Are Low Carb or Keto Diets Bad for Your Gut Health?

Why there’s confusion and what to consider.

Some articles circulate online claiming that low-carb or ketogenic diets starve your gut bacteria and are thus unhealthy. But these diets are not bad for your gut health. The claims are misleading: while they may starve some bacterial populations, they may promote the growth of others. These diets may not work for everyone. If you feel fatigued or irritable on a low-carb or keto diet, a moderate to higher-carb diet might be more ideal for your body. A diet low in fermentable carbs (called the low FODMAP diet) is worth considering if you suffer gas, bloating, diarrhea, constipation, or other digestive challenges.

Dr. Michael Ruscio, DC: Are low-carbohydrate or ketogenic diets bad for your gut health? Hi, this is Dr. Ruscio, and let’s discuss this question: if low-carb or ketogenic diets are bad for or could be damaging your gut health. In short, the answer is no. Low-carb or ketogenic diets are not bad for your gut health. But there’s confusion around this. Allow me to dispel briefly why there’s confusion and what this means for you.

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DrMR: Again, in short, if you’re doing a lower-carb or a ketogenic diet and deriving benefit from it, continue with it. Now, do not continue with that dietary approach if you’re feeling fatigued or irritable or having insomnia. A lower-carb or ketogenic dietary approach is not going to be the best approach for everyone, but for some, it will be helpful. Unfortunately, you may come across information on the internet that tells you a number of things. One of which could be that a low-carb diet can starve your gut bacteria and be bad for your gut bacterial health. And this is really misleading because while, yes, a lower-carb or ketogenic diet may starve certain populations, or you may see a dwindling of certain populations of bacteria, other populations of bacteria may actually bloom on a lower-carb diet.

There’s a debate in terms of what population of bacteria are better than the others. That is so littered with speculation that you as the healthcare consumer should not bother wading in that confusion.

What’s the Impact of a Low-Carb Diet?

Rather, we can look to outcome studies—meaning, what happens when people eat a certain way—for the answer to the question of what kind of impact a lower carb diet could have.

I’m going to extrapolate a little bit from research on the paleo diet. Research has been published showing that a paleo-type diet—which is oftentimes, but not always, lower in carb—reduces colorectal cancer risk to a similar degree as the Mediterranean diet. So some good evidence there. Now there are a number of trials showing that a lower-carb diet, oftentimes looking at this through the context of either a paleo diet or an overtly low-carb diet, can be beneficial for weight loss, metabolism, and cholesterol levels. So more good news there. It can improve your metabolism.

Now, how this ties in with gut bacteria… some studies have actually looked at a moderate, low, and very low-carb grouping of patients, and tracked what happened in their microbiotas and how that correlated with weight change. And ironically, in one study, in particular, found that the very low-carb diet group saw the most weight loss and had a measurable shift in their intestinal bacteria. And the higher the carb intake went, the less the change on the intestinal bacteria, and the less the weight loss.

So this thinking—that if a diet impacts or causes a dwindling of certain bacterial populations, it’s going to automatically be bad for you—is really an erroneous premise that doesn’t have good evidence to support it.

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Further yet—and I review this evidence extensively in Healthy Gut, Healthy You—when you look at fiber content in the diet, as compared to what’s known as all-cause mortality or morbidity (so, your chances of dying from any disease or having any disease), there is no general trend showing that eating more fiber is better for your health.

There are about as much data showing that higher fiber intake is good for you as there are data showing higher fiber intake has no impact on your health. So there’s no need to subject yourself to a higher fiber diet if you don’t feel good on that diet. Now, fiber and carb intake in the diet don’t share this direct relationship. But oftentimes what you’ll see is, when someone is doing a ketogenic diet, their fiber intake may become a little bit lower. If that ends up being you, that’s nothing to necessarily worry about. I do think it’s a good idea to make sure you’re not eating an unhealthy version of a low-carb or ketogenic diet and making sure that you get an ample amount of vegetables in your diet and some fruits. When we look at the data comparatively, vegetarian diets do tend to have a higher fiber intake than lower-carb diets, but what that means, I think, is up for debate.

Feeding Bacteria Isn’t Everything

And finally, if you’re someone looking for digestive relief by employing a low-carb or ketogenic diet, (if we look at a low FODMAP diet as a proxy) we see that those on a low FODMAP diet—a diet that by design reduces the amount of fermentable carbohydrates, so many forms of fruits and vegetables are reduced—we actually see that that can be very helpful for many people with digestive maladies like gas, bloating, constipation, heartburn, diarrhea, loose stools, abdominal pain, and what have you.

More evidence showing that a diet that, by design, restricts foods that feed healthy bacteria can actually cause you to be healthier. That tells us there’s quite an ample amount of evidence showing you do not need to directly feed gut bacteria in order to be healthy. The posit that I put forth in Healthy Gut, Healthy You is that it’s not about just feeding gut bacteria and hoping that those gut bacteria make you healthy, but rather eating in such a way that makes you the host as healthy as you can be. And when you are healthy as the host that houses the microbiota—your bacteria—a healthier host means a healthier environment, means healthier bacteria can grow. That is what you should be focusing on. Whether it be feeling better on a lower-carb or ketogenic type diet, or on the other end of the spectrum, someone who feels better on a moderate to higher carbohydrate diet.

The carbohydrate diet needs to be personalized to the individual. If you as an individual are feeling better on a lower-carb diet, and that’s creating a healthier you and healthier environment, then ostensibly you will see healthier bacteria grow. Conversely, if you’re someone who feels better on a higher carb diet, then you will likely have a healthier internal ecosystem that will harbor healthier bacteria, and that will work well for you.

In short, there is no good evidence to my knowledge—and this is after a fairly extensive review of the literature—showing that a lower-carb or ketogenic diet is bad for your gut health. In fact, there’s more evidence showing that it could be quite good for your gut health. But remember not to be dogmatic regarding diet. If you try that type of diet and don’t feel well, then you may want to try a moderate to higher-carb diet and see how you feel. So the answer: No, a low carb or ketogenic diet is not bad for your gut health.

This is Dr. Ruscio, and hopefully, this information helps you get healthy and get back to your life.

What do you think? I would like to hear your thoughts or experience with this.

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