practiced, isn’t working. 

Dr. Gardner, along with co-authors Matthew Landry, Ph.D., and Catherine Ward, Ph.D., collaborated to design the study they called “Cardiometabolic Effects of Omnivorous vs. Vegan Diets in Identical Twins.” It was conducted from May through June 2022, during which time 22 sets of identical twins were randomly split into groups that would determine their food choices for the following eight weeks. 

Overall, the study found that following a vegan diet improves cardiovascular health. “Vegan eaters’ low-density lipoprotein—which is the bad cholesterol—dropped on average by 15.2 milligrams over eight weeks [while] the omnivore participants’ low-density lipoprotein fell by 2.4 milligrams over eight weeks,” the study concluded. “Vegan participants shed an average of around 4 more pounds than omnivores, and their insulin dropped by 20% more.”

While both eating plans were as comparable as possible calorically and nutritionally, the omnivore diet was most similar to participant pre-study eating habits. Both were rich in vegetables and whole grains, but one group included a moderate amount of meat while the other followed a strict plant-based regimen.

Even though the food was flavor-optimized (no one was force-fed tofu), it’s a big deal to overhaul one’s normal eating habits for two months with no meat, no dairy and no exceptions. If you watched the docuseries, you may remember the range of emotions both before and during the big reveal, during which twins were randomly assigned to their groups. 

Beforehand some participants were curious, some were “fine with going vegan,” while others waited in what was hopefully mock terror to open the slip of paper assigning their group. 

One participant, Aleksandra Shai Chai, told the Washington Post that she needed a moment to process the idea that she would be stuck eating a vegan diet for eight weeks. She hoped hers would say “omnivore.”

Instead, it read “vegan.” Her twin sister, Mariya Foster, would eat a diet of meat and vegetables, while Shai Chai replaced her favorite foods—bacon, sushi and steak—with tofu, beans and vegetables.

During the initial four weeks, both groups received pre-made meals and dietary guidelines, which transitioned to self-cooked meals for the subsequent four weeks. Throughout the study, a registered dietitian was available for guidance, Stanford University noted in a news release.

The results were significant. After the eight-week period, participants adhering to the vegan diet exhibited lower insulin levels, reduced weight and diminished lipoprotein cholesterol levels, associated with cardiovascular risks.

“The findings suggest that a healthy plant-based diet confers significant cardiometabolic benefits compared to a healthy omnivorous diet,” the researchers concluded.

While Aleksandra Shai Chai says she didn’t love the diet, upon learning the study’s results she felt thankful that she had briefly changed her eating habits. 

Excited about the global exposure the documentary provided, Landry plans to focus on identifying the best diets for chronic disease prevention, with an emphasis on promoting the holistic benefits of plant-based diets on overall well-being and health.

Habits Die Hard 

Behavioral psychologists will confirm, knowing what to do has little effect on lasting habit change. What looks good on paper seldom evolves into real-life practice long term. For many people, the idea of giving up meat feels like losing a slice of Americana. Can you imagine life without barbecue?

Or without sushi. Shai Chai reported that after four weeks of a vegan diet, she had more energy and slept better. Still, she missed her favorite foods. While some participants continued to follow a vegan diet after the study concluded in July 2022, Shai Chai said she immediately started eating sushi again—which she said tasted better than ever. 

But she hasn’t completely reverted to her pre-vegan days. She says she is more mindful of eating healthy and has developed new meal ideas.

Short of a health goal or diagnosis, changing lifelong eating habits usually takes an aha moment, and that’s where the study authors really got it right. The docuseries portrayed a diverse group of people, which increases relatability among the audience. And the changes were both challenging and revealing. 

This opportunity to get a close-up look at such a personal process gave viewers a better understanding of what to expect, and more importantly, what transitioning to plant-based eating entails. 

Twins Study Backlash

In the complicated world of nutrition, not all experts are on the same page. There is vast and fervent disagreement about what constitutes a healthy diet. So naturally, there has been some backlash against the Stanford study, including by Oprah-level nutrition experts like Drs. Mark Hyman and Peter Attia.

Dr. Attia, an omnivore advocate and longevity expert went straight for the jugular with his recent article,  A study comparing the effects of vegan and omnivorous diets fails Science 101. In it, he accuses the authors of promoting plant-based diets as the solution to all of the health woes of modern society, yet in doing so commits a “categorical failure to isolate and test a specific independent variable–which, as emphasized in any sixth-grade science class, is perhaps the most basic requirement for hypothesis testing.”

He maintains that the study didn’t continue for long enough to reveal whether the vegan dieters would stick to their new eating habits, especially since their self-reported enjoyment of the food ranked lower than the omnivore twins.

Dr. Hyman, although not vegan, is known as a plant-based food advocate. Yet, he criticizes the absence of important body composition markers, which he says were not reported in the study. 

In a recent podcast, The Doctor’s Farmacy, Why the Vegan Twin Study Got it Wrong, he claims that “somehow the authors didn’t seem inclined to want to publish” some of the most important findings of the study because they contradicted their point of view.

These contractions are surprising to no one that follows nutrition news. US News and World Report conducts an annual survey of popular diets—30 in all—each with its own unique menus and guidelines.

Some critics of the twin study assume the authors had a hidden agenda. The researchers are accused of not telling the whole story, “cherry picking” the facts. 

Metro caught up with lead researcher Christopher Gardner to ask his response to criticisms from influencers like Mark Hyman and Peter Attia who question the study methodology. “We are disappointed that these two critics didn’t reach out to us to discuss their criticisms before publishing them,” Gardner responded.

“The majority can be easily addressed. It is unfortunate that their approach contributed to, and perpetuates, what seems like a lack of consensus and controversy on nutrition topics. I find that once most of these issues are put in the proper context and discussed openly, there is far more consensus than controversy.”

Are Criticisms Misdirected?

Dr. Attia and others have criticized the Stanford twin study for the eight-week timeline, which may be too short to assess long term adherence. His article says    it takes more than eight weeks to adapt to the new diet pattern—“acquire new tastes, learn new recipes, and so on.”

“The cost of the study would skyrocket if you kept following each study participant for months or years to find out how long they maintained the diet change,” said Gardner. “For LDL-cholesterol and most of the cardiometabolic risk factors we measured, eight weeks was more than enough.”

The study was designed to answer a specific set of questions rather than addressing every facet of the dietary concern—i.e. weight loss, blood glucose levels, permanent behavior modifications.

“A study duration needs to be of sufficient duration to see a change in the primary study outcomes,” said Gardner. “The primary outcome (as indicated in advance and registered on ClinicalTrials.Gov) was LDL-cholesterol.

“When someone changes from one stable diet pattern to another stable diet pattern, changes in blood cholesterol maximize and then restabilize after about two weeks. Further change is negligible after those two weeks,” continued Gardner. “The same can be said for blood pressure and glucose. For LDL-cholesterol, and most of the cardiometabolic risk factors we measured, eight weeks was MORE than enough.”

Which leads us back to the number one goal for most dietary changes: What about weight loss? 

To this point, the researcher advises weight loss to be much longer—six months minimum but preferably a year or more. Weight loss is typically rapid for about three months, then tapers off until six months, and then typically restabilizes. Or for some people this is the start of weight regain.

Carnivorous critics of “You Are What You Eat” have dismissed it as vegan propaganda. Others point out that the study received partial funding from Kyle Vogt, a vegan tech entrepreneur and executive director of the Netflix series. 

However, these funding sources don’t undermine the study’s scientific integrity. According to Walter Willett, a professor at Harvard’s school of public health, the study was well-conducted, with both vegan and omnivore diets being notably healthy.

Its findings align closely with existing research in nutrition and environmental health, emphasizing the importance of reducing meat and dairy consumption while increasing plant-based foods, particularly in affluent countries.

“There is no body of evidence that conflicts with the finding that a healthy plant-based, vegan diet as implemented in this study, is better than a typical omnivore diet,” Willett is quoted as saying.

Clearly there’s more to a vegan lifestyle than eight weeks without meat. And more than one reason to swing vegan. 

Shift Toward Sustainability

Health is just one of the reasons why people go animal-free. In the Netflix series, vegan advocates also highlight the adverse environmental impacts of meat and dairy farming, including climate change, deforestation and animal abuse. While this has been called vegan propaganda by some critics, these claims—though seemingly exaggerated—are sadly spot-on. 

Meat, especially beef, is by far the food sector’s biggest greenhouse gas emitter, and no solution to these emissions exists that would offer significant reductions—except scaling down production.

The environmental impacts of animal products markedly exceed those of plant-based alternatives; 83% of the world’s farmland is used for meat, aquaculture, eggs and dairy, contributing to 58% of food emissions, despite only providing 37% of our protein and 18% of our calories (Godfray et al., 2018). 

Statistics aside, convincing Americans to reduce their intake is challenging, especially with decades of government policies favoring the meat and dairy industries. Biological and technology “advances” to increase growth and production have lowered production costs to keep animal products cheap and plentiful. 

On average, Americans consume approximately 220 pounds of meat, 280 eggs and 660 pounds of dairy annually, surpassing consumption levels in most other countries. As such, Americans eat 40% more meat and 80% fewer vegetables than a healthy diet requires, according to MedicalNewsToday.com.

Even though 25% of people surveyed say they want to cut back, according to SentientMedia.org, per capita consumption continues to rise. Why are people eating against their own best interests? One concern regularly expressed is about getting enough protein. 

The beef industry suggests we need meat for protein because plants just don’t cut it. Vegan elite athletes like Venus Williams and Rich Roll beg to differ. Still, this old myth is kept current through meat and dairy industry marketing campaigns, no matter how long ago it was debunked. 

Influence on everyday food choices stems from multiple sources, including marketing, lobbying and agricultural policies. Large food corporations shape consumer preferences through advertising and product placement, and political decisions impact food production subsidies and regulations. Additionally, economic factors such as pricing and availability can further sway individuals’ food choices.

Dr. Attia writes in his twins study critique: “It seems that every few years, a new documentary promotes plant-based diets as the solution to all of the health woes of modern society, and recently, another has been added to the list.”

While “solution to all the health woes” may be an overstatement, a plant-based diet does address a number of health woes, and many environmental woes as well. With such a vast disparity between meat-industry influence and vegan benefit awareness, a new documentary every few years seems hardly enough. 

Behavioral Interventions

Most people struggle to stick to diets, including vegetarian and vegan diets. For example, only one of the people in the vegan cohort decided to stay vegan after the completion of the study. 

However, going vegan for two months did have a positive effect on most of the participants assigned to the vegan diet—all but one said they planned to eat more plant-based foods than they had prior to the study. For those looking to improve their health—and shrink their carbon footprint—by eating more plant-based meals, it may make sense to shift away from meat and dairy in stages.

“Each step in the direction of a healthy plant-based diet will have benefits for personal and planetary health,” said Willett. “One doesn’t need to be a strict vegan to have major benefits.” But if you’re going to eat less meat and dairy, it’s important to prioritize healthy plant-based protein sources like nuts, soy foods, grains, beans and other legumes, Willett said. 

Other barriers on the path to veganism are a perceived lack of convenience of meatless meals, lack of familiarity, and a negative perception and expectation towards the taste of plant-based meals, according to studies. But at the same time, there are so many new and delicious plant-based foods coming onto the market in response to growing demand. In other words, as plant-based diets become more mainstream, delicious options are more available, making it easier than ever to go vegan. 

A Stanford press release describes the twin study as step one in a process of expanding beyond the plate into the study of ”behavioral interventions” to assist people and making choices more aligned with health goals. 

A separate team of researchers led by the Stanford School of Medicine have honed in on a set of simple food swaps that can make a big difference in environmental impact—without a drastic dietary overhaul. The suggestions include exchanges as easy as replacing beef with chicken in a burrito or selecting plant-based milk over dairy. If universally adopted, such choices would lower the U.S. dietary carbon footprint by more than 35%, the researchers found.

“Many people are concerned about climate change, but sweeping dietary change can be hard,” says the study’s lead author, Anna Grummon, Ph.D., an assistant professor of pediatrics and health policy. “Instead, we’ve identified simple, achievable substitutions—small changes—that can still produce a meaningful impact.”


  1. Thank you, Metro & Elizabeth Borelli, for this write up. I’m Carolyn, one of the twins in the Netflix docuseries and a participant in the study. Since the release of the show on Jan. 1, I’ve been asked by strangers every day about various aspects of the series. I appreciate this write up and especially appreciate Christopher Gardner’s invitation to continue these conversations in our communities and especially in our families.

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