Keto Diarrhea: Why Your Gut Is Changing

Quick answer: Keto diarrhea usually comes from a sudden swing in what your gut is handling: far fewer starches, a lot more fat, and often less fiber. The bacteria that lived on carbs lose their food and the population shifts, and that disruption can go either way, loose or backed up. It usually settles in the first weeks. The two culprits worth checking first are fat bombs and sugar-alcohol "keto" products.

In this article

• What's actually changing in your gut
• The usual suspects
• What to do this week
• What the research says
• KetoDick's take
• When it's not just keto
• Key takeaways

What's actually changing in your gut

Your gut is home to a huge population of bacteria, and different species prefer different food. Some do well on starches and sugars. When you cut carbs hard, those species run short, and others that handle fat and protein do better. KetoDick describes it in exactly those terms: take away their carbs and some starve, add fats and proteins and others bloom, and the disruption causes gas, bloating, inconsistency, and possibly constipation (video UygKGoP_BtA).

In his carbs video he adds a detail people find surprising: a good part of what you pass is dead bacteria, so when the population turns over, you notice (video 1vyxuWlxT5M). That's why the same forum thread can be full of people with loose stools and people who can't go at all. It's one shift showing up two ways.

The fat side matters too. If you went from a low-fat diet to cooking everything in butter and drinking cream in your coffee, your digestion is suddenly handling a lot more fat than it's used to. Jump in slowly and it has time to catch up.

The usual suspects

Before blaming "keto" in general, look at what's new on your plate.

• Fat bombs. They're mostly concentrated fat, often coconut oil, butter or cream cheese. KetoDick calls them mostly empty calories and, if you make them at all, says to keep them tiny, no bigger than a quarter (video xyEZL0HuejI). A big dose of fat at once is a common trigger. See do fat bombs help weight loss?.
• Sugar alcohols and added fibers in "keto" products. Bars, sugar-free chocolate and "net carb" breads lean on these. KetoDick says the soluble fiber and sugar alcohols in packaged net-carb foods cause digestive problems, and suggests keeping them to a few items a week (video UygKGoP_BtA). He also notes sugar alcohols vary in how well they're digested (video ffwBPoKxlDI). More in sugar alcohols and sweeteners.
• A carb meal after weeks without one. A viewer at three and a half weeks ate a bowl of cereal and paid for it. His answer: a lot of people get gastric distress going from keto to not, and it mainly has to do with the gut biome (video xyEZL0HuejI).
• Not enough vegetables. Low fiber can produce loose, inconsistent stools as well as constipation (video 1vyxuWlxT5M).

What to do this week

1. Cut the fat bombs and the bulletproof coffee for a few days. Get your fat from food: eggs, fatty cuts of meat, vegetables cooked in butter or oil.
2. Pause sugar-alcohol products. If things settle, bring them back one at a time so you know which one it was.
3. Bring vegetables back, gradually. Leafy greens and low-starch vegetables, a serving at a time. Fiber firms things up as well as moving them along.
4. Drink water and replace salt. Diarrhea pulls out fluid and electrolytes, which keto's early water loss is already doing. See hydration on keto.
5. Give it two to three weeks. He says you aren't really on the diet until week three, four or five (video cTGYIFOTPl8). The gut is on the same timeline.

What the research says

• Keto does shift the microbiome. In a human diet study, a ketogenic diet changed gut bacteria, including lower bifidobacteria (Ang 2020). The study shows a change; it doesn't tell you whether your stools will be loose.
• Fiber feeds the bacteria. Fermentation of fiber produces short-chain fatty acids that act as signals to your body (Koh 2016), and low-fiber diets are linked to reduced diversity (Makki 2018).
• Fiber and regularity. EFSA's panel judged 25 g a day adequate for normal laxation in adults (EFSA 2010).

The evidence pages this blog draws on hold no trial of diarrhea rates on keto and no study of individual sugar alcohols, so no percentages or per-gram thresholds are given here. Anyone quoting you a precise number should show you the study.

KetoDick's take

His frame is that the gut change is part of what keto is doing, not a sign it's failing. He says the gut biome is part of what keto aims to change, and that the bacteria living on glucose and starches die back when you stop feeding them (video 1vyxuWlxT5M).

He draws a line at duration, though. A few weeks of upheaval is one thing; six months of loose "keto poops" isn't healthy for your intestines, and the fix is to go looking for fiber, not just keep limiting carbs (video 1vyxuWlxT5M). That's the practical test: improving week to week is the adjustment; stuck for months means something about the plate needs to change.

When it's not just keto

Call your doctor rather than waiting it out if:

• It lasts more than a couple of weeks, or you see blood or black stools.
• You have fever, severe cramping, or signs of dehydration such as dizziness or very little urine.
• You take diabetes or blood pressure medication. Diarrhea plus keto's early water and salt loss can stack on top of those medications' effects, and your doses may need a look anyway when you cut carbs.
• You have IBS, Crohn's or another bowel condition, where fiber and fat changes should be planned with your doctor or dietitian.
• You're pregnant.

More in side effects of keto.

Key takeaways

• Cutting carbs changes which gut bacteria thrive; the disruption can show up as loose stools or constipation.
• Check fat bombs, bulletproof coffee and sugar-alcohol products first.
• Bring vegetables back gradually; fiber helps in both directions.
• Replace fluid and salt while it lasts.
• Weeks of improvement is normal; months, blood, fever or dehydration means a call to your doctor.

Related

• Guide: Carbs · Fiber on keto · Sugar alcohols and sweeteners · Too much fat
• Blog: Keto constipation · Keto and the gut microbiome · Fat bombs and weight loss · Keto-labeled products
• Videos: Managing your carbs and fiber · Keto basics for beginners · Common beginner questions · Beginner questions by a random guy