Have you seen the recent documentary on Netflix called “You Are What You Eat: A Twin Experiment?” The documentary looks at how different diets affect sets of twins’ health. 

The show concluded that a plant-based (vegan) diet is healthier than an omnivorous diet. While that may (or may not) be true, we can’t confirm that from this study. Why? Many scientists have identified shortcomings in the design of their study. Scroll down to decide for yourself!

Study Design: Barbie Dolls and The Independent Variable

Let’s run through a quick thought experiment before we jump into the documentary.

Imagine that your daughter’s birthday is coming up, and you’re trying to determine which type of Barbie doll she prefers to play with. You hypothesize that she likes blonde Barbies over brunette Barbies. To find out, you allow her to play with two groups of Barbie dolls – one group being blonde, the other being brunette – to see which group she plays with for the longest. Each of the Barbie dolls is wearing a unique outfit from the other Barbies. Most of the blondes are wearing pink and most of the brunettes are wearing blue.

You find that your daughter plays with the blonde Barbie dolls significantly more than she plays with the brunette Barbie dolls. So, you assume that she likes blonde Barbies more than brunette Barbies – right? Not necessarily!

Why? Since the blondes were typically wearing pink and the brunettes were typically wearing blue, you couldn’t know for sure whether your daughter enjoyed playing with the blonde Barbies because they were wearing pink or because of the color of their hair.

How would you know for sure that your daughter prefers blonde-haired Barbie dolls? You would have had to make the Barbies identical to each other aside from their hair color. AKA, you should have put all of the dolls in identical outfits so that hair color is the only difference tested in your experiment. In science, we call this (the one detail that is changed) an “independent variable.”

Let’s take a look at the study design in the Netflix documentary and decide whether it incorporated an independent variable.

Their Study Design vs. Our Barbie Doll Study Design

The producers of the show designed the study. Their aim was to find out how adhering to specific diets – namely, a healthy vegan diet and a healthy omnivorous diet – would affect the metabolic and cardiovascular health of their participants. 

In an attempt to control for genetic differences in the diets’ effects, the producers used twins in their study. Over eight weeks, one twin would eat the vegan diet and the other twin would eat the omnivorous diet. Several sets of twins were enrolled. Their metabolic and cardiovascular health markers were tested before and after the diet period. 

The participants who followed the vegan diet saw more significant improvements in LDL-C than the omnivorous group. LDL-C is an important cardiovascular health marker – having unhealthy levels is associated with heart attack risk, among other things.

So, it sounds like the vegan diet was healthier than the omnivorous diet, right? Not so fast! Let’s check to make sure there is an independent variable. 

First – props to the producers for using twins. This controls for genetic differences between study participants.

But is the diet the only thing that was changed between the groups? Not quite. 

Where the Documentary Falls Short: No Independent Variable

Here’s where things get precarious. The participants following the vegan diet consumed fewer saturated fats and fewer calories than the participants following the omnivorous diet. 

Think back to our Barbie doll thought experiment. How could we tell if our daughter preferred blonde Barbies if each group of Barbies wore different outfits? Maybe the outfits played a role in her doll preference. Similarly, in the documentary, how can we tell it was veganism itself that made the difference in the participants’ LDL-C levels? Maybe it was the lower calories and saturated fats that affected the change. 

Ideally, the producers would have controlled for calories and saturated fats – meaning they could have made both twins consume the same amount of calories and saturated fats. That way, we could see whether it was veganism itself that created the advantageous results.

Does this mean the documentary’s findings are useless? Not completely! We do see that people who eat vegan diets typically consume fewer calories and saturated fats than those who eat omnivorous diets. However, the key word is typically – not “always.” Just keep in mind that you may be able to get the same effect while eating an omnivorous diet by cutting calories and saturated fats. We just don’t know for sure. 

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