Why a keto cheat day is a bad idea

A keto cheat day is a bad idea. In fact, I think cheating on a keto diet opens up much bigger problems than cheating on any other type of diet. I posted a blog covering a study indicating as much earlier this week and the keto police arrested me. Check that out here.

Apparently, since the higher fat diet was 30% fat, this study has absolutely nothing to do with keto. The most frequent comments were that my conclusions didn’t represent the data. Keto is different than a higher fat diet with 30% carbohydrates. I asked how, and got no answers.

The reasons I received no answers is that a keto diet isn’t different. My conclusions are fully supported by the data. And in this blog I’ll lay out the rest of the data showing why.

Before we get to that, I’ll briefly summarize the data in the blog above. Researchers found that 5 days of eating a weight maintenance diet high in fat(55% fat, 30% carb, 15% fat) doubled fasting LPS/endotoxin. But, this was despite no increase in “leaky gut”

Thus, the theory is that increasing the amount of LPS in the gut increases passive absorption in to the blood. That’s the essence of the study.

Is keto any different?

We know for a fact that eating a keto diet changes the microbiome, and in short time. But does it increase the amount of LPS in the gut? The answer to that is a resounding YES! A review in Nutrients provides an excellent graphic illustrating what happens to the microbiome during a keto diet.

Keto cheat day

It’s important to point out that the 2 primary phyla of bacteria in the human gut are Firmicutes and Bacteroidetes. Different diet components alter the Firmicutes:Bacteroidetes ratio. A keto diet increases Bacteroidetes.

Another review on the effects of a keto diet on the microbiome elaborates on the human data.

A study in children with refractory epilepsy after a week of KD showed a reduction in richness of gut microbiota, also revealing an increment in Bacteroidetes… At genus level, Bacteroides, Bifidobacterium, and Prevotella resulted increased after KD…

In another study, in children affected by refractory epilepsy, after 6 months of treatment with KD, the fecal microbiota profiles showed lower diversity after KD therapy and revealed significantly decreased abundance of Firmicutes and increased levels of Bacteroidetes

So why is the increase in Bacteroidetes a concern in a keto diet? To quote another study

Bacteroidetes bacteria are the main contributors to LPS biosynthesis in the human gut microbiome.

Looking at the data, it’s pretty clear that a keto diet increases Bacteroidetes and increases LPS in the gut. So a keto diet creates the effect described by the authors of the high-fat study mentioned above.

Keto cheat day: Just say no

To be clear, I don’t think there’s data to say a keto diet is bad. In fact, it’s clear that people who lose weight on a keto diet improve their health tremendously.

Be that as it may, it seems clear that a keto diet likely increases LPS in the gut. This may be a beneficial effect because many Bacteroidetes seem to decreases LPS-induced inflammation in the gut. Essentially, they promote bowel tolerance to LPS.

But LPS in the blood is different. LPS shouldn’t be in the blood. And increasing fasting serum LPS 2-3x induces metabolic dysfunction. This is called metabolic endotoxemia.

Bacteroides, members of the Bacteroidetes phylum, are particularly problematic.

Species of the genus Bacteroides have the most antibiotic resistance mechanisms and the highest resistance rates of all anaerobic pathogens. Clinically, Bacteroides species have exhibited increasing resistance to many antibiotics…

When the Bacteroides organisms escape the gut, usually resulting from rupture of the gastrointestinal (GI) tract or intestinal surgery, they can cause significant pathology, including abscess formation in multiple body sites (e.g., the abdomen, brain, liver, pelvis, and lungs) as well as bacteremia.

I don’t think sticking to a keto diet causes problems. The big issue is increasing carbohydrate consumption during keto. A high carb meal in this insulin resistant state seems to cause leaky gut. Check out how here.

Another problem is that a keto diet without fiber thins the mucus layer, increasing the likelihood that LPS enters the blood. Consuming fiber helps re-thicken the mucus layer. But this depends on what’s in your keto diet.

High sulfur-based aminos coupled with high fat in the diet increases hydrogen sulfide in the colon. Hydrogen sulfide blocks butyrate metabolism, which is what increases the thickness of the mucus layer.

The important take-home here is that you can make a good or bad keto diet.


There’s no discounting the benefits that many who undertake a keto diet see. Even outside the realm of epilepsy and other medical conditions, people who lose weight going keto certainly improve their health.

But it’s important to realize that a keto cheat day may have greater repercussions than cheating on other diets. Many of the beneficial effects of switching to keto likely lie in the changes to the microbiome. Unfortunately, this benefit may become a big problem when you cheat.

The data is pretty clear that there is a concern here. But that doesn’t mean you shouldn’t go keto. It means that if you go keto, stay keto.

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