“But where do you get your fiber?”

If you’re eating keto or carnivore, you’ve heard this concern more times than you can count. Right up there with “What about your brain?” (which runs beautifully on ketones, by the way). Friends, family, even doctors express genuine worry about your fiber intake, repeating decades of dietary dogma that positions fiber as essential for gut health, disease prevention, and regularity.

Here’s what they’re missing: The “fiber imperative” only applies when you’re eating carbohydrates.

When you restrict carbs below 50 grams daily—or eliminate them entirely on a carnivore diet—your body’s entire digestive physiology transforms so dramatically that conventional fiber recommendations become irrelevant. Growing evidence shows that individuals thrive on minimal or even zero dietary fiber when carbohydrates are severely restricted. Let’s examine what the science actually reveals about fiber, gut health, and why the rules change completely when you’re in ketosis.

The fiber hypothesis: Observational data, not hard science

The modern obsession with dietary fiber traces back to observations made in 1960s Uganda. Irish surgeon Denis Burkitt noticed that rural Africans consuming over 100 grams of fiber daily had dramatically lower rates of “Western diseases”—heart disease, diabetes, colon cancer, diverticulitis—compared to Europeans eating just 15 grams. Along with colleague Hugh Trowell, who formally defined “dietary fiber” in 1972, Burkitt hypothesized that fiber deficiency caused these conditions.

The medical establishment embraced this idea enthusiastically. By 2005, the Institute of Medicine established current recommendations: 25 grams daily for women, 38 grams for men. A 2019 meta-analysis in The Lancet examining 185 prospective studies found higher fiber intake associated with 15-30% reductions in all-cause mortality, cardiovascular disease, type 2 diabetes, and colorectal cancer.

Sounds definitive, right? But here’s the problem: Nearly all of this evidence is observational, riddled with confounding variables, and completely irrelevant to low-carb dieters.

Why the fiber studies don’t apply to you

Here’s what mainstream nutrition gets wrong: People who eat high-fiber diets differ in countless unmeasured ways from those who don’t. They exercise more, smoke less, have higher education levels, avoid processed foods, and generally practice healthier lifestyles. Fiber intake correlates tightly with other beneficial nutrients—magnesium, B vitamins, polyphenols, and resistant starches. Is fiber itself protective, or is it merely a marker for overall dietary quality?

A 2013 review in the American Journal of Clinical Nutrition identified this “healthy user bias” as a fundamental flaw in fiber research. When individuals adopt fiber-rich eating patterns, they simultaneously replace refined carbohydrates, added sugars, and ultra-processed foods. The observational data cannot isolate fiber’s specific contribution—and it doesn’t even try.

Even more damning: there’s a massive difference between fiber from whole plant foods versus isolated supplements. A comprehensive 2015 review found that 90% of fiber supplements lack the physical characteristics needed for any clinical benefits. The 2019 Lancet authors quietly acknowledged their evidence “relates principally to fiber-rich foods as most studies were undertaken before synthetic and extracted fiber were widely used.” Processing destroys the gel-forming capacity that supposedly matters—beta-glucan baked into bread shows no cholesterol-lowering effect, while the same compound in juice does.

Translation: The fiber hypothesis was built on comparing people eating whole foods versus junk food, not on fiber itself. And none of these studies examined populations eating well-formulated ketogenic or carnivore diets.

Your gut works completely differently in ketosis

When you restrict carbohydrates below roughly 50 grams daily, your body undergoes profound metabolic transformation. This isn’t tweaking your diet—it’s fundamentally changing your fuel source. Insulin levels plummet, triggering your kidneys to excrete substantial water and sodium. Glycogen stores deplete, releasing roughly 3 grams of water for every gram of stored glucose. In metabolic ward studies, participants on 800-calorie ketogenic diets lost weight that was 61% water versus just 37% water on isocaloric mixed diets—a striking difference that affects every aspect of digestion.

Your gut microbiome shifts dramatically. A 2023 systematic review examining ketogenic diet studies found the most consistent change was reduced Bifidobacterium populations—bacteria that ferment fiber into short-chain fatty acids (SCFAs) like butyrate, acetate, and propionate. Total fecal SCFA concentrations decline. The bacterial community transitions from carbohydrate-fermenting species to those metabolizing proteins and fats.

Conventional wisdom says this is alarming. We’re told we need those fiber-fermenting bacteria producing SCFAs to nourish colonocytes, regulate immune function, strengthen gut barrier integrity, and influence metabolic signaling. But here’s what the fiber evangelists won’t tell you: Your body produces SCFAs without dietary fiber.

Studies show that when fermentable carbohydrates are scarce, gut bacteria switch to fermenting amino acids and proteins, generating propionate from aspartate, alanine, and threonine, and butyrate from various amino acids. Roughly one-third of colonic acetate comes from acetogenic bacteria synthesizing it from hydrogen and CO₂—not from carbohydrate fermentation at all. A well-formulated ketogenic diet providing adequate protein supplies plenty of substrates for SCFA production through these alternative pathways.

Even more compelling: Ketone body production itself compensates. When your liver produces beta-hydroxybutyrate at rates exceeding 100 grams daily during nutritional ketosis, this systemic ketone availability may reduce dependence on colonically-produced butyrate for gut health. Think about it: a keto-adapted individual might allocate 25 grams of circulating beta-hydroxybutyrate to maintain colon health—5-10 times more butyrate-equivalents than could ever be produced from fiber fermentation.

Your body adapted to this metabolic state over millions of years of evolution. It knows what to do.

The shocking truth: Fiber often causes constipation

Here’s where mainstream nutrition advice becomes genuinely harmful. The most provocative finding in fiber research comes from a 2012 study published in the World Journal of Gastroenterology that directly contradicts everything you’ve been told.

Researchers at Singapore’s University Hospital followed 63 patients with idiopathic constipation and asked them to stop eating fiber for two weeks, then reduce intake to personally tolerable levels. The results demolished conventional thinking.

Among the 41 patients who stopped fiber completely, bowel movements increased from once every 3.75 days to daily. Bloating disappeared in 100% of participants. Straining during defecation ceased entirely. The 16 patients who merely reduced fiber saw significant but less dramatic improvements. The 6 who continued high fiber intake? No improvement whatsoever—still constipated, still bloating, still straining.

Read that again. Eliminating fiber cured constipation that adding fiber had failed to resolve.

The mechanism explains why. Insoluble fiber increases stool bulk without improving evacuation in people with slow colonic transit or pelvic floor dysfunction. Fiber fermentation produces hydrogen, carbon dioxide, and methane gases that become trapped, exerting pressure on bowel walls and causing pain. The study authors concluded that “idiopathic constipation and its associated symptoms can be effectively reduced by stopping or even lowering the intake of dietary fiber.”

This isn’t an isolated finding. A 1997 study in the American Journal of Gastroenterology found that 80% of patients with slow transit constipation and 63% with defecation disorders did not respond to fiber treatment. For these individuals, the standard medical advice to “eat more fiber” actively worsened their symptoms.

Patients with irritable bowel syndrome tell the same story. Low-FODMAP diets—which restrict fermentable oligosaccharides, disaccharides, monosaccharides, and polyols, many of which are fibers—rank first among dietary interventions for IBS in network meta-analyses. While doctors still reflexively recommend increasing fiber for IBS, clinical trials show that soluble fiber like psyllium may help some patients, but insoluble fiber and rapidly-fermentable fibers (inulin, fructooligosaccharides, galactooligosaccharides) frequently worsen bloating, gas, and abdominal pain.

The lesson: Fiber’s effects depend entirely on context. It’s not a universal requirement—it’s often a problem.

Historical proof: Humans thrive without fiber

If fiber is essential for human health, how do we explain entire populations and rigorous clinical protocols where it’s absent?

The Inuit

The Inuit traditionally consumed diets consisting almost entirely of marine mammals and terrestrial animals—less than 10% carbohydrate, essentially zero fiber. Genetic studies have identified the CPT1a L479 variant in Inuit populations representing metabolic adaptation to very low-carbohydrate, high-protein intake. This selective sweep occurred 6,000-23,000 years ago. Early explorers consistently described the Inuit as among the healthiest populations they encountered. While modern researchers point to confounding factors, one fact remains undeniable: these populations consumed virtually no fiber for millennia and thrived.

The Maasai

The Maasai of East Africa traditionally consumed diets dominated by milk, blood, and meat—minimal to zero fiber. When Maasai communities transitioned to diets higher in refined carbohydrates, researchers documented increased obesity, diabetes, and heart disease. Their ancestral low-fiber pattern wasn’t merely “not harmful”—the shift away from it caused disease.

The Bellevue Hospital Meat Diet Study

The most rigorously documented experiment involved explorer Vilhjalmur Stefansson and colleague Karsten Anderson, who consumed an exclusive meat diet for one year under medical supervision at Bellevue Hospital in 1928-1929. The study, published in the Journal of Biological Chemistry, is worth reading in full. The results? No scurvy. No vitamin deficiencies. No kidney damage. No blood pressure increases. The medical researchers concluded: “No evidence that any ill effects had occurred from the prolonged use of the exclusive meat diet.” Macronutrients averaged 78-85% fat, 15-25% protein, and 1-2% carbohydrate—essentially zero fiber for an entire year.

Let that sink in. Under controlled hospital conditions, two men ate nothing but meat for 365 days and remained in excellent health.

Modern medicine uses zero-fiber diets—successfully

Here’s the ultimate irony: The same medical establishment warning you about low fiber actively prescribes zero-fiber diets as therapeutic protocols.

“Elemental” diets contain liquid formulas of pre-digested nutrients—amino acids, medium-chain triglycerides, maltodextrins—absorbed in the upper gastrointestinal tract. Zero fiber by design. A 2024 comprehensive review documented their effectiveness: in small intestinal bacterial overgrowth (SIBO), 80-85% of patients normalized breath tests after 14-21 days. For Crohn’s disease, elemental diets achieved 63.8% remission rates comparable to corticosteroids but with favorable side effect profiles.

These clinical protocols deliberately exclude fiber to allow gut rest and healing. They work. Consistently. In peer-reviewed studies.

Low-residue diets restricting fiber to less than 10-15 grams daily are standard medical practice for colonoscopy preparation, inflammatory bowel disease flares, acute diverticulitis, and post-surgical recovery. Systematic reviews and meta-analyses show low-residue approaches are as effective as or superior to clear liquid diets for bowel preparation, with better patient tolerability and fewer adverse effects.

So doctors tell healthy people they must eat 25-38 grams of fiber daily, then prescribe zero-fiber diets to sick people to help them heal. The cognitive dissonance is remarkable.

The carnivore diet: Anecdotal evidence meets real results

The carnivore diet—consuming exclusively animal products with zero fiber—has exploded in popularity, and for good reason: people are reporting transformative results.

A 2021 Harvard and Cedars-Sinai survey collected data from 2,029 adults following carnivore diets for at least 6 months. The findings were striking: 95% reported improved overall health and 66-91% reported improved well-being across multiple domains. BMI decreased from 27.2 to 24.3. Among those with diabetes, HbA1c decreased 0.4% and medication use dropped 84-100%.

Yes, this was self-reported survey data with inherent selection bias. But dismissing thousands of people reporting dramatic health improvements as mere placebo effect requires believing that these individuals are either lying or deluded. The more parsimonious explanation: zero-fiber carnivore diets work for many people.

A 2024 case series documented 10 inflammatory bowel disease patients achieving “complete freedom from symptoms” and discontinuing all medications on carnivore-ketogenic diets. These weren’t mild cases—these were people suffering with Crohn’s disease and ulcerative colitis who found relief by eliminating plant foods entirely, including fiber.

The honest assessment: We need randomized controlled trials. The evidence level is preliminary. Nutrient analyses of carnivore meal plans find thiamin, magnesium, calcium, and vitamin C below recommended levels in some cases (though recommendations themselves are debatable). LDL cholesterol tends to elevate substantially, though HDL and triglycerides typically optimize. Long-term cardiovascular outcomes remain unknown.

But here’s what we know for certain: Thousands of people are thriving on zero-fiber carnivore diets, reporting improvements in inflammatory conditions, autoimmune disorders, digestive issues, mental health, and metabolic markers. That deserves rigorous investigation, not dismissive skepticism.

Individual variation: Why one-size-fits-all fiber advice fails

One of the most important recent findings is that fiber’s effects vary dramatically between individuals based on baseline gut microbiome composition. This completely undermines the idea of universal fiber requirements.

A landmark 2021 study published in Microbiome journal fed genetically identical mice four different fiber diets. Despite identical genetics, mice with different gut microbiomes showed completely different metabolic responses to the same fibers—varying effects on adiposity, liver triglycerides, and glucose handling. The authors’ conclusion deserves emphasis: “A one-fits-all fiber supplementation approach is unlikely to elicit consistent effects.”

Human studies confirm this variability. In an 18-adult trial providing 40-50 grams of fiber daily for two weeks, individual variation explained 78% of microbiome differences, while fiber intervention explained just 1.5%. The same fiber dose produced vastly different SCFA responses in different people. Another study found apple fiber had “striking” effects on obese microbial communities but minimal impact on lean individuals.

This context-dependency extends to clinical conditions. Fiber helps some people without motility disorders but actively worsens symptoms in those with slow transit. IBS subtypes respond differently. Pre-existing microbiome composition determines SCFA production capacity.

The implication is clear: There is no universal fiber requirement. It depends on your physiology, your diet composition, your metabolic state, and your individual gut ecosystem. The people telling you that “everyone needs 25-38 grams of fiber daily” are ignoring the actual science.

What this means for your keto or carnivore diet

The evidence points to several clear principles:

First, truly low-carb diets fundamentally change the fiber equation. The metabolic shifts—ketone production, alternative SCFA pathways, changed gut transit, microbiome adaptation—mean that standard 25-38 gram fiber recommendations (established in the context of 200-300 gram daily carbohydrate intake) simply don’t apply to you. You’re playing a different metabolic game entirely.

Second, you can include fiber on keto if you want to—but you don’t have to. The DIETFITS study found that adults following healthy low-carb diets averaged 18.6 grams of fiber daily from non-starchy vegetables (5.1g), nuts and seeds (2.6g), and other low-carb sources. With 30-50 grams of total carbohydrates daily, there’s room for fiber-rich foods like leafy greens, cruciferous vegetables, avocados, nuts, seeds, and berries. These foods provide valuable minerals like magnesium and potassium alongside fiber.

Third, many people feel better on lower fiber intake. If you experience bloating, gas, abdominal discomfort, or worsening constipation on keto despite adequate hydration and electrolytes, fiber itself may be contributing to your symptoms. The Singapore constipation study and IBS research demonstrate conclusively that fiber reduction helps specific populations. This is particularly relevant for those with inflammatory bowel conditions, SIBO, or IBS. Listen to your body, not conventional dogma.

Fourth, electrolytes are non-negotiable. Much digestive discomfort attributed to “low fiber” on ketogenic diets actually stems from inadequate sodium, potassium, and magnesium. Your kidneys excrete these minerals rapidly when insulin drops. Sodium requirements increase to 4-6 grams daily (versus standard 2.3g recommendations), and magnesium at 500-1000mg acts as a natural laxative. Get your electrolytes dialed in first before worrying about fiber.

Fifth, carnivore dieters can thrive with monitoring. While the long-term evidence base is still developing, current data shows many people do exceptionally well on zero-fiber carnivore approaches. If you’re pursuing very low-fiber or zero-fiber eating, be mindful of micronutrient intake (particularly thiamin, magnesium, calcium, vitamin C) and consider working with a knowledgeable practitioner who understands these dietary approaches. Track your biomarkers, pay attention to how you feel, and adjust accordingly.

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The bottom line: You don’t need fiber on keto or carnivore

So where do you get your fiber on a ketogenic or carnivore diet?

The answer is simple: You don’t need to worry about it.

The obsession with fiber is based on observational studies of people eating high-carbohydrate diets rich in processed foods. When you’re eating truly low-carb—especially when you’re in nutritional ketosis or following a carnivore approach—your body operates in a completely different metabolic state. Your gut microbiome adapts. Your bacteria produce short-chain fatty acids from protein instead of carbohydrates. Your liver floods your system with ketone bodies that may serve many of the same functions attributed to fiber-derived butyrate.

The science is clear: Fiber necessity is conditional, not absolute. On high-carbohydrate diets emphasizing grains and starches, fiber from whole foods correlates with better health outcomes in observational studies—though whether fiber itself causes these benefits or merely marks overall dietary quality remains questionable. On truly low-carbohydrate ketogenic diets, the rules change completely.

Your body produces SCFAs through alternative pathways. Ketone bodies compensate for reduced colonic butyrate. Gut transit normalizes with proper electrolyte intake. Clinical studies show that constipation often improves when fiber is reduced or eliminated. Medicine successfully uses zero-fiber protocols for healing gut conditions. Historical populations thrived on essentially zero-fiber diets. Modern carnivore dieters report remarkable health transformations.

These aren’t fringe findings—they’re published in peer-reviewed journals from mainstream institutions.

The most important principle is personalization. Some people feel great including 15-20 grams of fiber daily from low-carb vegetables and nuts. Others optimize at 5-10 grams. Many thrive on zero fiber. Your individual response matters far more than population-level recommendations from studies that never examined low-carb diets in the first place.

Pay attention to your body. If you feel energetic, digest well, have regular bowel movements, and your biomarkers look good on low or zero fiber, you’re doing fine. The dogma that “everyone needs high fiber always” crumbles under scrutiny—particularly for those eating keto or carnivore.

You don’t owe anyone an explanation for your dietary choices. The next time someone asks “But where do you get your fiber?” you can smile and explain that the question itself is based on outdated assumptions that don’t apply to low-carbohydrate eating. Or you can simply say: “I don’t need much—my body works differently now.”

Because it does.

Bibliography

Cummings JH, Engineer A. Denis Burkitt and the origins of the dietary fibre hypothesis. Nutr Res Rev. 2018;31(1):1-15. https://pubmed.ncbi.nlm.nih.gov/28583217/

Ho KS, Tan CY, Mohd Daud MA, Seow-Choen F. Stopping or reducing dietary fiber intake reduces constipation and its associated symptoms. World J Gastroenterol. 2012;18(33):4593-4596. https://pmc.ncbi.nlm.nih.gov/articles/PMC3435786/

Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press; 2005.

Landry MJ, Crimarco A, Gardner CD. Changes in Quantity and Sources of Dietary Fiber from Adopting Healthy Low-Fat vs Healthy Low-Carb Weight Loss Diets. Nutrients. 2021;13(10):3625. https://pmc.ncbi.nlm.nih.gov/articles/PMC8539701/

Lennerz BS, Mey JT, Henn OH, Ludwig DS. Behavioral Characteristics and Self-Reported Health Status among 2029 Adults Consuming a ‘Carnivore Diet’. Curr Dev Nutr. 2021;5(12):nzab133. https://pubmed.ncbi.nlm.nih.gov/34934897/

Louis P, Flint HJ. Formation of propionate and butyrate by the human colonic microbiota. Environ Microbiol. 2017;19(1):29-41. https://enviromicro-journals.onlinelibrary.wiley.com/doi/full/10.1111/1462-2920.13589

McClellan WS, Du Bois EF. Clinical Calorimetry XLV. Prolonged Meat Diets with a Study of Kidney Function and Ketosis. J Biol Chem. 1930;87(3):651-668.

McRorie JW Jr, McKeown NM. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract. J Acad Nutr Diet. 2017;117(2):251-264. https://pmc.ncbi.nlm.nih.gov/articles/PMC5415939/

Nakazawa Y, Yamada K, Hiramatsu Y, et al. The effect of ketogenic diet on gut microbiota in epilepsy. Front Nutr. 2023;10:1124092. https://pmc.ncbi.nlm.nih.gov/articles/PMC9876773/

Nasser J, Mehravar S, Pichetshote N, et al. Elemental Diet as a Therapeutic Modality: A Comprehensive Review. Dig Dis Sci. 2024;69(9):3183-3197. https://pubmed.ncbi.nlm.nih.gov/39001958/

National Institutes of Health. High Fiber Diet. StatPearls [Internet]. Updated July 2024. https://www.ncbi.nlm.nih.gov/books/NBK559033/

National Institutes of Health. Low Residue Diet. StatPearls [Internet]. Updated February 2024. https://www.ncbi.nlm.nih.gov/books/NBK557693/

Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393(10170):434-445. https://pubmed.ncbi.nlm.nih.gov/30638909/

Voderholzer WA, Schatke W, Mühldorfer BE, et al. Clinical response to dietary fiber treatment of chronic constipation. Am J Gastroenterol. 1997;92(1):95-98. https://pubmed.ncbi.nlm.nih.gov/8995945/

Waslo R, Streif A, Teigland C, et al. A carnivore-centered ketogenic diet for the treatment of inflammatory bowel disease. Front Nutr. 2024;11:1467475. https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2024.1467475/full

Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137-4153. https://pubmed.ncbi.nlm.nih.gov/34256014/

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