One identical twin went vegan while the other didn’t. See what happened

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Healthy twins who ate a vegan diet for eight weeks had lower “bad” low-density lipoprotein, or LDL, cholesterol, better blood sugar levels, and greater weight loss than siblings who ate a diet of meat and vegetables, a new study found.

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“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,” said lead study author Christopher Gardner, a research professor of medicine at the Stanford Prevention Research Center in Palo Alto, California.

A vegan diet differs from a vegetarian diet in that it eliminates not only animal flesh but dairy, eggs or any other ingredient derived from animals. A strictly plant-based diet can be higher in fiber, vitamins, minerals, and phytonutrients compared with other dietary patterns, said Gardner, who is also the director of the Nutrition Studies Research Group at Stanford.

“The results of this study confirm the benefit of current dietary guidance to reduce cardiovascular disease risk,” Alice Lichtenstein, director and senior scientist at Tufts University’s Cardiovascular Nutrition Laboratory, told CNN in an email. She was not involved in the study.

“The diet with more unsaturated relative to saturated fat, more whole grains relative to refined grains, fewer calories, more fiber and vegetables, and less cholesterol resulted in a more favorable cardiovascular disease risk factor profile than the comparison diet,” she said.

The study was unusual because it used genetically identical twins, most of whom shared similar lifestyle behaviors, including haircuts and clothes.

“The twin study design is elegant because it largely controls genetic and environmental factors that may influence the trial results,” said Dr. Frank Hu, a professor of nutrition and epidemiology and chair of the department of nutrition at the Harvard T.H. Chan School of Public Health in Boston. He was not involved in the study.

“However, the recruitment of identical twins into dietary intervention studies is challenging; this is why the design has been rarely used in nutritional studies,” Hu said. “Also, the findings from identical twins may not be generalizable to the general population.”

Identical twins help scientists control variables that may otherwise influence trial results. (The twins seen above did not participate in the new study.)

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The study, published Thursday in the journal JAMA Network Open, involved 22 pairs of identical twins. In each pair, one twin was assigned a vegan diet, while the other was assigned an omnivore diet. They each followed their respective diets for eight weeks. During the first four weeks, all meals were provided to each twin so they could see the types of food they should be eating.

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

The meat and veggie group also had meals delivered that were a step up from their normal fare.

“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 said. “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.”

Once each pair of twins learned the types of foods to eat, they were asked to prepare their own diet-appropriate meals and snacks for the second half of the study, Gardner said.

Biological markers including blood and feces were gathered from each twin at baseline and again at week four and eight. Surprisingly, twins on the vegan diet tested younger on measures of biological versus chronological age, data which will be presented in a future study, Gardner said.

However, the improvement in cardiovascular biomarkers such as lower LDL cholesterol, insulin resistance and the loss in weight were to be expected, Gardner said.

“The vegetarians got more fiber, less saturated fat. That’s going to explain the LDL cholesterol. More fiber is going to explain less fasting insulin because they just have a slower release of glucose in the bloodstream,” he said. “And all the vegetables and fruits and grains are bulkier than meat and so people could be more satiated and eating fewer calories.”

Restricted diets

But as with all diets that restrict people from foods they are used to eating, staying the course can be tough.

“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.”

Despite the quick improvement in health, people don’t have to become a vegan to benefit from the findings of the study, Gardner said. Cutting back on eating meat and animal byproducts can be done slowly, bit by bit.

Harvard’s Hu agreed: “While this study showed a vegan diet may provide additional advantages compared to a healthy omnivorous diet, it does not mean that everyone should become a vegan or vegetarian.

“Dietary choices are influenced by a variety of factors such as individual health conditions, personal preferences, cultural traditions, and ethical and environmental considerations,” he said.