April 15, 2024 | 10 minute read

The Netflix docuseries You Are What You Eat—A Twin Experiment featuring insights from the Stanford Twin Study has garnered plenty of viewers. If you’re looking for a reality TV show to provide insight into how people adapt to a new style of eating, you may enjoy following along. The series is designed to be an entertaining vehicle to share a pro-vegan diet perspective. You should not tune in expecting a scientifically accurate summary of the research.

Let’s unpack key findings from the study and how the docuseries succeeds—and fails—to convey them.

About the Stanford Twin Study

Twenty-two sets of identical twins were recruited for a Stanford University study and assigned to follow either a healthy vegan diet or a healthy omnivorous diet (plant-focused, but not exclusively plants) for eight weeks. Working with twins gives researchers the unique opportunity to control for genetic differences that might influence the effects of these two diets on health.

Vegan vs Omnivore Diets: What’s on the Plate?

The vegan diet included:

• Vegetables
• Fruits
• Beans
• Whole grains
• Nuts and seeds

The vegan diet did not include animal products, such as meat and dairy. The omnivorous diet included a mix of both plant-based foods and animal products. Both diets encouraged choosing minimally processed foods and limiting refined grains and added sugars.

Groups kicked off with four weeks of meals delivered to participants’ doorsteps, making it easier for them to see what the principles of their diet looked like in practice. For the second half of the study, participants bought and cooked food themselves, with guidance from health educators to keep it on track.

What Were the Results? 

After eight weeks on their assigned diets, the following results were noted:

LDL-cholesterol levels

• Vegan diet: LDL decreased significantly; saturated fat intake reduced by about 50%
• Omnivorous diet: Little change in LDL; slight decrease in saturated fat intake

Note:  Saturated fat in the diet is a major driver of LDL levels. 

Weight loss

• Vegan diet: Lost an average of 3 pounds; calorie intake dropped ~300 calories/day
• Omnivorous diet: Lost little, if any, weight; calorie consumption dropped ~100 calories/day 

Fasting insulin levels

• Vegan diet: Dropped somewhat
• Omnivorous diet: Did not drop

Note: The challenge with interpreting this result is that there are no accepted standards that define a normal level. So, it’s not clear whether differences here are clinically meaningful.

Other parameters

Changes in blood triglycerides, blood sugar and TMAO levels were modest and unrelated to participants’ assigned diet. TMAO is a compound produced in the gut. High levels are related to diets high in animal foods and may be associated with increased risk of heart disease and colon cancer.

Vitamin B12 is found only in animal foods and fortified plant foods. People on the vegan diet consumed below recommended levels. However, blood levels of B12 were maintained by body stores so did not significantly decrease. Long-term use of a vegan diet should include fortified foods or a supplement.

Putting the Study Results in Context

Reading the details of the research study provides some evidence for potential health advantages of a vegan diet. It also shows the importance of looking beyond the presence or absence of animal foods as a sign of diet quality. Food choices and overall pattern both matter.

• Saturated fat: Omnivorous diets could be created with lower levels of saturated fat than seen here. Although participants on omnivorous diets made some dietary improvements, on average they still did not reach recommended amounts of whole grains, vegetables or dietary fiber.
• Vitamin B12: Vegan diets may be deficient in B12, but low blood levels won’t show up right away. Fortified foods or B12 supplements can be recommended.
• Protein: Although total protein consumption remained within the generally recommended range, it was significantly lower on vegan diets. These diets can meet a wide range of protein needs, but people may need a learning curve to include more plant-based sources of protein.
• Calories: Short-term weight loss and reduced calories don’t necessarily predict long-term results. Typically, the greater the change someone tries to make in their diet, the harder it is for them to figure out what to eat. That can lead to short-term weight loss, which is often regained as people get more comfortable with new food choices. Regardless of diet type, you can reduce calories by limiting foods and drinks that are concentrated in calories and by choosing appropriate portions.

The Twin Study as Portrayed in You Are What You Eat—A Twin Experiment

The Netflix documentary series uses the Stanford University Twin Study to present an engaging story about dietary habits. But the Netflix team only follows four pairs of twins from the study, not all 22 sets. This selective reporting means that the story diverges from the actual study in several key aspects.

• Exercise coaching for the four pairs of twins depicted in the series was not part of the study’s protocol, which focused solely on diet.
• The focus on body composition changes, showcased through DEXA scans, gives the impression that these results are representative of study results. However, body composition was not assessed in the original study, and is only available for this small subset of participants selected by the Netflix team. The outcomes portrayed in the documentary, such as changes in muscle and visceral fat, may not accurately represent results of others in the study.
• Additional outcomes, such as sexual arousal testing among women, telomere lengthening and cognitive testing, were not included in the original study. That means the methods used for these tests did not go through the multi-layered review process involved with conducting and publishing a peer-reviewed study. Testing for gut microbiome changes was noted in the methods of the published Twin Study, but results were not part of the published study and will be presented in future publications. The docuseries shares these findings as showing significant diet-related differences, but we don’t have a basis to know the quality of the testing or analysis.

Furthermore, interwoven with the story of the Twin Study are a variety of interviews sharing a perspective strongly favoring vegan diets, emphasizing benefits for health and the environment. These views are consistent with the mission of the Oceanic Preservation Society, the organization that funded the documentary, but were not based on evidence from the study.

Despite these discrepancies, the documentary provides some valuable insights. It highlights:

• The need to look beyond the scale to consider changes in both muscle and body fat
• Challenges that can arise in starting a more plant-based diet, such as uncertainties about carbohydrate-rich food and adequacy of calorie and protein consumption
• Individual differences in approach to and barriers faced in changing diet

The series offers intriguing perspectives on individual responses to diet and exercise, but viewers should be careful not to assume that methods in the documentary are consistent with those of the peer-reviewed study or that results depicted mirror those of the entire study.

Finding Reasonable Take-Home Points 

Despite messages conveyed in the Netflix docuseries, here’s what the Twin Study authors concluded:

Vegan diets show several health advantages compared with a healthy omnivorous diet in this study. However, research suggests that cardiovascular benefits can be achieved with modest reductions in animal foods and increases in healthy plant-based foods compared with typical diets.

Participants on vegan diets were less satisfied with their diets overall compared to those on omnivorous diets. For some people, vegan diets fit lifestyle and preferences well. But for others, strict limits on foods may create barriers to finding choices in certain situations and limit enjoyment of eating.

For health-care providers:

Within a clinical setting, patients should be supported in choosing a dietary pattern that fits their needs and preferences. Dietary patterns can be adapted to meet people’s health conditions and their personal, family and cultural food preferences. Rather than pushing people toward one specific kind of diet, health professionals can help people create and maintain healthy eating habits through two-way conversations focused on support and problem-solving.

A Checklist for Critical Thinking

With the increasing popularity of documentaries and articles about scientific topics, people now have more opportunities to hear about scientific research. This is a positive development, which empowers people to understand the influence that eating and lifestyle choices can have on their health. But it also requires weeding through the false headlines and biased documentaries so you can get to the truth.

Dr. Christopher Gardner, PhD, the senior author of the Twin Study, says that much of the controversy in nutrition would turn into consensus if these two basic questions are asked:

• “With what?”—What other foods were people eating? And what is being compared? Is one diet compared to another healthy option or to a typical U.S. diet? Are the very highest and lowest levels compared, or do we have information about moderate consumption?
• “Instead of what?”—We don’t eat foods in isolation, so ask “If we ate this instead, what did we NOT eat? Did eliminating something make the difference?

In addition to asking these questions, AICR offers the following helpful advice. When you are reading health-based articles or watching documentaries, ask:

• Who funded the study or documentary? Is there a bias?
• Is it referenced? Does it cite scientific journals or other trusted sources?
• Is scientific jargon used properly or is it used to confuse you?
• Is this one study an outlier that requires further research? Are there consistent study results from multiple high-quality studies that you can read?
• Are the headlines meant to grab your attention or do they accurately represent the key point of a study and how it fits in overall research on the topic?

Dr. Gardner notes: “Scientific papers are better sources of information than media documentaries, but the language and format are inaccessible or unintelligible for most people (and boring). A documentary or video can be more entertaining and engaging, but likely some of the accuracy of information is glossed over or avoided for the purpose of keeping the viewer engaged.”