Episode 144 – You Are What You Eat: A Twin Experiment – Dietitian’s Review

Key Topics Covered

You are what you eat: a twin experiment netflix cover.

When reviewin this, we are going to be talking interchangeably between the documentary and the study that it was based on. 

What Was the Study?

  • They got 22 sets of twins (44 participants total) and aimed to compare a vegan diet to an omnivorous diet.
  • The study is titled – ‘Cardiometabolic Effects of Omnivorous vs Vegan Diets in Identical Twins’.
  • It was an RCT with the timeframe being 8 weeks total. The first 4 weeks were delivered meals, the last 4 were self-made. 
  • The conclusion was “those consuming a healthy vegan diet showed significantly improved low-density lipoprotein cholesterol concentration, fasting insulin level, and weight loss compared with twins consuming a healthy omnivorous diet.
  • Another key factor worth being aware of is the specific wording from the study “Although weight loss was not discouraged, our diet design did not include a prescribed energy restriction and was not intended to be a weight loss study. Participants were told to eat until they were satiated throughout the study.”

Unpacking the Study

Macronutrients distribution of you are what you eat study.
Fat distrubtion of the 'you are what you eat' study.
Carbohydrate distribution of the 'you are what you eat' study.

  • The main results reported in this paper was that the reduction in LDL, fasting insulin, and weight was greater in the vegan group.
  • Note that the weight change was not massive. We are talking about a 1.5kg difference.
  • A very important part of this conversation from both angles is that intake was ad-libitum.
  • Calorie intake was reported throughout. We know that self-reported data isn’t perfect. But it’s still helpful. Calorie intake was around 200 calories lower in the vegan group throughout the 8 weeks.
  • Protein also happened to be lower.
  • Saturated fat was higher in the omnivorous group.
  • Fibre intake was about ⅓ higher in the vegan group.

Interpreting This Data

  • As an independent variable, reducing saturated fat and replacing it with polyunsaturated fat typically seems to reduce cholesterol. 
  • Higher fibre intake also contributes to reductions in cholesterol.
  • The reduced calorie intake is a huge factor in the reduction in body weight, fasting insulin and LDLs too.
  • This can be interpreted two ways:
  1. This can be achieved with omnivorous diets with some additional specific interventions (increased fibre, decreased saturated fat & reduce overall energy intake). Regardless, we typically encourage relatively low saturated fat intake and high fibre intakes no matter if someone is on a plant-based or omnivorous diet.
  2. Most people would benefit from eating more fruits, vegetables, whole grains, and minimizing their intake of high-fat meats. 
  3. Alternatively, you could look at this and also acknowledge that being on a plant-based diet means you will be way more likely to achieve these things – plus they will kind of just happen by default. For a lot of people going vegan with a focus on wholefoods is an easier to understand intervention than “I am going to reduce my saturated fat intake, increase my fibre intake, and go into a small caloric deficit”. The “how” is a lot more obvious in the first one and it’s also a bit more exciting
  • For a lot of people going vegan with a focus on wholefoods is an easier to understand intervention than “I am going to reduce my saturated fat intake, increase my fibre intake, and go into a small caloric deficit”. The “how” is a lot more obvious in the first one and it’s also a bit more exciting.

Documentary Breakdown – Part 1

Twins from 'you are what you eat' preparing food.

  • My first note was how much the documentary in a way kind of devalued how strictly controlled most people would picture a study like this. 
  • There are so many examples. The first one is that the paper talks about how all food is provided for the first 4 weeks and then it is consumed ad libitum. But there are parts in the documentary where they are getting the DEXA scans and they are being told to do certain things with their diet e.g. the leanest twins are told to focus on building muscle and getting more calories in and the heavier twins are getting different advice. Is it really ad libitum if you are intentionally trying to eat more calories?
  • There was footage of the twins who were chefs eating non-pre-prepared food during the first 4 weeks while at work. It’s assumed that people aren’t 100% adherent, but I’d never seen it literally being filmed and also not being commented on.

Documentary Breakdown – Part 2

Twins from 'you are what you eat' eating food.

  • Although we have the results of the paper – it was also interesting seeing some case studies of individuals in the documentary getting DEXA scans at the end.
  • With the lean twins who were told to eat more – the omnivore gained 7.1lbs of lean mass and the vegan one gained 2.3lbs of muscle. That was interesting to see.
  • But this full section also further exposed some big issues.

Example One

  • One example is a set of male twins.
  • The one who was in the vegan group had a young family and moved house multiple times during the study. He missed many training sessions and just did some bodyweight exercises at home.
  • The omnivorous one was lifting consistently in the gym.
  • When the DEXA showed that they both lost weight but the omnivore one gained lean mass, while the vegan one lost lean mass, that is far less about the diet and more about the exercise.

Example Two

  • The twins that we had previously mentioned were eating food off the plan at work didn’t really follow the protocol.
    • They stuck to vegan vs omnivore – but randomly started doing cardio every day and completely changed their diets. They said the diets were too carb-heavy and followed a different intake to what was prescribed.
    • They had a wild result. The one on the vegan group lost 7.6lbs of weight, of which only 1lb was body fat. The omnivorous one lost 3.5lbs, all of which was lean mass. 

Example Three

  • Another set of twins had a similar issue with a huge difference in outcomes.
  • The one in the vegan group just focused on walking, while the one in the omnivorous group was lifting weights

Final Notes

Twins from 'you are what you eat' eating food/

  • There are many ‘abstract readers’ who have pointed to this study as the be-all and end-all of omnivorous vs vegan. From both sides. 
  • Those in the omnivorous camp point out that the plant-based group lost more lean mass. The people in the plant-based group will point to things like LDL, visceral fat, fasting insulin, weight and TMAO. 
  • On the one hand, it’s very impressive to get 22 sets of twins willing to do a study like this. But on the other hand it’s hard to watch people interpret this study as if it was strictly controlled and all the food/exercise were matched the way we would like

Ethical Aspects

Factory farming chickens.

  • They included many aspects outside of nutrition such as climate change, ethical dilemmas, factory farming, fish farms, deforestation, bacteria/virus issues, and many others.
  • This is personally why I (Leah) went vegan almost a decade ago now.
  • The health arguments for veganism never really struck a cord me because I always felt that whether I had a vegan or omni diet, I would be able to maintain a health-promoting diet. I still feel this way 10 years later and after practicing as a dietitian for half that time (specifically in the vegan space).
  • On the environmental side of things, it is pretty clear that we are not going to be able to sustain such a high global consumption of animal products forever.
    • Either we need to completely shift animal agriculture (i.e. lab-grown meat) or we need to make some pretty big population changes.
  • BUT, I have always been a firm believer in “we need millions of people eating less meat, more plants – not a handful of perfect vegans”.


  • The fact that twins were used for this study is great. But it also doesn’t make it perfectly like for like since adherence to the training and nutrition was a big difference here
  • You could look at the outcomes from multiple angles. These same results can be achieved on an omnivorous diet. Being plant-based likely does make it easier to see reductions in things like body weight, fasting insulin and LDL cholesterol. And alternatively, muscle loss on a plant-based diet could be avoided by increasing the protein intake.

Relevant Links

Studies Mentioned: