One twin went vegan and the other ate meat. Here's what happened.

At the beginning of 2022, Stanford University researchers selected 22 pairs of genetically identical twins from a twin research registry. They randomly assigned each twin to follow either a vegan or omnivore diet. The goal of this experiment was to gain insights into how diet affects health. The results, published in December 2023 in the JAMA Network Open journal, were remarkable.

Selected participants received 21 weekly pre-packaged meals from Trifecta, an organic meal-delivery service that specializes in providing nutrient-dense, pre-prepared “deep chilled” meals designed to support weight loss and fitness goals. In the assortment of meals the participants received, the vegan meals consisted of oatmeal, tofu, broccoli, spinach, beans, lentils, and brown rice. Whereas, the omnivore meals included foods like eggs, chicken, turkey bacon, vegetables, and jasmine rice. Participants regularly logged their meals on Cronometer, an app that tracks diet and health data, per the Washington Post.

For eight weeks, participants followed their assigned diets while dieticians regularly checked in on their progress. They also provided blood and stool samples and underwent physical and cognitive tests to evaluate how the diets impacted their balance and memory. The researchers found that those on the vegan diet had lower cholesterol, insulin levels, and body weight compared to 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,” lead study author Christopher Gardner, a research professor of medicine at the Stanford Prevention Research Center in Palo Alto, California told CNN.

This isn’t the first study to suggest that vegans may be healthier than meat eaters. A similar study was conducted in 2021 by the Department of Twin Research & Genetic Epidemiology at King's College in London, on a pair of genetically identical twins named Hugo and Ross Turner. The twins followed two distinctly different diets for 12 weeks, Hugo ate strictly vegan food, while Ross continued to eat meat and dairy products, reported BBC Global.

Both twins consumed the same number of calories daily and followed identical gym routines. Initially, Hugo’s body struggled to adjust and he felt strong cravings for meat and dairy products, but once his body adjusted to wholesome food, he felt more energetic and satiated throughout the day. Researchers found that although the vegan diet provided more satiety, the meat-eaters had an advantage over the healthy gut microbes. They also proposed that, while vegans are healthier than meat eaters, sometimes they end up eating ultra-processed foods.

As for this latest research, Gardner, who’s a vegetarian, hopes nutritionists use the findings to encourage people to eat vegan foods. 

Editor's note: This article was originally published on July 20, 2024. It has since been updated.