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What The Carnivore Diet Does To Your Microbiome, According To A Gut Expert

Will Bulsiewicz, M.D., MSCI
September 17, 2019
Will Bulsiewicz, M.D., MSCI
By Will Bulsiewicz, M.D., MSCI
Dr. Will Bulsiewicz, M.D., MSCI is a gastroenterologist and internationally recognized gut health expert. He completed a bachelor’s degree from Vanderbilt University, a medical degree from Georgetown University, and a master's in clinical investigation from Northwestern University.
September 17, 2019

"Hey, Doc, I heard the carnivore diet is good for your gut. Should I do it?"

High-protein, low-carb diets are kind of having a moment right now, so it probably comes as no surprise that I get questions like this quite often.

As a gastroenterologist, it is my job to critically assess the impacts of different diets on the gut microbiota (the microorganisms living inside your digestive tract). Diet is at the foundation of how I treat my patients to foster digestive health. So, while this answer might seem simple, it's important for me to walk you through the various things I consider before sharing my opinion on a specific diet and how it will affect your gut.

Let's take a trip inside my mind—here's the lowdown on the carnivore diet and your gut.

First, what is the carnivore diet?

It's pretty much exactly what it sounds like—a heavy-on-the-meat diet that consists of, well, meat, as well as a few other proteins like fish and eggs. You'll also find butter, heavy whipping cream, and some hard cheeses in the mix. But no legumes, vegetables or grains on this diet. In fact, if the paleo diet is low-carb and the keto diet is very low-carb, then consider the carnivore diet a no-carb diet.

What's the 411 on gut health?

Before we delve into how the carnivore diet and gut health work together—if at all—it's important that you first understand why gut health matters. Excuse me while I geek out.

I truly believe that all health starts in the gut. It's not just about digestion. In recent years, the immune system, metabolism, hormonal balance, mood, brain function, and even genetic expression have all been connected to the health of our gut. When people take proper care of their guts, their health tends to follow. Disease gets reversed—or, better yet, prevented—and the body functions the way nature intended. You feel energized and strong, and you're basically living your best life.

So if this is true, then it's important to know what determines a healthy gut. Thankfully, there's an answer from Rob Knight, M.D., and The American Gut Project. This groundbreaking 2018 study involved more than 15,000 microbiome samples from more than 11,000 human participants across 45 countries. It is by far the largest database connecting the gut microbiome to diet and lifestyle and therefore the best tool for understanding these connections.

Here's what Knight and researchers discovered when they analyzed their database to determine the clear-cut, most powerful determinant of a healthy gut microbiome: the diversity of plants in your diet. This was more important than age, gender, nation of origin, and even recent antibiotic exposure.

Plants and your microbiome.

Makes sense, right? Every single fruit, vegetable, whole grain, seed, nut, and legume has a unique mix of vitamins, minerals, and phytochemicals (aka healing chemicals exclusively found in plants). Some examples are the antioxidant anthocyanidins in blueberries for memory, beta-carotene in carrots for healthy eyes, and lycopene1 in tomatoes for the heart. All of the colors you find in plants are more than just easy on the eyes; they're fuel for your health.

And wait, there's more: When we're talking gut microbes, there is one nutrient more essential than the rest, and that is fiber. Prebiotic fiber is food for your gut microbes. When they feast on it, they release postbiotic short-chain fatty acids (SCFAs) that have healing effects throughout the body. They strengthen the good gut microbes and weaken the inflammatory bad ones. They also close the holes in the colon that are often referred to as "leaky gut," optimize the immune system, lower cholesterol, and regulate blood sugar. Not to mention that SCFAs protect humans from killers like heart disease, stroke, and cancer and even cross the blood-brain barrier to improve brain function. In short, they are powerful!

But there's only one way to get SCFAs, and that's through prebiotic fiber. And there is only one place you can get fiber from—plants! Yes, plants have cornered the market on fiber and SCFAs. There are some out there who may claim that you can find butyrate—one of the SCFAs—in high-fat dairy (like butter)2 or in a supplement. But just to set the record straight, ingested butyrate is not the same as butyrate produced by our microbes. When you eat your butyrate, it's absorbed almost immediately in the small intestine without ever reaching the large intestine where you need it. This is why I stress the importance of fiber for SCFAs. There is no evidence that butter or a supplement is able to adequately recreate these benefits.

OK, so what about the carnivore diet?

By now, you might be wondering where I'm going with all of this. Let me bring it full circle: Remember how the carnivore diet is all meat, eggs, and some dairy? Yeah, well, that means you can think of it as the extreme, polar opposite of a whole foods, plant-based diet.

If you've been paying attention, then you know there's a problem here. I just told you that high-quality research from one of the leading scientists on the planet has shown us that the most powerful predictor of gut health is the diversity of the plants in our diet. In this regard, the carnivore diet is one-size-fits-all, and it's not a size you want. You literally could not have less plant-based diversity than the carnivore diet. Plant diversity = ZERO.

There was also that part about the healing benefits of prebiotic fiber, how it fuels the good gut microbes by producing SCFAs and has benefits throughout the entire body, including the brain. It's an essential nutrient for a healthy gut. But again, we have a problem. Plants have a monopoly on fiber. It doesn't exist in meat, eggs, or dairy—the constituent parts of a carnivore diet. In other words, you could not have less fiber in your diet than you will have on the carnivore diet. So, if you believe that fiber is critical to gut health—as I do—then you could venture a guess as to how good the carnivore diet is for gut health.

Why the carnivore diet is bad for your gut.

Here's the beautiful thing: We don't have to venture a guess when we have high-quality research to just give us the answer. In a 2014 study3, researchers monitored changes to the microbiome day by day during five days on a whole foods, plant-based diet versus five days on a diet composed entirely of animal products—meat, eggs, and some dairy. You could call the latter the "carnivore diet," even though it didn't have a name yet

Here's what happened when participants ate only animal products:

  • Dramatic changes in the microbiome in less than 24 hours.
  • Increased growth of inflammatory bacteria (Alistipes, Bilophila, and Bacteroides) and decreased growth of anti-inflammatory bacteria (Roseburia, Eubacterium rectale, and Ruminococcus bromii).
  • Dramatic increases in Bilophila wadsworthia, a bacteria strongly associated with the development of inflammatory bowel diseases, like Crohn's and ulcerative colitis.
  • Significantly lower levels of SCFAs butyrate and acetate. (Duh!)
  • Increased antibiotic resistance in the gut.
  • Production of more secondary bile salts, which are known to cause colon and liver cancer.

Yeah, so in other words, in just five days on a carnivore-like diet, the study revealed the human body began to replace "good" anti-inflammatory bacteria with "bad" inflammatory bacteria, starve our gut of healing SCFAs, and lay the foundation for antibiotic resistance, inflammatory bowel disease, and colon cancer. None of that equates to a healthier gut.

Which brings us right back to the original question (see what I did there?): "I heard the carnivore diet is good for your gut. Do you recommend it?"

It would seem the answer is pretty clear. But in case you need me to spell it out, that's a hard "no."

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Will Bulsiewicz, M.D., MSCI author page.
Will Bulsiewicz, M.D., MSCI

Dr. Will Bulsiewicz, M.D., MSCI is a gastroenterologist and internationally recognized gut health expert who wants to help you tap into the incredible healing power that lives inside you—your gut microbiota. His medical training involved 16 years at America's elite institutions. He completed a bachelor’s degree from Vanderbilt University, a medical degree from Georgetown University, and a master's in clinical investigation from Northwestern University. Bulsiewicz was also the chief medical resident at Northwestern and the chief gastroenterology fellow at UNC, and received the highest award given by both his residency and fellowship. He also completed an epidemiology fellowship at UNC's prestigious Gillings School of Global Public Health.