The study was recently published on November 30th 2023. It’s now become a Netflix documentary with a lot of chatter about how good the vegan diet is.  But the documentary didn’t really look at all the results of the study, or some of its flaws.  The title of this study in question is:

Cardiometabolic Effects of Omnivorous vs Vegan Diets in Identical Twins

Basically, the study is looking at a vegan diet (no animal products at all: no meat, poultry, pork, eggs, dairy, seafood- only plants) vs an omnivore diet (both animal products and plants) and their effects on your risk factors for heart disease.  It states: “In this randomized clinical trial of healthy, adult identical twins, the 8-week change in LDL-C level [“bad cholesterol”]—the primary outcome—was significantly lower for twins receiving the vegan diet compared with twins receiving the omnivorous diet”. It also noted that insulin and weight also went down more in the vegan group (plants only) than the omnivore group (plants and animal).

SOLD!!! Right? Going vegan saw more improvement in LDL, insulin, and weight compared to eating meat.  Not so fast. Let’s look a little closer at all the details and results.

Study size: 44 people (22 sets of twins) and while I’m impressed they found this many identical twins, and using twins is really smart- as they are genetically identical…it’s not a very big group of people. Additionally most of them were women (75% or 34 of the 44 people).  The study also noted they were pretty healthy to start- could that have skewed the results at all?

Duration: Only 8 weeks, that’s not very long.  There can be a lot of good results in the beginning of a diet, but really, is it sustainable in the long term? A vegan diet is not very easy to follow (there are a lot of products with animal products in them you might not even know about) and you need to do it right, or you’ll have gaps in your nutrition.  You can’t just cut foods out, you need to add specific foods to make sure you get all your vitamins, minerals, and protein. Case in point: the study found the vegan group ate less protein overall and consumed less B12- a vitamin that is vital for DNA production and red blood cell formation.

Human Error: The first 4 weeks the study provided food to the participants in the form of prepared meals. During the second part the participants were to continue their assignment (vegan or omnivore), but they were now making their own meals. The participants were educated and encouraged to eat in a similar way to match what they were provided for the first 4 weeks, but ultimately it was up to them. The study checked in on participants with 3 unannounced 24 hour recalls (where they ask the participants what they ate in the previous 24 hours). However, 24 hour recalls are notorious for their inaccuracy as it’s up to the person to remember what they ate and how much, which they might not remember, or estimate the wrong portion size (there is a big difference between 2 tablespoons of olive oil and 3).  Possibly the omnivore group was a little more relaxed in the second 4 weeks…

Lab results:  From this chart below, we can see that the omnivore twin results are in black, and the vegan twin results are in orange.  Time to break it down.

LDL– Point Vegan as their LDL, better known as “bad cholesterol”, went down.

HDL- Point Omnivore.  HDL or “good cholesterol” went down.  We don’t want this going down.

Triglycerides– Point Omnivore.  Triglycerides went up in the vegan group; that’s the opposite direction you want them to go.

Glucose- Point Omnivore- though barely.  

Insulin- Point Vegan.  There was a good drop from baseline for insulin in the vegan group; this is good.

Vitamin B12– Point Omnivore- you don’t want B12 going down though this is probably expected.  B12 is only found in animal products, so you have to try really hard to get it in as a vegan.

Weight– Point Vegan.  The vegan group did lose more weight than the omnivore diet, but both groups did lose weight.  What would happen if the study went longer?  I’ve seen lots of studies where weight loss might be fast in the beginning for one group, but end up being the same for both groups by the end of the study.  It should also be noted that while the vegan group did lose more weight overall, they lost more muscle mass than the omnivore group, which is not what you want.

The clinically significant results (meaning, they were big enough changes to really mean something) were the LDL, insulin, and weight results, which all were in favor of the vegan group.  Although there were some points scored for the Omnivore diet, they weren’t as impressive.

So did the vegan diet win?  Sort of?  They definitely improved some of their labs and weight, but not ALL their labs went in the right direction.  Plus, it’s a pretty short time frame to say anything definitively.  The study also reported the twins on the vegan diet felt more restricted, which will most likely lead to them falling off the diet faster (aka, it’s not sustainable, and that means their labs and weight might just revert back to their original numbers once off the diet).

Bottom line:  This study did find improvements in LDL, weight, and insulin- which are good things!  However, it warrants further research.  What happens after a year? Are the participants still on the vegan diet?  What happened to the labs and weight for both groups after a longer period of time?  What are the results like when starting with a population that’s less healthy- maybe already have heart disease?  When it comes to reading the results or a study, be a little skeptical and look at ALL the results, and how the study was designed.  Another tip- who funded the study- any conflicts of interest? 

My takeaway?  At Simple Start Nutrition we ALWAYS encourage fruits and vegetables- terms like “plant based” or “plant forward” mean that type of style.  AKA- you focus a lot on plants (fruits, veggies, whole grains, plant based fats) and less on animal products (meat, dairy, butter, etc), but they’re still in there.  This can be a more realistic approach- more sustainable.  Not quite vegetarian or vegan, but easier to follow and likely more enjoyable if you don’t want to give up dairy (ice cream) or meat (like bacon:).

As always, we recommend you talk to a dietitian if you want to make any big changes to your diet, and going vegan is a big one. Even making little changes can be easier with a dietitian on your side.  So reach out whenever!

Stay Happy, Stay Healthy

This blog was written by Chris Henigan MS, RD, LDN, co-founder of Simple Start Nutrition.  For more blog posts and nutrition information follow @simplestartnutrition on Instagram and Facebook.  Also feel free to contact us, by visiting our website (simplestartnutrition.com) to schedule an appointment to discuss your nutrition related needs.