While some keto or low-carb proponents claim fiber is useless at best and actively harmful at worst, I come down on the side that says fiber is probably helpful for most people. Some folks have persistently better responses to low- or no-fiber keto diets, and I won’t argue with that—I’ve seen it happen and I’ve read the studies where de-emphasizing fiber can actually improve constipation, for example.
I’ll just say that I have an opposite reaction, and, most importantly, I love eating a variety of plant foods that also happen to contain a ton of great nutrients in addition to fiber.
Do I buy into the idea that fiber is important because it is every human being’s responsibility to produce as much colonic bulk as humanly possible? No.
Do I think we should be consistently pushing the limits of our digestive tracts, performing feats of bathroom heroism so momentous they border on Herculean, and making sure the toilet bowl buckles beneath us? No.
The real value of fiber lies not in its coarseness, its tendency to form colonic bulk, to keep us topped off. The true value lies in its fermentability. A fermentable fiber is a prebiotic fiber—fiber that feeds our gut bacteria.
I will, however, explain why we need to be feeding our gut bacteria. Our gut bacteria form a physical barrier against incursions and colonization by pathogenic bacteria; they take up room along the gut lining so pathogens can’t. If we don’t feed our gut bacteria with prebiotics , it won’t be around to protect us. After antibiotic treatment where both good and bad gut flora are indiscriminately targeted and wiped out, pathogenic obesity-promoting bacteria take advantage of the open space . That’s a worst-case scenario, but it shows what can happen when the harmony of the gut  is disturbed by antibiotics or, to a less extent, a lack of fermentable prebiotic fibers.
When our gut bacteria eat prebiotics , they also give off metabolites like butyric acid—a short chain fatty acid that our colonic cells use as an energy source and which improves metabolic health .
Gut bacteria also convert antinutrients like phytic acid into nutrients like inositol. The almond meal-obsessed keto eater would do well to have a powerful gut biome set up to convert all that phytic acid to inositol.
Now, some writers will come up with specific blends of fibers, powders and gums to create the “optimal” prebiotic diet for your gut bacteria, but that’s pretty silly. The gut is a complicated place. We’ve barely begun to even identify all its inhabitants. To think we know the precise blend of isolated fiber that will make them flourish, and then act on that, is a mistake.
1) Almonds and Pistachios
Nuts are usually favored in health-conscious circles for a few reasons. They like the monounsaturated fat. They like the mineral profile, or the complete protein, or their ability to dissemble into nut meals and form baked goods. But what gets short shrift is the fiber content. Now, I can’t speak for other nuts, but almonds and pistachios in particular contain fiber with potent prebiotic effects. People who eat almonds and to an even greater extent pistachios  end up with improved gut bacteria profiles.
2) Green Bananas
Ripe bananas are difficult to squeeze into a ketogenic diet. The green banana—an unripe one—is mostly resistant starch, a type of starch that cannot be digested and travels untouched until colonic bacteria metabolize it. It’s one of the best stimulators we know of butyric acid production. And sure, you could do a spoonful of raw potato starch  to get your resistant starch , but the beauty of the green banana is that it also provides potassium, another nutrient that some find difficult to obtain and stay keto.
3) Wild Blueberries
Blackberries, boysenberries, raspberries, and strawberries are all loaded with fiber, and you should eat them. They’re lower carb than you think, they’re loaded with polyphenols, and topped with some real whipped cream they make a fantastic dessert. But wild blueberries are special. They’re smaller than other berries, which increases the amount of skin per ounce you get, and skin is where all the polyphenols and fiber lie. Heck, even the blueberry’s polyphenols have prebiotic effects  on the gut biome.
A few years ago, I wrote a whole post on mushrooms . Suffice it to say, they’re quite wonderful, bordering on magical. I did not discuss the fiber they contain. It turns out that all the various mushroom polysaccharides/fibers, including beta-glucans, mannans, chitin, xylans, and galactans also act as potent prebiotics  that improve the health of the host.
Your standard avocado has about 12-15 grams of fiber, if you eat the whole thing. I generally enjoy one a day myself.
Great with chili powder, salt, and lime juice, jicama  is about 11 grams of carbs per cup, but half of those are inulin, a potent prebiotic fiber with a tendency to really ramp up butyrate production.
Onions are another fantastic source of inulin. They go into almost every dish of every cuisine, so there’s no excuse not to be eating onions.
I’ve been known to treat garlic like a vegetable, roasting an entire cast iron pan full until brown and sweet and chewy. They’re another great source of prebiotic fiber.
Broccolini is a major part of my favorite meal of the day—my Big-Ass Keto Salad. Broccoli (and cruciferous vegetables in general) has been shown  to have modulatory effects on the gut biome.
12) Dark Chocolate
13) Animal Fiber
Obligate carnivores like cheetahs who don’t eat any plants (willingly) still have gut bacteria. These gut bacteria thrive on “animal fiber,” the gristle and cartilage and other bits of connective tissue that comprise a good 20-30% of the walking weight of a prey animal. Humans are not obligate carnivores, but eating the entire animal has been a mainstay of advanced hominid existence for millions of years. I find it very likely that something, someone, somewhere inside our guts is breaking down the animal fiber we eat—so you’d better be eating some!
Not so tough, is it? It’s not like I’m suggesting you load up on bran muffins, psyllium smoothies. I don’t want you dumping flax meal into everything or munching on those awful fiber gummies. Just eat some basic, healthy, low-carb plant matter—foods that don’t really scream “fiber”—and the rest will take care of itself.
What’s your favorite low-carb source of fiber? Let me know down below.
Thanks for reading, everyone.
Hernández E, Bargiela R, Diez MS, et al. Functional consequences of microbial shifts in the human gastrointestinal tract linked to antibiotic treatment and obesity . Gut Microbes. 2013;4(4):306-15.
Ukhanova M, Wang X, Baer DJ, Novotny JA, Fredborg M, Mai V. Effects of almond and pistachio consumption on gut microbiota composition in a randomised cross-over human feeding study . Br J Nutr. 2014;111(12):2146-52.
Jiao X, Wang Y, Lin Y, et al. Blueberry polyphenols extract as a potential prebiotic with anti-obesity effects on C57BL/6 J mice by modulating the gut microbiota . J Nutr Biochem. 2019;64:88-100.
Jayachandran M, Xiao J, Xu B. A Critical Review on Health Promoting Benefits of Edible Mushrooms through Gut Microbiota . Int J Mol Sci. 2017;18(9)
Nielsen ES, Garnås E, Jensen KJ, et al. Lacto-fermented sauerkraut improves symptoms in IBS patients independent of product pasteurisation – a pilot study . Food Funct. 2018;9(10):5323-5335.