Some people criticize the keto diet for being low in dietary fiber. While it’s true that most low-carb ketogenic foods don’t have as much fiber as grains, fruits, or legumes, there are still plenty of keto-friendly foods that can supply your diet with fiber.
If you’ve ever experienced constipation, diarrhea, or other gut issues on keto, fiber intake is one factor to consider. On the other hand, depending on your individual microbiome, a high-fiber diet may not be the best solution to your gut problems.
Read on to learn how to cultivate healthy gut bacteria, how much fiber to eat, and how to get enough fiber on the ketogenic diet using healthy, keto-friendly fiber sources.
The term dietary fiber refers to the indigestible carbohydrates and lignins (plant fibers) found in plants. Most fiber comes from the cell walls of plants, where it functions similarly to a skeleton and helps plants maintain their shape and structural integrity.
There are two main types of dietary fiber, based on their solubility in water: soluble fiber and insoluble fiber.
Soluble fiber is soluble in water. It forms a viscous, gel-like substance when it is mixed with water. Gut bacteria in your colon ferment soluble fiber into gases and other byproducts, including short-chain fatty acids. Soluble fiber is also called prebiotic fiber, because it feeds your gut bacteria.
Fruits, vegetables, grains, and legumes are high in soluble fiber. Eating soluble fiber slows gastric emptying and increases your feeling of fullness.
In contrast insoluble fiber, which does not dissolve in water, is inert to digestive enzymes in your upper gastrointestinal tract. While some forms of insoluble fiber (like resistant starch) can ferment in your colon, most insoluble fiber moves through your digestive system relatively unchanged, absorbing water as it goes, eventually adding bulk to your stool and easing your bowel movements.
Insoluble fiber occurs in nuts, seeds, grains, legumes, some vegetables, some fruits, and in the skin of fruits like kiwi, grapes, plums, and tomatoes.
#1: Improves Gut Health (Sometimes)
You have around 100 trillion bacteria living in your intestines, give or take a few trillion. Dietary fiber feeds the beneficial, probiotic gut bacteria that comprise your microbiome[*].
However, if your microbiome is out of balance, eating prebiotics can also feed gut bacteria that may be harmful to your health[*]. That’s why not everyone benefits from upping their fiber intake.
Keeping your gut flora in balance decreases inflammation in your body, helps you maintain a healthy body weight, reduces your risk of many diseases, and even supports cognition and brain health[*].
#2: Appetite Reduction
Soluble fiber absorbs water that’s in your intestine, slowing the absorption of food in the process. This type of fiber also contributes to feelings of fullness.
When you feel full from eating fiber, you’re less likely to overeat because your appetite is reduced. The appetite lowering effects of fiber are not universal, but it’s worth a try if you have trouble with cravings or eating too much food[*].
#3: Lowers Blood Sugar
As followers of the keto diet followers already know, high blood sugar levels are toxic to your health. Excessive carb intake spikes your blood glucose, which can cause insulin resistance. And if you’re insulin resistant, your fasting blood glucose levels stay high. It’s a vicious cycle that can lead to type 2 diabetes..
Because fiber can slow your digestion, it also smooths out the elevation of blood glucose after you finish the meal[*]. In other words, when you consume carbohydrates, eating extra fiber lowers the glycemic index of the meal.
#4: Weight Loss
#5: Decreases Risk of Some Cancers
Dietary fiber can decrease your risk of colon cancer, ovarian cancer, endometrial cancer, breast cancer, and pancreatic cancer[*][*][*][*][*]. In one study, eating about 18 grams of fiber per day reduced all-cause cancer mortality risk by 24%, and lowered the chances of dying from colorectal-anal cancers by 58%[*].
#6: Relieves Constipation
Technically, the National Institute of Health (NIH) defines constipation as having fewer than three bowel movements per week, or dry, hard, stools that are difficult to pass. But if you aren’t having a bowel movement once or twice per day, you are at the very least mildly constipated.
If you’re constipated, fiber can be your friend[*][*][*][*]. Soluble fiber can enhance your microbiome health, while insoluble fiber bulks up your stools and moves them along. However, not everyone benefits–some people experience worsening of symptoms, or other side-effects from increasing their fiber intake[*].
Along with increasing fiber in your diet, be sure to drink enough water and get regular physical activity. If upping your fiber intake, hydrating, and exercising don’t help, talk to your doctor.
#7: Reduces Cholesterol
According to a meta-analysis of 76 controlled trials, adding an extra two to ten grams of soluble fiber each day can reduce your LDL and total cholesterol[*]. Other reviews of studies have found similar results[*][*].
It depends on who you ask. Fiber guidelines and studies don’t necessarily agree as to the precise amount of fiber you need for optimal health. And while some guidelines assume you’re eating whole grains that contribute to your dietary fiber intake, that isn’t true of people following a low-carb diet.
At the higher end, the USDA dietary guidelines for Americans recommend approximately 25-31 grams per day of fiber for teenage and adult men and women, while the National Academy of Science recommendation is 38 grams for men and 25 grams for women[*][*].
But do you need that much fiber to stay healthy? Perhaps not. Several different large reviews of dozens of studies have found that eating more fiber than the average person can reduce your risk of dying from both heart disease and cancer by at least 10%[*][*][*]. The benefits of eating fiber in these studies occurred with a total daily fiber intake between 18-26 grams, much lower than the USDA and NAS recommendations.
On the low side of the spectrum, a separate large study demonstrated a 23% reduction in cancer death risk and 13% reduction of all-cause mortality among healthy adults with a daily intake of about 9-15 grams of insoluble fiber plus 5-8 grams of soluble fiber per day, for a total of 14-23 grams of fiber per day[*]
To figure out the correct amount of fiber for your body, try experimenting. If you’re on the keto diet, start with 15-20 grams of total dietary fiber per day from a mix of soluble and insoluble fiber for several weeks, then consider adding 3-5 grams at a time as needed to see how you feel. Remember that some people actually feel worse when they boost their fiber intake[*][*][*].
Fiber vs. Net Carbs
Because dietary fiber is not assimilated by the digestive enzymes in your small intestine, it does not raise your blood sugar. Although fiber is technically a carbohydrate, it doesn’t count towards your net carb intake for the day.
To calculate net carbohydrates, subtract dietary fiber from total carbs. To figure out how many net carbs to eat, try the Perfect Keto Macro Calculator.
Does Everyone Need Fiber?
If you follow the ketogenic diet, moderate fiber intake is probably sufficient. You may recall from an earlier section that many of the benefits of high fiber diets come from the production of short chain fatty acids (SCFAs) in your gut, which alter gene expression and reduce inflammation in your body[*][*][*].
Another significant benefit of the ketogenic diet is that you avoid the big spikes in blood glucose levels caused by excessive carbohydrate consumption. Because some of the benefits of eating dietary fiber come from improved glycemic control, your fiber needs may be further reduced on the keto diet since your glycemic control comes from eating very few carbs.
#1: Chia Seeds
Chia seeds provide calcium, phosphorus, and manganese, as well as omega-3 fatty acids.
An ounce of chia seeds has a mighty 10.6 grams of dietary fiber and 1.7 grams of net carbs.
Flaxseeds provide alpha-linoleic acid, an anti-inflammatory omega-3 fatty acid. They also contain plenty of thiamin, magnesium, phosphorus, copper, manganese, and selenium.
Just one ounce of flaxseeds has an incredible 7.6 grams of fiber and 0.5 grams of net carbs.
Avocados are one of the top sources of fiber on the keto diet, as well as a perfect source of healthy fats. This fruit is savory, creamy, and loaded with fresh flavor.
A large avocado weighing about 200 grams has a whopping 13.5 grams of dietary fiber and just 3.6 grams of net carbs.
#4: Raw coconut
Coconut is a delicious and versatile food item that offers manganese, zinc, copper, selenium, iron, folate, and phytosterols, plus medium-chain triglycerides and other healthy saturated fats.
One cup (83 grams) of raw, shredded coconut provides an impressive 7.2 grams of dietary fiber and 5 grams of net carbs.
#5: Pumpkin Seeds (Pepitas)
Whole roasted pumpkin seeds, also called pepitas, are an excellent snack item. They are high in protein and fat and also provide zinc, copper, potassium, manganese, and magnesium.
An ounce of pepitas has 3 grams of dietary fiber and 1 grams of net carbs.
Pecans are among the lowest of low-carb nuts, and they also provide healthy fats, thiamin, manganese, and copper.
A one-ounce serving of pecans has 2.7 grams of fiber and 1.2 grams of net carbs.
#7: Collard Greens and Other Leafy Greens
Collards and other leafy greens are high in fiber and low in starchy carbs. They’re also packed with folate and vitamins K, A, and C. Most leafy greens like collards and spinach cook down dramatically, so if you want to up your fiber intake more easily, cook your greens.
A 100 gram serving of collard greens has 3.6 grams of dietary fiber and 2.1 grams of net carbs.
Along with plenty of fiber, almonds offer vitamin E, riboflavin, calcium, iron, magnesium, potassium, phosphorus, copper, and manganese.
An ounce of almonds has 3.3 grams of fiber and 2.1 grams of net carbs.
#9: Broccoli and Other Cruciferous Vegetables
Cruciferous vegetables like broccoli, cauliflower, and cabbage contain sulforaphane, diindolylmethane, and other cancer-fighting compounds. They’re all high in fiber and relatively low in net carbs.
A small cooked broccoli stalk (140 grams) contains 4.6 grams of fiber and 5.5 grams of net carbs.
#10: Bell Peppers
Bell peppers come in different color varieties like green, yellow, red, orange, and purple Each one varies slightly in its nutritional profile, but all of them are ultra-healthy and loaded with vitamins C and E.
A one-cup (149 gram) serving of chopped raw red bell peppers has 3.1 grams of fiber and 6.3 grams of net carbs.
Mushrooms are high in potassium and Vitamins C and B, and also help improve your immune function[*].
A cup of diced raw portobello mushrooms (83 grams) contains 1.3 grams of fiber and 3.1 grams of net carbs, while an equivalent amount of raw oyster mushrooms has 2 grams of fiber and 3.6 grams of net carbs.
Although fiber supplements are popular, you don’t need to supplement fiber if you eat a well-rounded diet with healthy whole foods. Most people who take fiber supplements are compensating for a diet that lacks healthy sources of fiber like vegetables.
When you supplement fiber instead of eating enough whole food fiber sources, you miss out on the beneficial micronutrients and phytonutrients found in plant foods.
Remember that prebiotics like soluble fiber feed all bacteria, beneficial and otherwise. Adding prebiotics to an unhealthy gut is like adding fuel to a fire.
If prebiotics or fiber supplements cause symptoms like major gas, bloating, cramping or pain, go see your functional medicine doctor. It could be a sign of dysbiosis or SIBO.
If you have gut dysbiosis or small intestinal bacterial overgrowth (SIBO), you may consider trying a low FODMAP diet (which is also low in fiber) for a few months before upping your fiber consumption again.
The Takeaway: Fiber On Keto
On a low-carb, high-fat keto diet, your fiber needs are significantly reduced because you aren’t as reliant on fiber for glycemic control or to produce short-chain fatty acids (SCFAs).
If you are constipated or having other problems with your bowel movements, be sure to obtain regular physical activity and stay hydrated. Being outside in the sun can also help your gut health, in part by providing vitamin D. Adding some extra fiber may help too, but that’s not the case for everyone.
Some people experience diarrhea when starting the keto diet because of the increased production of bile, but it typically goes away within a few weeks. Diarrhea on keto is usually not due to a lack of dietary fiber.
To address gut bacteria problems, begin by eliminating processed foods and sugary drinks. Then you can try adding more fiber to your diet if necessary.
Remember that adding fiber to your diet provides energy for all bacteria, not just beneficial strains. If adding fiber doesn’t address your symptoms, speak to a functional medicine about how to balance your microbiome using a low FODMAP diet and other tools.
And if you do decide to add fiber, ensure it comes from real food, not supplements.