clock menu more-arrow no yes mobile

Filed under:

When one twin goes vegan and the other doesn’t

What a plant-based diet can and cannot do for you, explained by Netflix’s You Are What You Eat.

Two women, identical twins, wear aprons and stand in a kitchen with their arms over each other’s shoulders and their heads together. In front of them is a counter with trays of food to be prepped.
Pam and Wendy, one of the twin pairs in the Stanford study and the accompanying Netflix series You Are What You Eat: A Twin Experiment.
Kenny Torrella is a staff writer for Vox’s Future Perfect section, with a focus on animal welfare and the future of meat.

A recent Stanford University study won’t end the raging war over what we should eat for optimal health, but it does give vegans a leg up.

Published in the medical journal JAMA Network Open in November, the first-of-its-kind study recruited 22 pairs of identical twins, split up by diet. In each twin pair, one was randomly assigned to eat a healthy vegan diet for eight weeks while the other was assigned a healthy omnivore diet that included meat, eggs, and dairy. Those in the vegan cohort ended the study with much better health outcomes, particularly lower fasting insulin and lower cholesterol — a key indicator for heart health.

While most nutrition research is published with little fanfare, the twin study has received outsize attention thanks to an accompanying Netflix documentary series — You Are What You Eat: A Twin Experiment — that follows the culinary journeys of four of the twin pairs participating in the study. It was released in early January — right in time for New Year’s resolutions — and quickly became one of the streaming platform’s most watched television programs in the US.

“Identical twins are the perfect natural experiment because each individual has identical genes in every cell of its body, so they are a perfect way to tell nature from nurture,” Tim Spector, a professor on twin research at King’s College London who was not involved in the study, said in the Netflix series. Researchers also noted that the twins said they had similar lifestyles and were raised in the same households.

The study results are in line with a large body of nutrition research on the benefits of healthy plant-based diets. But I worry its findings could be undermined by the sometimes oversimplified nutrition advice served up by the cast of mostly vegan advocates starring in You Are What You Eat. It’ll likely be effective in persuading a lot of its viewers to give plant-based eating a try, but in others, it could instill some skepticism.

What happens when you go vegan for two months

For the first four weeks of the Stanford study, each participant ate preprepared frozen meals; for the final four, they had to provide their own food following a few basic principles: Choose minimally processed foods and consume a variety of vegetables, starches, proteins, and healthy fats.

The researchers were most interested in what would happen to the participants’ low-density lipoprotein cholesterol levels (LDL-C), known as “bad cholesterol,” an important marker of heart health. LDL-C levels for the omnivorous group barely budged, ending up at 116.1 milligrams per deciliter on average, which is above the optimal 100 mg/dL maximum. LDL-C levels for participants on the vegan diet fell by 13 percent on average, to 95.5 mg/dL. Fasting insulin levels fell by 20 percent more for the vegan group (high fasting insulin is a risk factor for developing diabetes).

The vegan participants also lost 4.2 more pounds on average than the omnivore participants, though the researchers didn’t intend to study weight loss and participants were told to eat until they were full.

The vegans, however, experienced a drop in their levels — though not to anywhere near the level of deficiency — of the critical vitamin B12, which is essential for nervous system function. Nutrition experts recommend that vegetarians and vegans take a vitamin B12 supplement, which the study authors didn’t ask of participants. (The study was only eight weeks long, but for long-term vegan diets, supplementing B12 is a must, and it’s easy to do for a few pennies per day.)

What plant-based food can and cannot do for you

Carnivorous critics have said You Are What You Eat amounts to little more than vegan propaganda, and at times, it can feel that way. The series’ vegan advocates talk at length about how meat and dairy production accelerates climate change and deforestation, pollutes air and water, and abuses animals — claims that can sound hyperbolic but are actually spot-on.

But You Are What You Eat’s health claims are more complicated. There’s no doubt that a minimally processed plant-based diet can offer immense health benefits, especially for people who aren’t consuming enough whole grains, fruits, and vegetables, but the series sometimes makes sweeping generalizations about the benefits of plant-based eating and the harms of meat and dairy consumption — like when cheese is misleadingly described as “biologically addictive.”

While sometimes plant-based advocates can oversell the diet’s health potential, many meta-analyses conclude that vegans and vegetarians, along with flexitarians (those who significantly reduce their meat intake), tend to have lower rates of heart disease, Type 2 diabetes, and some cancers.

Critics have also pointed out that the study was partially funded by tech entrepreneur Kyle Vogt, who is vegan and an executive director of the Netflix series, and that the lead Stanford researcher, Christopher Gardner, has received funding from Beyond Meat, a plant-based meat company. (The company didn’t fund this study, though the vegan cohort did consume Beyond Meat products.)

But none of that means the study amounts to bad science: It was “very nicely conducted, especially because both the vegan and omnivore diets were quite healthy,” Walter Willett, a professor of epidemiology and nutrition at Harvard’s school of public health, told Vox. And its findings are highly consistent with existing nutrition and environmental health research, which points to the need for people in rich countries to eat less meat and dairy and more plant-based foods.

“There is no body of evidence that conflicts with the finding that a healthy plant-based, vegan diet as implemented in this study, is better than a typical omnivore diet,” Willett added.

The biggest limitation of the study was its short length. Eight weeks is a blip in nutrition science, which is why researchers favor longer-term studies. Nutrition science is also messy and complicated, and a diet is only as good as it is easy to follow.

Most people struggle to stick to diets, including vegetarian and vegan diets. For example, only one of the people in the vegan cohort decided to stay vegan after the completion of the study. However, going vegan for two months did have a positive effect on most of the participants assigned to the vegan diet — all but one said they planned to eat more plant-based foods than they had prior to the study. For those looking to improve their health — and shrink their carbon footprint — by eating more plant-based meals, it may make sense to first try out a flexitarian diet, which is much lower in meat and dairy than the standard American diet.

“Each step in the direction of a healthy plant-based diet will have benefits for personal and planetary health,” said Willett. “One doesn’t need to be a strict vegan to have major benefits.”

But if you’re going to eat less meat and dairy, it’s important to replace them with healthy plant-based foods like nuts, soy foods, and other legumes, Willett said. Ultimately, you’re much better off eating a healthy omnivore diet than a plant-based diet low in actual plants and high in vegan doughnuts.

For most Americans, however, cutting back on meat and dairy and upping intake of healthy plant-based foods is a hard sell. Due to decades of government policy favorable to meat companies and technological advancements that cruelly push farmed animals to their biological limits, meat and dairy have never been cheaper to produce. That’s led the average American to consume around 220 pounds of meat annually — more than any other country — along with about 280 eggs and over 660 pounds of dairy.

Almost a quarter of Americans say they’re trying to cut back on meat, yet annual per capita consumption continues to rise.

Some of the hesitancy to eat more plant-based food may be purely mental: There are several moments in You Are What You Eat where people remark that a vegan dish is so good that they wouldn’t have guessed it was vegan.

Willett encourages people to follow dietary recommendations from the EAT-Lancet Commission — a body of nutrition, sustainability, and agriculture experts of which he’s a co-chair — that encourage no more than 57 pounds of meat per person each year, about a quarter of the average American’s consumption, and a significant reduction in dairy.

(If you’re looking to incorporate more plant-based meals into your life, sign up for Vox’s Meat/Less newsletter here.)

Making such changes isn’t always easy in the US, and You Are What You Eat is at its strongest when it critiques the heavy influence of the livestock sector on our food system, the ecological destruction it’s wrought, and the barriers consumers face to eating healthy plant-based foods. The responsibility for improving public health ultimately shouldn’t lie with the individual, but with the policymakers who’ve shaped it through regulations and subsidies that favor the meat and dairy industries at the expense of Americans’ health and the climate. While the series has its flaws, it gets the big picture right about a lot of what’s wrong with our food system, and how to make it better.

A version of this story originally appeared in the Future Perfect newsletter. Sign up here!

Sign up for the newsletter Today, Explained

Understand the world with a daily explainer plus the most compelling stories of the day.