A study of 22 pairs of identical twins found that a vegan diet may be superior to an omnivorous diet at boosting cardiovascular health.

The researchers instructed one of the twins to eat an omnivorous diet, and the other to eat a vegan diet, for eight weeks. The paper was published in the Journal of the American Medical Association on November 30.

During the first four weeks, the twins were served prepared meals. Over the final four, they fixed their meals themselves with the help of a registered dietitian. They were asked to log all food consumed over the study's period.

Both diets were healthy, with a variety of vegetables, legumes, fruits and whole grains. The only difference was the presence of meat products in the omnivorous diet — chicken, fish, eggs, dairy and cheese, for example.

By the end of the trial period, the twin who ate the vegan diet had lower levels of low-density lipoprotein cholesterol (LDL-C) — dubbed "bad" cholesterolas it can clog the arteries.

In general, a healthy level of LDL-C is under 100 milligrams per deciliter. At the start of the trial, the vegans had an average level of 110.7, and the omnivores 118.5. By the end of the trial, the vegans' level had dropped to 95.5 and the omnivores' to 116.1. When asked about the difference in initial cholestrol levels in the two groups, study author Christopher Gardner said they were due to chance.

Researchers also found that the vegan participants had lower fasting insulin levels by the end of the trial than their omnivore counterparts and lost 4.2 pounds more on average.

Props for using twins

Studies tracking the benefits of veganism can be complicated by confounding factors related to a participant's social environment or genetics. Conducting the study on identical twins largely controlled for this.

Along with keeping most variables the same aside from the inclusion of animal products in the omnivorous diet, the trial also controlled for the fact that not all vegan diets are necessarily healthy.

"Vegan diets are defined by what they exclude rather than what is consumed. There are good and bad vegan diets," said Tom Sanders, a professor of nutrition and dietetics at King's College London who was not involved in the study. "For example, a bad vegan diet would be lacking vitamin B12, and high in fat, salt and sugar."

The study accounted for this by closely monitoring what the participants ate, adding a degree of rigor that could not be accounted for if the diet was not controlled.

Dietary satisfaction lower among vegans

However, the study did have some complications. For starters, the sample size was small. Researchers also noted that the overall food intake of the vegans was deficit in some 200 calories per day than that of the omnivores.

"This could explain the non-significant reduction in weight and perhaps at least partly explain the reduction in LDL cholesterol," said Duane Mellor, a registered dietitian and senior lecturer at Aston University who was not involved in the research.

This could be because dietary satisfaction was lower among vegans, said Mellor. This was outlined in the study's supplementary materials.

"Participants consuming the vegan diet reported the largest decrease in diet satisfaction when eating out (in a restaurant) at weeks 4 and 8 relative to baseline," the study said. "Among participants in the omnivorous diet arm, diet satisfaction either increased at weeks 4 and 8 or was maintained from baseline reported levels."

When asked about these critiques, main author Christopher Gardner, a professor of nutrition studies at Stanford University in the US, told DW that the calorie deficit isn't due to the study's design, but rather because the vegan participants didn't consume all the food they were served in the first four weeks. All groups were delivered the same number of calories, he said — the people in the vegan group just ate less. 

"Vegan diets are very restrictive," said Gardner in an explanation posted to the platform X. "I assume those on the vegan diet might have been hungry for some fish or eggs or yogurt or meat. But those were restricted. They weren't hungry for more veggies & beans."

Outside researcher Mellor said the reported satisfaction levels show that if people are to adopt a healthier diet, "it should be based around their preferences and not based on one particular dietary approach or another favored by their health professional or online influencer," he said.

When asked about the real world application of this study, given the vegans' reported dietary dissatisfaction, Gardner agreed that "if satisfaction of the diet is low, it suggests that maintaining that kind of diet would be difficult."

"Over time, it may be that someone shifts back to the type of diet they are more accustomed to and familiar with," he told DW. "It would be great to do these studies for a year, 5 years, 10 years. Then we would be more confident about maintainability. Realistically, most people are only willing to join a nutrition intervention study for a few months."

With that said, curiously, the researchers found that an overwhelming majority of the twins assigned a vegan diet (91%) said they planned to continue the habits learned in the trial into their normal life, as opposed to the twins who were assigned a meat-eating diet (67%). 

Edited by: Sushmitha Ramakrishnan