The docuseries, You Are What You Eat: A Twin Experiment, released by Netflix on January 1, 2024 has already sparked a lot of contention amongst the nutrition community. On one hand, I'm thrilled when nutrition information is made more accessible to the general population through entertaining sources such as documentaries. On the other hand, I cringe in anticipation of the potential messaging full of bias, fear mongering, and sensationalism…none of which actually helps anyone with their health.

I'm excited to provide my Registered Dietitian review of both the study and the docuseries, the Stanford Twins Study. The purpose of the 8-week study published at the end of 2023 was to determine the cardiometabolic effects of a vegan diet versus an omnivore diet in 22 sets of identical twins.

Twins have the same genetic makeup, making them the perfect controls for one another. This afforded researchers the rare opportunity to gain valuable insight into how each diet may impact one twin versus the other. According to Christopher Gardner, nutrition scientist and one of the study’s investigators, individuals respond differently to the same food, so getting people who are genetically the same eliminates this confounder.

Nutrition studies as a whole are challenging. I learned this the hard way during my graduate school research study focused on female endurance athletes. Although the path forward is clear as mud when it comes to nutrition research, there have been notable victories in the field of nutrition research. For example, studies have determined that vitamin C prevents scurvy, that vitamin D deficiency leads to Ricketts, and that beri beri develops from a thiamine (vitamin B1) deficiency.

In all those examples, there is a link between a particular compound and a specific condition. In most cases, the relationship is rarely so straight forward. This is especially true when investigating conditions where multiple factors are involved, such as obesity, osteoporosis, diabetes, or heart disease. Layer in a diet that includes different types of foods and you have yourself a challenge!

All of this to say, I will continue to be an advocate for more nutrition focused research because understanding the role of food in health and disease is essential and warrants attention. I get excited when studies like the Stanford Twins Study are released because there is always something new to learn and takeaway from a research study.

Before I dive into my review of the Netflix documentary, I'd like to provide a quick analysis and overview of the Stanford Twins Study. The docuseries muddies some of the important takeaways from the study itself, so I'll walk you through the literature using my expertise as a dietitian.

Study Design & Intervention

When designing a nutrition research study, it's critical to determine the best study design to answer the research question. This study is considered a population-based randomized clinical trial where 22 pairs of twins (N = 44) were randomized to a vegan or omnivorous diet (1 twin per diet). The intervention lasted 8-weeks. For the first 4 weeks, diet-specific meals were provided via a meal delivery service. For the final 4 weeks, participants prepared their own diet-appropriate meals and snacks. All participants received a health educator counseling for diet and exercise. Individuals were told to eat until satiated.

As previously discussed, the use of identical twins is meant to take away the confounding variable of how genetics can play a role in someone's response to a dietary change. Randomized control trials are considered the gold standard when it comes to clinical research. They aim to find out which treatment or intervention is best by making a fair comparison between two or more treatments. In this case, the study aimed to find out if a vegan diet or an omnivore diet provide more beneficial cardiometabolic effects.

Main Outcomes & Measures

A well-designed study will always determine the outcomes and measures before the intervention is undertaken. This ensures we aren't "cherry-picking" for data once the study is completed. For this study, the primary outcome was measuring the difference in low-density lipoprotein cholesterol (LDL-C) concentration from before the study to conclusion of the study. The secondary outcome measures were looking at changes in cardiometabolic factors including plasma lipids, glucose, and insulin levels, plasma vitamin B12 level, and body weight/composition. Exploratory measures were adherence to study diets, ease or difficulty in following the diets, participant energy levels, and overall sense of well-being.

Results & Conclusions

Now onto the fun part! What did the results show and what do the results tell us?

LDL Cholesterol Levels

After 8 weeks, compared with twins randomized to an omnivorous diet, the twins randomized to the vegan diet experienced significant decreases in LDL-C concentration. This is a good thing! We already know that more plants & less animal fat improves LDL-C concentrations and heart health, so this is not a new finding from this study, but it's certainly nice to see this verified in the science.

HDL Cholesterol and Vitamin B12

The vegan diet group experienced a larger but nonsignificant decrease in fasting high-density lipoprotein cholesterol (HDL-C) and vitamin B12 levels. These are both unfavorable outcomes. Although not significant, these observations are relevant to the study objective given higher HDL-C levels are considered to lower the risk of heart disease. Vitamin B-12 is an essential vitamin that we cannot manufacture in our bodies, so we must get it from food. Vitamin B12 only exists in animal foods, so a vegan diet requires supplementation of Vitamin B12 to maintain sufficient levels. The finding related to Vitamin B12 decreasing in the vegan diet group follows what we know in nutrition science.

Fasting Insulin Levels

Fasting insulin levels significantly decreased in the vegan group compared to the omnivore group. Higher insulin levels may indicate insulin resistance, which can mean our blood sugar remains a little too high too frequently. This presents a greater risk for developing chronic disease, such as Type 2 diabetes. This is an interesting finding, and one that is continuing to be investigated, as many different dietary patterns have been shown to help support glucose and insulin levels in Type 2 diabetes, including the ketogenic diet.

Lean Body Mass (Muscle)

The omnivore twins gained more lean muscle compared to the vegan twins. They also ate more protein and calories compared to the vegan twins. We know that we need more protein on a vegan diet to maintain and gain muscle mass, so this study proves this point. Because the study provided different amounts of calories and protein to the vegan and omnivore diet groups, we cannot determine causation for more lean body mass in the omnivore group. I'll discuss this more when I point out the limitations of the study.

Weight Loss

The vegan diet produced greater weight loss than the omnivore diet. However, this did not account for muscle vs fat loss. As mentioned above, the omnivore twins actually gained more muscle mass and we know that muscle is heavier than fat. We also know that a higher percentage of lean body mass is favorable for overall health. This study proves the point that weight loss comes down to eating fewer calories than our body burns. We can achieve this in many ways, so it doesn't necessarily mean that an omnivorous diet cannot be used to achieve weight loss. It should also be made clear that participants lost weight on both the vegan and omnivore diet.

Diet Satisfaction

Participants receiving the omnivorous diet had nominally higher diet satisfaction at weeks 4 and 8 compared with vegan participants. This is an important finding considering dietary interventions will only work if they're considered sustainable. One way to make a dietary intervention sustainable is to also make it enjoyable.

Study Strengths

Overall, this study was ethically sound and meticulously designed, so it's important to highlight some of the strengths of this study:

• Enrolling identical twins was beneficial because they were able to eliminate the confounding influences of age, sex, and genetic factors that may affect clinical outcomes.

• The initial 4-week period of food delivery facilitated participants’ high adherence to the diet, whereas the latter 4 weeks of self-provided foods increased generalizability.

• The study used LDL-C as the primary outcome measure, which is a well-established cardiometabolic clinical value.

• They assessed an extensive set of well-studied secondary clinical outcomes to evaluate overall cardiometabolic health, including fasting insulin, weight loss, and HDL-C.

• Previous trials have reported similar metabolic and weight loss benefits of vegan diets yet tended to focus on very low–fat vegan diets, and study populations with diabetes or overweight. This study used a more moderate- and higher-fat vegan diet, the generally healthy population without diabetes or overweight, and a healthy omnivorous comparison diet (eg, higher in vegetables and fiber than the baseline diet).

• To provide fair and objective comparisons and avoid “straw man” comparators, the study emphasized high-quality, exemplary dietary choices to participants on both diets.

Study Limitations

You can't analyze a research study without pointing out the limitations! During my first few semesters of grad school, I despised when we went over study limitations because I always felt like I was saying the same things such as, population size is too small, study duration is too short, study subjects are not diverse, etc. However, the more literature reviews and research I read, the more I was able to pick up on the subtle nuances that can be really problematic.

For the Stanford Twins Study, here are a few of the limitations I uncovered:

• The diet was not calorie controlled, so results cannot show causation between the two diets and weight loss. The vegan diet did not include as many calories as the omnivore diet.

• Self-reported nutrition intake from a 24-hour recall or input into a nutrition tracker database may not be accurate. The participants may have over or underestimated portion sizes, omitted certain foods, inflated consumption of foods they "should" have been eating, etc.

• The published study did not include the gut health and body composition data that was measured. This was covered in the Netflix documentary, but not in the published research article.

• The study did not continue to follow the twins to see if they continued with healthier eating habits and exercise regimens. This limits the stability and sustainability of the diet behaviors tested.

• The adult twin population was generally healthy and may not be generalizable to other populations.

• The sample size (N = 44) is small. Always important to point this out!

• The study duration was short (8 weeks). We know that dietary changes take time, and sometimes adverse or beneficial impacts take significant time to uncover.

• There was no diversity in education and socioeconomic status among participants. We know that health literacy and socioeconomics are major determinants of health.

A Registered Dietitian's Takeaway From the Study

This study did show us that a plant-based diet can be used to optimize cardiovascular health, especially in terms of lowering LDL-C and fasting insulin levels, as well as supporting weight loss. However, the study revealed an interesting discovery that's important to highlight. While the vegan diet group experienced some health benefits compared to the omnivore diet group, the vegan group reported lower diet satisfaction.

As a Registered Dietitian, my nutrition philosophy is built on the foundation of having a positive relationship with food and your body. You need to be enjoying the food you're consuming and interacting with on a daily basis. Without that solid foundation, I find that my clients don't continue engaging in certain supportive behaviors, despite the health benefits they experience. In other words, enjoyment, pleasure, and sustainability are key to long-term adherence.

This study adds a valuable piece to the library of nutrition science we have indicating that plant-based diets can offer significant cardiometabolic advantages. However, it also underscores the importance of balancing health benefits with personal preferences and lifestyle considerations. Health does not exist in a vacuum - it's multi-faceted and even more complicated by the dynamic environment we live in. It’s a reminder that the journey to health through diet is not just about the nutrients we consume but also about the enjoyment and satisfaction we derive from our food choices.

A Registered Dietitian's Review of You Are What You Eat: A Twin Experiment

Now that we have a comprehensive understanding of the study itself, let's dig into the documentary and how the science was presented to create entertainment.

First, let's take a look at who was interviewed throughout the documentary. Five minutes into the documentary, I actually paused the show and told the people I was watching the series with that I could immediately tell this series was going to be heavily biased towards a vegan or plant-based lifestyle. Now you know what it's like to watch something like this with a Registered Dietitian haha!

All of the nutrition and science experts, activists, politicians, farmers, and chefs listed below tend to be more pro-plant-based, so it was quite obvious the direction this documentary was going to go.

• Christopher Gardner: Leads the Stanford Nutrition Study, providing key insights into the study’s design and findings.

• Irwin Goldstein: Physician and director at San Diego Sexual Medicine, leading the sexual health portion of the study.

• Erica Sonnenburg: A microbiome scientist from Stanford University, discussing the impact of diet on gut health.

• Lucia Aronica: An epigenetics specialist at Stanford University, providing insights into the relationship between diet, genetics, and epigenetics.

• Dr. Michael Greger, Marion Nestle, and Nicole Avena: Discussing the health risks associated with excessive animal product consumption.

• Nimai Delgado: A lacto/vegetarian professional bodybuilder, offering perspectives on building muscle on a plant-based diet.

• Ayesha and Dean Sherzai: Neurologists discussing the impact of diet on cognitive health.

• Cory Booker: U.S. Senator discussing the impact of food choices on health, society, and the environment.

• Sherri White-Williamson: Lawyer and environmental justice activist, addressing the negative impacts of confined animal feeding operations.

• Don Staniford: Researcher and activist campaigning against salmon farming, highlighting environmental and health concerns.

• Leah Garces: CEO of Mercy for Animals, working on alternative farming practices like mushroom farming.

• Shakara Tyler: of the Detroit Black Community Food Security Network, working to reclaim equal access to culturally-relevant plant foods.

• Eric Adams: Mayor of New York City, sharing his experience of managing Type 2 diabetes through diet and lifestyle changes.

• Thomas Locke: A regenerative cattle rancher discussing sustainable farming practices.

• Danielle Daguio: From Keep Growing Detroit Farm, providing insights into urban farming and access to fresh foods.

• Daniel Humm: Chef and owner of Eleven Madison Park, sharing his experience transitioning to a plant-based menu.

• Craig Watts: A former chicken farmer who transitioned to mushroom farming, reflecting on the emotional impact of factory farming.

The guests and narrators featured on the docuseries both enhanced and discredited the presentation of the information. They enhanced the information as interviewing experts can contribute to a more interesting and in-depth discussion. The docuseries skillfully wove together a story of different perspectives from advocates, farmers, industry innovators, and academics. By curating the featured guests, the docuseries beautifully highlighted the influence of human dietary choices on our environment, which is something we cannot ignore if we want a future on this planet.

However, the slew of plant-forward guests and narrators discredited the presentation by revealing an obvious bias. In this case, the majority of the guests and narrators featured are either vegan themselves, advocates for animal welfare, or some combination of environmentalists and activists geared towards a plant-based lifestyle. This one-sided portrayal towards veganism could potentially alienate a segment of the audience. It also doesn't leave room for a more holistic presentation of the science. The dramatic way in which Eric Adams recounted his story and experience of reversing his Type 2 diabetes with plant-based eating certainly caught my attention!

There are some gaps in the docuseries' narrative, particularly in terms of personalized nutrition and the broader cultural and policy implications of adopting plant-based diets. The series presents a harsh dichotomy of dietary choices, which oversimplifies the choices we have as consumers. As a dietitian, I'm rarely labeling someone as omnivore or vegan. They're just labels and sometimes these labels can be really harmful and restrictive. Instead, I look at my client's and patient's diets on a spectrum. The reality is more nuanced, with various degrees of plant-based diets that can still offer health benefits without completely eliminating animal products.

In terms of policy implications, I would have liked to have seen a more in-depth discussion associated with policy integration of these dietary insights. The interview with Cory Booker, a vegan US senator who continues to advocate for a more sustainable food system, and was the closest we got.

The docuseries also oversimplified the term "healthy diets." The series portrayed vegan as equivalent to healthy and omnivore as equivalent to unhealthy, which is very misleading. A vegan diet can be just as processed and unbalanced as someone eating both animal and plant based foods. The study clearly laid out what is considered a "healthy vegan" and "healthy omnivore" diet, but the docuseries was unable to properly tease this out. Perhaps this was intentional given their bias towards veganism.

As a Registered Dietitian, I value the series for initiating critical conversations about nutrition and health. I'm constantly preaching the powers of medical nutrition therapy as a tool to prevent chronic disease, such as cardiovascular disease. Studies like this uncover how nutrition can be used to replace some medications, such as statins, which are used to lower LDL-C levels in patients. This is a large reason why I wish dietitians were built into the primary care medicine model and more accessible through insurance.

I enjoyed the unique inclusion of identical twins, which adds a fascinating layer to the both the science and the docuseries. It allowed the docuseries to highlight the importance of evaluating body composition over just body weight or body mass index, which is often overshadowed in mainstream health discussions.

You Are What You Eat: A Twin Experiment created an accessible resource for the general population to engage in learning more about diet, health, and sustainability. As a dietitian, I do fear that some may take the sensationalized content too seriously and engage in drastic dietary changes without the guidance of a healthcare provider. So here's my shameless plug that if you are curious about making dietary changes to support your health, please reach out to a Registered Dietitian or other healthcare provider!

I hope you found this review helpful!

Skylar Weir, MS, RD