Plant-Based Diet Improves Cholesterol, Blood Sugar, and Weight
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A new study of 22 pairs of identical twins found that compared with their meat-eating sibling, individuals who followed a vegan diet improved cholesterol, blood sugar, and body weight in just eight weeks.
The findings, published on November 30 in JAMA Network Open, suggest that anyone — even people who are already relatively healthy — can adopt a vegan diet to improve their long-term health in two months, according to the authors.
Investigators specifically set out to recruit “living, breathing, walking” identical twins — not people with preexisting heart disease risk factors such as having high LDL (bad) cholesterol, overweight, or obesity, says senior author Christopher Gardner, PhD, a professor of medicine at the Stanford Prevention Research Center in California.
“Yes, some of the twins had extra weight or high cholesterol, but a lot didn’t. They were already trim with normal cholesterol. And still, the vegan group lowered their LDL cholesterol by more than 10 percent and lost weight. We were surprised that it happened so quickly and in a group that didn’t really have high LDL cholesterol to begin with,” says Dr. Gardner.
Just 1 Percent of Americans Report Eating a Vegan Diet
A vegan diet excludes all meat (beef, pork, poultry), seafood, and fish, and animal products (dairy and eggs).
In the United States, 4 percent of people report they are vegetarians, and 1 percent are vegan, according to a Gallup poll published in August 2023. The polling company found that women, people who describe themselves as politically liberal, and people who report earning less than $40,000, a year are all more likely to eat a vegetarian diet.
Twinning Made the Study Unique — and Fun for Researchers
The study included 22 pairs of identical twins for a total of 44 participants. All were healthy and without heart disease from the Stanford Twin Registry, a database of fraternal and identical twins who have agreed to participate in research studies. In each pair, one twin was assigned a vegan diet, while the other followed an omnivore diet (a diet that includes meat, animal products like eggs and cow’s milk, and vegetables).
The twin aspect of the study was a lot of fun for the researchers, says Dr. Gardner. “It was just so fun to observe them when everyone got together. They had a great banter, wonderful stories — and they often dressed alike and had the same hairstyles!”
Including identical twins is a key aspect that made this study unique, he says. Most nutrition studies are observational studies, partly because it’s hard to get people to change how they eat and stick with it for weeks or months, says Gardner.
Most nutritional studies are observing the eating habits of vegans, omnivores, or pescatarians, for example, and then reporting whatever health outcomes the study is trying to isolate, he says. “And the obvious downside there is that the vegans were probably different than the omnivores for other reasons, too. They not only have behaviors that can influence their risk, but there are also genetic factors at play as well, and that’s true for randomized studies as well.”
By using identical twins, researchers could control for genetics and limit the other factors, because twins grow up in the same households and report similar lifestyles.
Both the Vegan and Omnivore Diet Were Healthy
The vegan diet was entirely plant-based and included no meat or animal products. The omnivore diet included chicken, fish, eggs, cheese, dairy, and other animal-sourced foods.
Another unique feature of the study was both diets were generally healthy and contained vegetables, beans, fruits, and whole grains and no sugars or refined starches.
“I’m opposed to dietary studies where the ‘favored’ diet is compared to eating a poor diet. That’s an easy thing to beat. We wanted to compare the benefits of a healthy vegan diet to a healthy omnivore diet because it makes the findings even more compelling,” he says.
During the first four weeks, a meal service delivered 21 meals per week — seven breakfasts, lunches, and dinners. For the remaining four weeks, the participants prepared their own meals.
Using the meal service had a few benefits, says Gardner. For one, researchers could be sure that there was no confusion about what could be included in the vegan diet, he says.
“It also gave people in the vegan group an opportunity to understand what could be included. It’s way more than ‘rabbit’ food or a salad with lettuce. It could be a tofu stir fry with lots of crunchy veggies, a lentil black bean burger, a Mediterranean salad with lots of beans and nuts — all very filling,” says Gardner.
A total of 21 out of 22 vegan eaters finished the study — another testament to how generalizable and accessible the diet is, he says.
Vegans Had Bigger Drops in Cholesterol, Fasting Blood Sugar, and Weight
Researchers observed the biggest improvements in heart health happened within the first four weeks of the diet change.
The average baseline LDL-C level was 110.7 milligrams per deciliter (mg/dL) for the vegans and 118.5 mg/dL for the omnivore participants.
It dropped to 95.5 mg/dL for vegans (about a 13 percent decrease) and 116.1 mg/dL for omnivores at the end of the study. The optimal healthy LDL-C level is less than 100 mg/dL.
The vegan participants also showed about a 20 percent drop in fasting insulin (higher insulin level is a risk factor for developing diabetes) and lost an average of 4.2 more pounds than the omnivores. At the start of the trial, the average BMI of participants was 25.9 kg/m2, slightly over the BMI healthy range, which is 18.5 to 24.9 kg/m2, according to the Centers for Disease Control and Prevention (CDC).
Further analysis into the participants’ biological ages (a measure of how old a person is physically and metabolically) found that at the end of the eight-week study, the twins who ate the vegan diet were biologically “a little younger” than their omnivorous counterparts, said Gardner in a video about the study.
“Based on these results and thinking about longevity, most of us would benefit from going to a more plant-based diet,” he says.
Why Would a Vegan Diet Improve Heart Health?
The vegan participants did the three most important things to improve cardiovascular health, according to Gardner: They cut back on saturated fats, increased dietary fiber, and lost weight.
Most Americans eat about half of the recommended amount of fiber, says Gardner.
RELATED: 11 High-Fiber Foods to Add to Your Diet
Dietary fiber intake is recommended at 14 grams per 1,000 calories of food. For example, at a 2,000 calorie reference level (which is appropriate for some but not all people) the daily dietary fiber intake should be 28 grams, according to the U.S. Department of Agriculture (USDA).
These findings support existing evidence that a vegetarian or vegan diet is associated with improvements in cardiovascular and all-cause mortality, according to a review published in February 2023 in the International Journal of Environmental Research and Public Health.
Although the exact mechanism behind the benefits isn’t fully understood, it’s suspected that the saturated fats in animal meat and products may cause low-grade systemic inflammation which could contribute to heart disease.
Ready to Try a Protein Flip?
If you’re not ready to fully adopt a vegan diet, you can still work on eating more veggies and plant-based protein and less meat by trying a “protein flip,” says Gardner.
“Most Americans think the center of the plate is chicken, beef, or pork and then the side dishes that go with it are grains and vegetables. The protein flip is putting plants at the middle of the plate — the meat is no longer the centerpiece, but rather a flavoring or a side dish,” he says.
One way to try this is with a bowl, which is popular in student dining halls or supermarket hot bars. “You can build your own. So maybe there’s meat there, but there could also be tempeh, or tofu, and grains and veggies along with it,” he says. The protein flip doesn’t cut meat out totally, but it can help cut down on meat consumption, says Gardner.
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