The keto, carnivore, and low-carb paleo diets are known to cause constipation. Carbs are one of the primary sources of dietary fiber — both soluble and insoluble. In a typical diet — for instance, a Mediterranean diet — there is a focus on getting plenty of fiber via whole grains, fruits and vegetables, lentils, etc. While there’s nothing to say that you can’t have these foods in your quota of “net carbs” (as is often calculated in a keto diet), sometimes, you might not be eating the right type of carbs for the job (poop).
If you’re having constipation on a low-carb diet, a lack of fiber can be the main culprit. Fiber makes you poop, in that it draws water to your stools, gives it bulk and softness, and also helps your “colon work like a muscle to move things through more easily,” per Dr. John Riopelle (via Everyday Health). Without fiber, your bowel movements will be, well, “backed up”. Simply adjusting to a low-carb diet can also throw your poop habits into chaos and cause constipation.
In the case of paleo, constipation could occur if the person isn’t doing the high-fiber eating plan right. Going low-carb doesn’t mean being low-fiber, especially on a paleo diet which typically prioritizes the consumption of fruits, vegetables, nuts, and seeds (fiber-rich foods).
In a carnivore diet, constipation happens because of low fiber and dehydration. When your body isn’t getting carbs, it breaks down glycogen stored in the muscles and liver. Dehydration is a side effect of excessive glycogen breakdown and it can also lead to constipation.