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Eating a high-fat diet low in fiber can lead to tummy trouble.

Eating a high-fat diet low in fiber can lead to tummy trouble.Getty Images; Shutterstock

If you’ve been following the ketogenic (“keto”) diet, your excitement for the eating plan might be tempered by a common — but not often talked about — side effect: constipation or diarrhea.

“Any time you make a big change to your diet, there’s the chance it will affect your gastrointestinal health,” says John Riopelle, DO, a gastroenterologist for Kaiser Permanente in Lone Tree, Colorado.

Given the standard American diet is high in carbohydrates, switching to a keto diet puts you at the opposite end of the spectrum. Based on a typical keto diet food list, this approach is high in fat (70 to 80 percent), moderate in protein, and very low in carbs — many people stick to 20 to 50 grams (g) daily. What’s more, everyone’s colon is unique, which is why some people may be stricken with constipation, others with diarrhea, and still, some may not notice a change at all, says Dr. Riopelle.

The biggest issue? “When it comes to gastrointestinal symptoms on a keto diet, I worry most about the lack of fiber in the diet, which can lead to constipation,” explains Lindsey Albenberg, DO, spokesperson of the American Gastroenterological Association and gastroenterologist at The Children’s Hospital of Philadelphia in Pennsylvania. And when you’re allotted so few carbs in your diet, you’re leaving out fiber-rich sources, like fruit, vegetables, whole grains, and legumes.

In fact, even without restricting carbs, Americans on average already fall short of their daily fiber quota. Men and women eat an average of 18 and 15 g, respectively, of fiber daily, according to recent data from the National Center for Health Statistics. The recommendation is 38 g of fiber daily for men and 25 g daily for women according to The Institute of Medicine.

Here’s why that matters: “Fiber is important for colonic health,” says Riopelle. The organ simply functions better when it has some fiber. “I explain to patients that the primary job of the colon is to be a sponge and absorb all the water you’ve consumed and fluids secreted during the digestive process. Fiber adds bulk to the stool and helps the colon work like a muscle to move things through [more easily],” he says.

But instead of constipation, perhaps things are moving through a little too fast, and now you’re constantly running to the bathroom. While diarrhea may not be as common as a reaction to the keto diet, it, too, is possible. “There may be a delay in enzymes that digest fat to respond to the increasing amount of fat in your diet. If fat doesn’t get broken down in your small intestine the way it’s meant to, it travels into your colon and activates bacteria that can lead to gas, bloating, and fat in the stool,” he says. Extra fat in the stool causes a looser stool.

Another potential problem is acid reflux. “Fats take the longest to empty out of the stomach, so they keep people fuller, longer. On keto, delayed stomach emptying may leave your stomach full all the time, triggering abdominal discomfort and an increase risk of regurgitation and heartburn,” says Dr. Albenberg.

That said, these effects are often short-lived. Your gastrointestinal (GI) tract will likely adjust, but the time it takes to do that differs for every person, says Riopelle. Still, there are warning signs to see a doctor, which we will discuss in a minute.

Is Keto Dangerous for Digestive Health?

Everyone knows that constipation is uncomfortable for sure, but can it harm your health? Aside from the risk of hemorrhoids due to constipation, the issue is that constipation is simply a sign that something’s off in your body, says Albenberg.

Then there’s the potential issue that changing your diet may affect your gut microbiome, the vast network of microorganisms in your digestive tract, which has been linked to immune and metabolic function, and may play a role in disease prevention — or progression, according to a review published in January 2016 in the journal Current Opinion in Gastroenterology. “Research in animals, and some human data, shows that high fat diets lead to fairly rapid and significant changes in the microbiome. Yet we don’t necessarily know what that means for long-term health,” says Albenberg. (It’s also important to note that the high-fat diets used in these studies are often Western-type diets that are high in both fat and sugar, so they don’t perfectly replicate the keto diet.) There is also an indication that these changes may be associated with increased inflammation, but it hasn’t been found to directly cause inflammation, she adds.

Adding to the complexity is that, in the context of epilepsy, it’s because of these alterations in the microbiome that keto may help reduce the frequency of seizures, notes a preliminary study published in May 2018 in the journal Cell that observed these effects on mice. Meaning in certain contexts and when medically appropriate, these changes can actually be helpful, says Albenberg. But many people today who are trying keto aren’t doing it as part of a seizure disorder treatment.

For those new to keto, it’s the lack of fiber to watch out for. “We know a high-fiber diet promotes diversity in the gut microbiome that’s been associated with health,” says Albenberg. Fiber travels through the colon and provides food for gut bacteria, which then produce short-chain fatty acids that nourish colon cells and are thought to be anti-inflammatory, she says.

It’s these short-chain fatty acids that have been shown to improve blood glucose regulation in people with diabetes, per a preliminary study published in March 2018 in the journal Science. What’s more, a certain type of fiber, called soluble fiber, may help control blood sugar and improve insulin sensitivity, according to the American Institute for Cancer Research, which can be important for those who have prediabetes or type 2 diabetes.

Then there’s cancer prevention. People who consume just 10 g of fiber daily have a 10 percent lower risk of colorectal cancer; eating three servings of whole grains daily dropped that risk by another 17 percent, according to a review published in the journal BMJ.

And finally, fiber aids weight regulation. Not only does it increase feelings of fullness (leading to lower caloric intake), but it also promotes the growth of good bacteria, which may help protect against obesity, noted animal research published in January 2018 in the journal Cell Host & Microbe. Of course, you’re not a mouse, but a human study published in February 2015 in the Annals of Internal Medicine found that people who made the sole change of adding more fiber to their diets were able to successfully lose weight.

On the flip side, it’s worth noting that while keto may take away many fiber-rich carbohydrates, it also eliminates the highly processed and refined carbohydrates (crackers, snack mixes, desserts). The potential health risks all come down to the individual. For instance, for an obese person who has an unhealthy diet and has been advised to jump-start weight loss quickly, a short course of the keto diet under doctor supervision may be appropriate, says Albenberg. Beyond that, though, “I don’t see keto as being a great long-term solution, simply because it is just too low in fiber,” she says.

When to Worry if You’re Having Tummy Trouble on the Keto Diet

If you’re new to the keto diet, it’s not abnormal that an intense dietary swing would change your bowel habits, so you may need to give your GI system time to adjust, says Amar Naik, MD, a gastroenterologist at Loyola Medicine in Chicago. You can do that safely, as long as you don’t have any red flags, like blood in your stool.

Also watch out for severe or constant abdominal pain, or diarrhea that’s more than six times a day or waking you up at night, says Riopelle. If symptoms persist more than two or three weeks, see your doctor to make sure that things are okay.

Your Gut-Friendly Action Plan for Following Keto

If you’re committed to the keto diet, here’s how to work through GI symptoms and help prevent them in the first place:

Choose keto-friendly high-fiber foods. Many people on a keto diet count so-called net carbs, which are grams of total carbs minus grams of fiber. That’s great news for people who are looking to get more fiber in their diet, as it will give you more wiggle room to meet or get close to your quota. Options include nonstarchy vegetables, like broccoli (1 g fiber, 2 g net carbs per ½ cup), artichokes (7 g fiber, 6.5 g net carbs per medium-sized artichoke), and collard greens (1 cup of cooked, chopped greens has 7.5 g of fiber and 3 g net carbs). Some fat sources also have a lot of fiber, like avocado (1 whole avocado has 13 g of fiber and 4 g of net carbs) and nuts (1 ounce [oz] of almonds has 3.5 g of fiber and 2.5 g of net carbs).

Load up on fermented foods. These contain natural probiotics to support gut health, and are handy if you’re on a restricted diet, says Albenberg. A couple of keto-compliant choices are cabbage-based: sauerkraut (1 cup has 4 g of fiber and 2 g net carbs) and kimchi (1 cup is about 2.5 g of fiber and 1 g net carb).

Take a fiber supplement. First, a warning: It’s difficult to get all the fiber you need from supplements alone, says Albenberg. So, make sure your keto diet is well planned so that it includes high-fiber foods, too. That said, go ahead and try a fiber supplement. Experiment with different types, as people have varying responses to each, says Riopelle.

Try a whole food “supplement.” Some foods have so much fiber, they’re basically a supplement. That includes 1 oz of chia seeds (10 g fiber, 2 g net carbs) and 2 tablespoons of ground flaxseed (4 g fiber, less than ¼ g net carbs).

Stay hydrated. Adequate water intake is key to moving things along. The Institute of Medicine recommends that women get 91 oz of total fluid from beverages and foods daily and men get 125 oz.

Ask your doc about using a short-term laxative. Normal bowel frequency ranges from three movements daily to one every three days, says Riopelle. If you’ve gone beyond what’s normal for you, you can talk to your primary care physician about whether a stool softener or stimulant laxative on a short-term basis might be right for you.

Don’t count on probiotics. It may be tempting to pop a probiotic supplement and count on it to sort out your digestive health. But there’s not much evidence that commercial probiotics impact GI health in a meaningful way — and many do nothing at all — says Albenberg. For this reason, focus your efforts on fiber.

Don’t count on coffee, either. For some people, a cup of coffee does stimulate a bowel movement for those who are stopped up, but it’s not true for everyone, says Riopelle. It’s okay if coffee has this effect, but the point is, this isn’t a reliable go-to fix.

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