Because fiber is generally considered one of the cornerstones of a healthy diet, ketogenic diets have been criticized for excluding high-carb fiber sources like beans, wheat bran, and oats. The truth is that a keto lifestyle isn’t necessarily low in fiber. In addition, responses to fiber vary widely from person to person––and more isn’t always better. Keep reading to learn about fiber, its potential benefits and downsides, and how to determine the right amount for your ketogenic lifestyle.
What Is Fiber?
Dietary fiber is the portion of plant foods that your body can’t digest or absorb. Instead of being broken down into glucose and absorbed into your bloodstream, fiber moves through your gastrointestinal tract until it reaches the colon. From there, fiber’s fate depends on its type.
There are two broad categories of fiber: soluble, which dissolves in water, and insoluble, which does not dissolve in water. Soluble fiber is further classified as being viscous (capable of forming gels) or nonviscous.
Your colon contains bacteria that can digest fiber. When soluble fiber reaches the colon, these bacteria ferment it into short-chain fatty acids (SCFAs), which provide energy to intestinal cells and support gut health and overall well-being, as we’ll soon discuss further. By contrast, insoluble fiber simply moves through the colon and adds bulk to your stool.
Plants contain a combination of both types of fiber, typically around one-third soluble fiber and two-thirds insoluble fiber. A few exceptions are apples, asparagus, black and pinto beans, blackberries, and psyllium, which are higher in soluble fiber than insoluble fiber.
Potential Benefits and Drawbacks of Fiber
In general, fiber offers health benefits, but in certain cases and for some individuals, it may have negative impacts.
Potential benefits:
- Decreases blood glucose and cholesterol levels: In studies, viscous soluble fiber has been shown to lower blood sugar and cholesterol levels, whereas other types of soluble fiber appear to have little impact on these biomarkers.
- May improve irritable bowel syndrome (IBS): A review of 22 trials found that supplementing with soluble, but not insoluble, fiber can improve symptoms like constipation and abdominal pain in people with IBS.
- May promote satiety: Some studies suggest that by forming a gel in the digestive tract, soluble fiber can increase feelings of fullness and help reduce food intake. Additionally, when viscous fiber is fermented in the gut, it can trigger signals from the colon that enhance satiety.
- Feeds healthy gut bacteria: Soluble fiber is a source of prebiotics or “food” for bacteria that live in your colon and form a large portion of your gut microbiome. As mentioned earlier, your gut bacteria ferment soluble fiber into SCFAs, which have been credited with several health benefits, such as reducing inflammation, supporting the immune system, and helping protect against conditions like diabetes, cancer, and neurological diseases. Growing research suggests that fiber from whole plant sources can promote a healthy gut microbiome, but less is known about the effects of fiber supplements or fiber added to food products.
Potential downsides:
- May worsen constipation: Results from studies exploring fiber’s impact on constipation have been mixed. In a review of randomized trials, fiber supplementation increased stool frequency but did not improve stool consistency, pain, and other symptoms in individuals with chronic constipation. In one study, 58 people with persistent constipation experienced significant improvement in their symptoms after reducing or eliminating fiber from their diet.
- May be ineffective for or aggravate inflammatory bowel disease (IBD): While soluble fiber is generally considered beneficial for gut health because it is fermented into SCFAs, some researchers are questioning whether it has the same positive effects for people with Crohn’s disease or ulcerative colitis. In these individuals, an imbalanced gut microbiome may lead to incomplete fermentation of fiber, potentially leading to a buildup of pro-inflammatory substances.
- Excessive gas or bloating: High fiber intake is commonly associated with increased flatulence and bloating in some people. The extent of these symptoms can vary depending on the type of fiber, the amount consumed, and the individual’s gut microbiome.
- May reduce mineral absorption and balance: Concerns have been raised that fiber may hinder the absorption of calcium and other minerals by forming complexes that the digestive system can’t absorb, potentially disrupting the body’s mineral balance. However, while some studies have shown that high fiber intake can reduce nutrient absorption, others have found no impact or even improved absorption. Notably, most research on fiber’s effect on mineral absorption has focused on fiber supplementation rather than increasing the intake of whole foods naturally rich in fiber.
The Fiber-Butyrate Connection: High-Fiber vs. Ketogenic Diets
Among the three main short-chain fatty acids (SCFAs)—acetate, butyrate, and propionate—butyrate is the most effective for nourishing colon cells and supporting gut health. It also provides several other benefits, including reducing inflammation, increasing insulin sensitivity, and enhancing brain function.
Butyrate is strongly associated with high fiber intake, since it’s produced when gut bacteria ferment soluble fiber. This is one of the main reasons that fiber-rich diets are often recommended. But is it possible to experience the benefits of butyrate without consuming a lot of fiber?
Ketogenic diets dramatically increase levels of the ketone beta-hydroxybutyrate (BHB), which is similar to butyrate in chemical structure. In addition, emerging evidence suggests that BHB can be converted to butyrate by certain gut bacteria. Finally, clinical trials have shown that BHB provides many of the same benefits as butyrate, including positive effects on the gut microbiome, depression, insulin sensitivity, and inflammation.
Therefore, even with a low fiber intake, a ketogenic lifestyle may provide comparable gut and overall health benefits to a high-fiber diet because it maintains elevated BHB levels. This is a topic that deserves further research.
Fiber Responses Vary Widely Among Individuals
Because the human body doesn’t require fiber to survive, it isn’t considered an essential nutrient. However, most health authorities worldwide recommend that people should aim for approximately 25 to 35 grams of fiber daily.
It’s important to keep in mind that responses to fiber are very individual. Some people thrive on a high-fiber diet, while others report that they experience digestive issues like gas, bloating, or constipation when they consume the recommended amounts of fiber.
Overall, research suggests that regular intake of fiber––particularly soluble fiber––is beneficial. But this doesn’t seem to be true for everyone.
As mentioned earlier, studies show that soluble fiber can be helpful for IBS. Yet based on anecdotal reports, many people with this condition experience improvement in symptoms when they cut back on fiber. And in a notable case series, researchers found that 10 individuals with Crohn’s disease and ulcerative colitis achieved clinical remission by following ketogenic or carnivore diets with little to no fiber.
On the other hand, some people report that they have better digestive function and enjoy their ketogenic lifestyle more when they consume whole foods that are high in fiber.
You can learn what works best for you by experimenting. Keep a food journal to monitor how you feel after eating fiber-rich foods. Are you having more regular bowel movements? Do you feel more satisfied and less hungry? Are you experiencing bloating, gas, or other unpleasant effects? Your body’s response is the best guide to your fiber needs.
If you feel your best and maintain good digestive function when you eat less fiber, that’s important feedback to consider when making food choices. As mentioned earlier, a ketogenic lifestyle maintains BHB at levels that can support gut health even if you consume little fiber. If you’re limiting fiber due to GI issues, you can gradually reintroduce non-starchy vegetables and other fiber-rich whole foods once your gut has healed, if desired.
And if you do well with higher fiber intake but want to stay in ketosis, there are plenty of fiber sources you can include in your ketogenic lifestyle.
Keto-Friendly Foods That Are High in Fiber
Non-starchy vegetables, most nuts and seeds, and some berries are high in fiber yet low in carbs.
Here’s a list of several high-fiber foods that contain no more than 6 grams of net carbs per serving (Serving size is 100 grams, unless otherwise noted):
Almonds: 7 grams of fiber (1.4 grams soluble) per 1/2 cup
Artichoke: 6.5 grams of fiber (1.7 grams soluble) per one medium artichoke
Asparagus: 3.5 grams of fiber (2.1 grams soluble) per cup/180 grams
Avocado: 7 grams of fiber (1.5 grams soluble) per half of one large avocado
Blackberries: 6.5 grams of fiber (4.5 grams soluble) per 2/3 cup
Broccoli: 4.5 grams of fiber (1.9 grams soluble) per 2/3 cup
Brussels sprouts: 3 grams of fiber (1.1 grams soluble) per 2/3 cup
Chia seeds: 5 grams of fiber (0.5 grams soluble) per tablespoon
Flaxseed (ground): 3.5 grams of fiber (1.25 grams soluble) per 2 tablespoons
Green beans: 4 grams of fiber (1.6 soluble) per 3/4 cup
Hazelnuts: 7 grams of fiber (1.5 grams soluble) per 1/2 cup
Kale (cooked): 5 grams of fiber (1.4 grams soluble) per 1 cup
Macadamia nuts: 5 grams of fiber (1.1 grams soluble) per 1/2 cup/56 grams
Psyllium husk (ground): 4 grams of fiber (3.6 grams soluble) per tablespoon
Raspberries: 5 grams of fiber (1.3 grams soluble) per 2/3 cup
Walnuts: 6 grams of fiber (1.2 grams soluble) per 1/2 cup
Note: Fiber listings are approximations based on published data. Actual fiber content may vary based on different varieties of plants, growing conditions, and other factors.
Key Takeaway
By adjusting your fiber intake based on how your body responds––and focusing on a whole-foods diet that meets your essential nutritional needs––you can maintain a ketogenic lifestyle that supports your digestive health and overall well-being.
