Vegan diet versus meat-based diet: Identical twins followed different meal plans, here's what happened

While identical twins share a significant amount of DNA, their dietary choices can lead to very different health outcomes for them.

In March 2022, 22 sets of identical adult twins participated in a randomized clinical trial in which one twin adopted a vegan (fully plant-based) diet — and the other ate an omnivorous (meat-eating) diet.

The findings, published in JAMA Network Open on Nov. 30, 2023, showed that the twins who consumed a healthy vegan diet had "significantly improved low-density lipoprotein cholesterol concentration, fasting insulin level and weight loss" — compared to the twins who ate diets containing meat.

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LDL (low-density lipoprotein), also known as "bad cholesterol," should be kept low to reduce the risk for cardiovascular disease, experts agree.

The reduced fasting insulin level, which dropped by 20%, indicates the twins had less insulin resistance, which means they were less likely to develop diabetes and metabolic syndrome. 

While identical twins share a significant amount of DNA, their dietary choices can lead to very different health outcomes. (iStock)

"Even when compared to a healthy omnivorous diet that includes plant foods, there can be additional health advantages to incorporating more plants in the diet, and eating less meat," lead researcher Christopher Gardner, PhD, a professor of medicine at Stanford Medicine, told Fox News Digital.

All participants were healthy and had no history of cardiovascular disease.

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The twins adhered to their assigned diets between May and July 2022, for an eight-week period.

Both the vegan and omnivorous diets were considered healthy — "replete with vegetables, legumes, fruits and whole grains and void of sugars and refined starches," the study article noted. 

The vegan diet contained only plant-based foods, with no meat or animal products. 

The omnivore diet included chicken, fish, eggs, cheese, dairy and other animal-sourced foods, the researchers noted.

Both the vegan and omnivorous diets were considered healthy — "replete with vegetables, legumes, fruits and whole grains and void of sugars and refined starches," the study article noted.  (iStock)

For the first four weeks of the study, the participants’ meals were provided by a delivery service. 

For the second half of the study, the twins all cooked their own meals.

Researchers gathered weight data and blood samples for the participants at the start of the study, then at the four-week and eight-week marks.

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When evaluating the outcome at the end of the eight weeks, the Stanford researchers partnered with a Kentucky-based company called TruDiagnostic, which measures biological age based on epigenetics, the study of how someone’s environment and lifestyle affect their genes.

"Following a vegan diet in this study has been linked with better cardiovascular health, but a completely vegan diet may not be the right answer for everyone."

"The TruDiagnostic group initially reported back to us that one of the groups had experienced a statistically significant shift that implied a decrease in biological age. Both the vegan and omnivorous diets were considered healthy — "replete with vegetables, legumes, fruits and whole grains and void of sugars and refined starches," the study article noted, as Gardner told Fox News Digital.

"At the time, they were blinded to the study groups and didn’t know if this was for the omnivorous or the vegan group."

Based on the study findings, the researchers recommend that people try experimenting by adding more vegetables, beans, fruits, nuts, seeds and whole grains to their diet.  (iStock)

It turned out the vegan group was the one that showed the decrease in age. 

"This was the most surprising finding for me," said Gardner. "I assumed the eight-week intervention from this study would be too short to cause any meaningful changes."

Looking ahead, he noted that further research is warranted to replicate these findings.

Dietitians weigh in

Michelle Routhenstein, a New York City-based cardiology dietitian who was not involved in the study, said there are many different factors that could explain the outcome.

"People seeking to change their diet should seek assistance from a registered dietitian to ensure they are meeting their nutrient needs and health goals."

"The cardiometabolic advantages observed in healthy, adult identical twins adhering to a healthful plant-based vegan diet, in contrast to those on a healthful omnivorous diet, can be explained by factors such as decreased saturated fat intake, increased dietary fiber, increased antioxidant content, incorporation of beneficial fats, enhanced insulin sensitivity and potential positive effects on the gut microbiome," she told Fox News Digital. 

"These elements collectively contribute to favorable alterations in lipid profiles, insulin levels and body weight, highlighting the potential benefit of a well-structured plant-based diet in addressing cardiometabolic risk factors among healthy individuals."

Although the vegans in the study (not pictured) also lost an average of 4.2 more pounds than the omnivores, weight loss isn’t always the most important factor when determining a diet's success, a dietitian noted. (iStock)

The key components of healthy dietary changes are education and personalization, said Routhenstein. 

"A fully vegan diet can be deficient in iron, calcium and Vitamin B12," Freirich told Fox News Digital.

"People seeking to change their diet should seek assistance from a registered dietitian to ensure they are meeting their nutrient needs and health goals — that is very different for a teenage male trying to gain muscle compared to a woman in her 30s trying to become pregnant," she added.

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Someone can become a vegan and still eat cookies, sodas, french fries and other processed foods all day long, she noted — but that wouldn't lead to healthier outcomes.  

Although the vegans also lost an average of 4.2 more pounds than the omnivores, weight loss isn’t always the most important factor when determining the success of a diet, noted Tanya Freirich, a registered dietitian nutritionist in Charlotte, North Carolina, who practices as The Lupus Dietitian.

Consuming more beans, lentils, chickpeas, mushrooms, nuts and seeds — as well as fewer processed meats for protein — is a great way for anyone to get started on improving their cardiovascular health, a dietitian said. (iStock)

"For example, bodybuilders would seem to be overweight if you were just examining their weight to height because their muscle weighs so much," Freirich, who was not involved in the study, told Fox News Digital. 

"Additionally, people can be healthy in a variety of shapes, sizes and weights."

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"Ideally, we should look more in-depth at the type of weight lost with each diet," Freirich said. "That is dependent on many other factors, such as exercise and the actual foods chosen in the vegan or omnivore diet, including portions and meal combinations."

Fox News Digital reached out to the North American Meat Institute for comment on the study findings.

Study had ‘many’ limitations

As all the study participants were generally healthy to begin with, Gardner acknowledged that the outcomes can’t be generalized to younger or older people, or to those with health issues.

The eight-week timeline was another limitation.

"Ideally, we would run studies for many months or years to better understand how sustainable these kinds of diet changes are," he said. "The reality is, very few people are willing to volunteer to be randomized to one diet or another if the study period is going to go on for months or years."

Gardner also emphasized that there are many different ways to follow an omnivorous or vegan diet. 

Although the vegans also lost an average of 4.2 more pounds than the omnivores, weight loss isn’t always the most important factor when determining the success of a diet, a nutritionist said. (iStock)

"It is fairly easy for a nutritionist to design both a healthy and an unhealthy version of both of those diet patterns," he told Fox News Digital. 

"If others reading about the study are interested and want to try this, they may choose an unhealthy vegan diet — for example, soda is vegan, many candies are vegan and white refined flour is vegan."

Based on the study's findings, the researchers recommend that people try experimenting by adding more vegetables, beans, fruits, nuts, seeds and whole grains to their diet. 

Across all popular diet plans, there is a "broad consensus" that focusing on more whole foods, more vegetables, less added sugars and less refined grains would lead to a "striking degree of health improvement," said Gardner, who is chair of the American Heart Association Nutrition Committee.

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"My best recommendation for [those] trying to change their diet for the better is to focus less on the label of being completely vegan or omnivorous, and instead focus on how many unprocessed foods, especially plants, you can include in your diet," said Freirich.

"Your diet needs to work for you and your lifestyle for your whole life, not only eight weeks."

Consuming more beans, lentils, chickpeas, mushrooms, nuts and seeds — as well as fewer processed meats for protein — is a great way for anyone to get started improving their cardiovascular health, Freirich noted. 

"These foods are generally lower in saturated fats and sodium and higher in fiber," she said. 

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"Your diet needs to work for you and your lifestyle for your whole life, not only eight weeks," Freirich noted. 

"Following a vegan diet in this study has been linked with better cardiovascular health, but a completely vegan diet may not be the right answer for everyone."

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