Stanford Twin Study: Are Vegan Diets Healthier?

Bret: 
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You’ve probably heard the results of the Stanford Twin Study with all the news it’s generated. The conclusion, at least as it’s portrayed in the media, is that a vegan diet is healthier than an omnivore diet. And we would all likely be healthier if we went vegan, or at the very least, focused more on plant foods and less on animal foods.

And that conclusion has gotten a fair amount of press, partly because it nicely fits the bias that many in nutrition and medicine have. We all have biases. So, that’s not a knock. It’s just an observation. But it turns out those conclusions are misleading, and this is a perfect study to highlight the problems with nutrition science, especially inaccuracies of interpretation and the misrepresentation of the findings.

So, while the study wasn’t a bad study, per se, we should be very cautious about how strongly we interpret the results. Now, I would argue that the strength of this study doesn’t deserve near the attention it’s getting. So, let’s walk through the study so you can see for yourself how the results were maybe overplayed. At the end, I’m going to offer a challenge, or actually an opportunity, to the Stanford researchers for how we can improve this and work together.

So, stay tuned for that. But first, the basics. The researchers at Stanford recruited 22 pairs of twins and randomized them to a healthy, minimally-processed vegan or omnivore diet for eight weeks. The primary outcome was LDL level with other labs, like HDL, triglycerides and insulin level, and body weight being secondary outcomes.

So, right away, we see this was small and short study, right? Only 44 people, 22 in each arm, for eight weeks. And second, the outcome measures were labs, not clinical outcomes, which we would expect for a study that’s only eight weeks long. But how much can we learn about someone’s health in only eight weeks?

Probably not much. And look, doing research, especially nutrition research, is hard. So again, it’s not a knock against the study, per se, but it’s a clear indication that we need caution interpreting such a limited intervention. And another critical point is that the study compared two diets and two diets only.

So, we can only conclude if one diet is quote unquote “better” or “different” than the other, but not compared to any other diets. So, let me explain. The subjects were randomized to a vegan diet or an omnivore diet with both diets containing fruit, veggies, nuts, seeds, whole grains. And if you look at the supplemental information, you can see that these were both high-carb diets with the vegans eating around 50% carbohydrates and the omnivores 41%.

So, from my standpoint, it’s very important to point out this was not a low-carb study, and that’s okay, right? The authors never claimed it was, but we can’t conclude anything about a vegan diet versus a low-carb omnivore diet or a vegan diet versus any other way of eating for that matter. We can only conclude a comparison of these two high-carb diets, something that I haven’t really seen expressed in media coverage.

Okay, so what did they find? First, the people on the vegan diet ate around 200 fewer calories per day. So, no surprise then that they lost a little more weight than the people on the omnivore diet. And they also had a small decrease in their LDL and their insulin levels. LDL was 95 compared to 116 on the omnivore diet, and insulin was 10 versus 13.

Now, those are statistically significant, albeit small differences. But we should wonder, was there a difference only because of the weight loss or was it due to the difference in plant and animal foods? Now, ironically, it’s the same question that sometimes comes up with keto diet studies, as those eating keto often eat fewer calories and lose more weight. But this study has no way of answering that question, and it may not matter unless you wanted to extrapolate the healthfulness of a vegan diet to everyone.

So, if all you concluded was how one diet compares to another, then maybe it doesn’t matter. But if you want to extrapolate the findings to all plant-based diets compared to all animal containing diets, then you would need to be very confident about the mechanism, which we can’t be from this study.

And there are a couple other issues worth mentioning. So, the omnivores ate more sugar than the vegans. The absolute amount was still much lower than levels for the standard American diet, but it was still a noticeable difference and statistically significant. So, that could be important. And the last thing I’ll mention is that the vegans enjoyed their meals less, and only one subject said they would stay on a vegan diet long term.

So, this goes back to being only an eight week study. As a doctor, I’m not too concerned about what happens to you in eight weeks. I’m more concerned about eight years, the rest of your life. So, if the diet is unsustainable for you, then I’m not interested in it as a clinician for that one person. It may be the right diet long term, and that’s great. But for everyone else in this study, it clearly wasn’t.

So again, it’s not that it’s bad science and should be thrown out, but I’m concerned about the way the findings are being overplayed in the media. We have to be responsible for how we discuss the results and understand the very limited scope of the study and very limited results. The conclusion should be among high-carb diets after eight weeks, vegans ate fewer calories, lost more weight, and lowered LDL and insulin a little more than those following a high-carb omnivore diet.

But we’re not able to extrapolate these findings to any other comparison of plant or animal foods. That’s a lot less interesting than just saying, vegan diet improves heart health, and we should all eat more plants. That’s much more catchy. But as I said, this study was narrowly designed, and therefore, we should have a narrow interpretation.

Here’s the exciting part. We would love to see an extension of this study looking at a true low-carb or keto omnivore diet compared to a vegan diet. Again, comparing weight, insulin resistance, blood pressure, body composition, other metabolic and lipid markers, and willingness to stay on the diet In fact.

We want to partner with Stanford to help fund and execute this study. So, if Dr. Chris Gardner or the other Stanford researchers are watching this, please reach out to us and let’s make this happen. And if they aren’t watching it, please share it around so it makes its way to them so we can make this a reality.

We want to help make nutrition science better and to find ways to truly help us understand what role different diets can play in our health. Come on Stanford, let’s team up, and let’s get this done. Thanks for listening to the Metabolic Mind Podcast. If you found this episode helpful, please leave a rating and comment as we’d love to hear from you.

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