The Quick Report

One Identical Twin Went Vegan – Doctors Say This Happened Next

For years now, many doctors have been advocating the benefits of plant-based and vegan dietetic choices for maintaining good health. Unfortunately, the majority of Americans maintain a diet that is contrary to what doctors advise. But a new study has shown overwhelming evidence that what doctors have been saying about diet is true — and absolutely works to improve health.

A medical research team from the Stanford Prevention Research Center in Palo Alto, California took a novel, if not ingenious, approach to show a contrast in diet choices. 


Led by Christopher Gardner, a research professor of medicine, the doctor and his colleagues recruited 22 pairs of identical twins. What better to test opposite diets on than those who have an identical genetic makeup?

When studying disparate individuals, it can be difficult to draw definitive conclusions. This is because things like genetics, upbringing, and lifestyle choices introduce many variables. Seeking to eliminate these variants, Doctor Gardner and his colleagues chose to study identical twins because they share the same genetic makeup. Twins also grew up in the same household and often have similar lifestyles. In essence, while not complete clones, identical twins are as close as you can come to leveling the playing field between individuals.

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Parameters of the Study

The twins were all listed and selected from the Stanford twin registry, a database of identical and fraternal twins who all agreed to participate in research studies.

The research team assigned one twin from each pair to eat only a strict vegan diet containing no animal products, such as eggs or milk. The other twin would consume an omnivore diet which included chicken, fish, eggs, cheese, dairy, and other foods and ingredients from animal sources.

The differences between a vegetarian and vegan diet should be noted. A vegetarian diet typically eliminates only animal flesh. It still allows dairy, eggs, and other ingredients derived from animal products. A vegan diet allows none of these.


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Overall, both diets were healthy. Both contained plenty of vegetables, beans, fruits, and whole grains. Both diets also limited sugars and refined starches. 

The participants followed their respective diets for eight weeks. During the first four weeks, a meal service delivered three meals a day. This was so the twins could learn and see the types of foods they should be eating. These meals were also a step up from their normal fare. 

Misconceptions of Vegan Diets

Dr. Gardner is also the director of the Nutrition Studies Research Group at Stanford.

“I feel like a lot of people who do a vegan diet think ‘Oh great, soda is vegan. Pancakes are vegan,'” Gardner said. “No, they are refined ultraprocessed grains. So, we tried to get a healthy vegan diet and show them quickly what that was for four weeks.”

For the remaining four weeks, the participants were asked to prepare their own diet-appropriate meals and snacks.

The clinical trial ran from May to July 2022. Out of the 44 individuals involved in the study, only one did not follow through to completion. Researchers gathered biological markers from the participants including blood and feces. These were collected at baseline and again at weeks four and eight.

The findings of the study were published on Nov. 30 in the journal JAMA Network Open.

“The twins were also a riot to work with,” Dr. Gardner noted in a university news release. “They dressed the same, they talked the same and they had a banter between them that you could have only if you spent an inordinate amount of time together.”

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Study Proves Diet Choices Matter to Good Health

What the research on identical twins proved overwhelmingly and beyond a doubt was that diet makes a significant difference in overall health. Further, a vegan diet can vastly improve a person’s heart health. The twins on the vegan diet showed significant improvements in cholesterol, insulin, and body weight. 

The other twins who ate a healthy diet containing animal protein did not show the improvements seen with the twins who consumed a vegan diet.

“Our study used a generalizable diet that is accessible to anyone,” Gardner pointed out. “This suggests that anyone who chooses a vegan diet can improve their long-term health in two months, with the most change seen in the first month.”

For the vegans, average “bad” LDL cholesterol levels dropped steadily. With the omnivores, it stayed about the same. In fasting insulin levels, vegans saw a 20% decrease. On average, vegans lost 4 more pounds than the omnivores.

“There was a 10% to 15% drop in LDL cholesterol, a 25% drop in insulin, and a 3% drop in body weight in just eight weeks, all by eating real food without animal products,” Dr. Gardner said. “Not only did this study provide a groundbreaking way to assert that a vegan diet is healthier than the conventional omnivore diet, but … A strictly plant-based diet can be higher in fiber, vitamins, minerals, and phytonutrients compared with other dietary patterns.” 

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The Importance of Balance

“I’m really into equipoise or balance in my studies — I don’t have a wonderfully healthy vegan diet and a crap strawman diet to be knocked over,” Gardner continued. “People who ate the omnivorous diet ate more vegetables, more whole grains, less added sugar, and less refined grains than they did in their usual habitual diet. Nor did they get crappy meat, it was all good quality. So, they actually had some dietary improvements.”

“To be honest, twins on the vegan diet had less satisfaction because it was so restrictive. So, there’s certainly the flip side of this: ‘Oh, I could have eaten more, but I just wasn’t hungry for more grains and more vegetables,'” Gardner added. “Despite the quick improvement in health, people don’t have to become a vegan to benefit from the findings of the study. Cutting back on eating meat and animal byproducts can be done slowly, bit by bit.”