Stanford study inspired Netflix documentary
Stanford University scientists are the driving force behind a hit Netflix documentary “You Are What You Eat: A Twin Experiment” that tries to answer the question of how to eat for better health.
But how solid is the science in the show, which features Bay Area residents as test subjects, and what does it tell us about how to nourish ourselves?
At the heart of the Stanford research and the Netflix series is the question of whether a vegan diet is healthier than one that includes meat and dairy.
The docuseries, one of Netflix’s most-watched shows in the United States for the second week in a row, is largely based on a 2022 Stanford study that sought to answer the question by introducing a new twist.
Researchers recruited 22 sets of identical twins and had one of each pair eat a vegan diet for eight weeks while the other followed a healthy omnivorous diet. Each set of twins had access to the same fitness trainer for the duration of the study so they could exercise at similar levels. The series followed the twins as they cooked, worked out, and underwent full-body scans, blood draws and other medical testing.
The approach allowed scientists to examine how diet alone impacts health, separate from the influence of genetics and lifestyle, which are typically hard to control for in human studies.
Using identical twins “is a step up in scientific rigor,” said the study’s lead researcher, Christopher Gardner, director of nutrition studies at the Stanford Prevention Research Center.
The study, which was published in JAMA Network in November, analyzed changes in the twins’ body composition, microbiome and biological clock. Both types of diets were approved by dietitians to be nutritious and balanced.
At the end of the study, the twins on the vegan diet showed greater improvements in several key health measures than the twins on the omnivorous diet:
• The vegans had bigger declines in low-density lipoprotein cholesterol (LDL-C, often called “bad cholesterol”). The vegans’ cholesterol decreased by 15.2 milligrams per deciliter (mg/dL), while the omnivores’ cholesterol decreased by 2.4 mg/dL.
• The vegans on average lost 4.2 pounds more than their omnivorous counterparts.
• The vegan twins reported a more than a 20% drop, relative to the omnivorous twins, in fasting insulin levels. A high level of fasting insulin can indicate insulin resistance, a characteristic of Type 2 diabetes.
Some of the results are not entirely new or surprising. They reiterate what many previous studies have shown — that vegan diets are associated with better cardiovascular health and lower risk of heart disease than many other diets. This is probably because vegan diets contain more fiber, vitamins and minerals from vegetables, fruits, whole grains, nuts and seeds.
“If you look at the (Dr. Dean) Ornish diet or the Mediterranean diet, there’s definitely data showing that more plants and nonanimal based sources of omega fatty acids like nuts and legumes are heart-healthy,” said Dr. Diana Thiara, who leads UCSF’s weight management program and was not involved in the study.
The Ornish and Mediterranean diets focus on fruits and vegetables, whole grains and legumes, and eliminate or limit meat and some dairy.
And because the study included just 44 people, all of whom were generally healthy adults to begin with, experts cautioned against applying the results too broadly.
“The twin study is a super small sample size, which is great for a Netflix documentary, but it’s a little hard to extrapolate population-level impact on that few number of people,” Thiara said.
Both the vegan and omnivorous groups ate a healthier diet during the study — specifically, more vegetables and less refined grains — than they were eating prior to the study. So that could’ve accounted for some of the health improvements.
The vegans also consumed fewer calories than the omnivores during the study. But that’s mostly because the vegan diets were higher in water content and fiber, which tend to make people feel fuller, Gardner said. And the study was a “free-living” study, which means participants were told to eat until they were full, not until they consumed a certain number of calories, he said. The meal kits the twins received for the first half of the study were actually designed to have the same number of calories, but it was up to them how much they ate.
Gardner, who’s been studying diets for 30 years, hopes the takeaway from the study and the documentary is to eat healthier, not necessarily to go vegan, which can feel unsustainable to many people. That can be as simple as eating less saturated fat and animal fat and more fiber. Gardner himself has been a vegetarian for 40 years, and about 15 years ago gave up eggs and dairy.
“Really, what we want is for people to eat healthy for the rest of their life,” he said. “And you wouldn’t have to be that extreme. Just go in that direction.”