The exploding popularity of ketogenic diets is undeniable. Whether someone’s goal is fat loss, improving blood sugar control, supporting athletic performance, or potentially improving management of a neurological or neurodegenerative condition, patients are flocking to this low-carb high-fat way of eating. While a vast body of research supports this approach for numerous applications—with some experts positing that carbohydrate restriction should be the default treatment or first approach for type 2 diabetes and metabolic syndrome—there are concerns about certain aspects of this type of diet. For example, with carbohydrate intake being so low, there’s concern that people may not be getting enough fiber, which could have an impact on various issues related to the gut microbiome. So let’s take a look at fiber intake on ketogenic diets.
Ketogenic diets are not, by definition, low in fiber. In fact, when grains, beans, and starchy vegetables are eliminated from the diet, people often find themselves eating more fiber than before, in the form of nuts, leafy greens, cruciferous vegetables, and other vegetables high in fiber. Because fiber from whole foods has little to no impact on blood sugar levels, some people use the concept of “net carbs” when calculating their carb intake on keto. Net carbs represent the total amount of carbohydrate in a food minus the fiber and sugar alcohols. (Some people are more sensitive than others to sugar alcohols, however, and should be cautious when consuming large amounts of these.) Using net carbs allows a more liberal carbohydrate intake as long as the foods are rich in fiber.
Ketogenic diets must be tailored for an individual’s health status and goals. There’s no one set way to implement this nutritional strategy, but generally speaking, there’s plenty of room for high-fiber foods on keto. Let’s look at some keto-friendly fiber-rich foods. The good news is, all the low carbohydrate foods that are high in fiber don’t just provide fiber; they’re high in various micronutrients, too.
Almonds: Per 1oz serving, almonds provide about 6 grams of total carbohydrate, of which 3.5g are fiber. Almonds are also rich in monounsaturated fat, making them a perfect snack for keto dieters.
Avocado: Now that the fearmongering about dietary fat that dominated the nutritional landscape for decades is subsiding, people are re-embracing healthy fats, and avocados are in high demand. A 3.5oz serving of avocado (100g) has 8.6g total carbs and 6.8g fiber. Avocados are also a great source of potassium, which may be a concern for some people after cutting higher carb foods like potatoes and bananas out of their diet.
Spinach: Spinach is over 90% water. An entire 10oz package (284g) of spinach brings 10.3g of total carbohydrate and 6.2g of fiber, for a net carb count of 4.1 grams. And very few people are going to sit down to an entire package at once anyway, so a more typical serving is even lower in carbohydrate. The same could be said for typical serving sizes of several vegetables that are mostly water, like cucumbers, radishes, and mushrooms.
Blackberries: A whole cup of blackberries (144g) provides 13.8g total carbs, of which 7.6g are fiber, for a net carb count of just 6.2g—a useful comeback to those who insist fruit cannot fit into a ketogenic diet. Eating an entire cup in one sitting isn’t recommended, but low-sugar, high-fiber berries can certainly be part of a nutritious ketogenic diet.
These are just a few examples of high-fiber, keto-friendly foods. There’s a wide array of others, which highlights the falsehood of ketogenic and low-carb diets being characterized by unfounded and incorrect stereotypes about these ways of eating. Contrary to popular belief, the original Atkins diet wasn’t all about bunless bacon cheeseburgers. Dr. Atkins, a cardiologist by training, was adamant about consumption of salad greens and low-carbohydrate vegetables. In fact, the two week “induction” period that’s the prelude to the diet—and is the strictest with regard to carbohydrate intake—allowed for as much as 3 cups of salad vegetables (loosely packed), or 2 cups of salad vegetables and close to 1 cup of cooked very-low-carb vegetables. Clearly, fiber can absolutely fit into ketogenic diets.
All this being said, the interesting thing is, for at least some individuals, reducing fiber intake—and in some cases, eliminating it altogether—actually improves idiopathic constipation and some associated symptoms, such as anal bleeding, bloating, and strain in bowel opening. According to a paper from the American Journal of Gastroenterology, “A diet poor in fiber should not be assumed to be the cause of chronic constipation. Some patients may be helped by a fiber-rich diet but many patients with more severe constipation get worse symptoms when increasing dietary fiber intake.” For patients who already have an adequate intake of dietary fiber yet still experience constipation, more fiber may actually do more harm than good: if stool is already not being eliminated in a timely manner, then increased dietary fiber may result in larger, bulkier stools, which could end up making things worse.
Nevertheless, constipation is a fairly common side-effect of ketogenic diets, especially if someone was relying on high-fiber cereals or fortified baked goods to “keep things moving.” Individuals who experience constipation on keto may wish to supplement with magnesium citrate to loosen the stools, or with psyllium husks taken in water.