Fiber

The Most Underrated Nutrient for Gut Health (and Why We Shouldn’t Be Avoiding Plants)

In the age of dietary extremes, fiber has somehow become controversial. With the rise of carnivore diets and plant-free eating trends, there’s a growing narrative that fiber—and even plants themselves—are unnecessary or harmful.

But here’s the truth, backed by both science and evolutionary biology: fiber is one of the most essential nutrients we’re not getting enough of, and our gut health depends on it.

Let’s talk about why fiber isn’t just filler—it’s fuel for the 100 trillion microbes that live inside you.

First, What Is Fiber?

Fiber is a type of carbohydrate found in plants that your body can’t digest. Fiber isn’t digestible by us—but that’s exactly the point. It’s food for the trillions of microbes living in your gut.

These microbes (your gut microbiota) play a critical role in:

• Digestion
• Immunity
• Hormone regulation
• Mood (via the gut-brain axis)
• Inflammation control
• Nutrient production (like B vitamins and vitamin K)

These beneficial microbes feed on fiber. Specifically, prebiotic fiber—the kind that resists digestion and travels to the colon, where it’s fermented into short-chain fatty acids (SCFAs) like butyrate, which:

• Reduce inflammation
• Strengthen the gut lining
• Support immune health
• Improve blood sugar regulation
• Even influence brain health and mood

In fact, a landmark 2021 review in Cell Host & Microbe concluded that dietary fiber is one of the most critical components for shaping a healthy microbiome—and that low fiber intake is directly linked to chronic disease, gut dysfunction, and immune dysregulation.

The Carnivore Diet & “Anti-Nutrients” 

Carnivore diets have exploded in popularity recently, often promoted as cures for autoimmune issues, mental health struggles, and digestive distress. And while an elimination diet can offer short-term symptom relief, it’s not a sustainable or supportive way to nourish the entire body ecosystem—especially the gut.

One of the biggest critiques of plants in carnivore spaces is the presence of “anti-nutrients” like lectins, oxalates, and phytates. These compounds can slightly reduce nutrient absorption—but they also:

• Have antioxidant effects
• Stimulate the immune system in beneficial ways
• Act as prebiotics (feeding beneficial gut bacteria)

More importantly, avoiding all plant foods means depriving your gut microbiota of the fiber it needs to thrive.

What Happens When You Don’t Eat Enough Fiber?

• Your microbial diversity drops
• Inflammation increases
• Gut barrier weakens
• Digestion slows
• Constipation, bloating, and blood sugar instability become common
• Your microbes may start eating away at your own gut lining (literally), due to lack of food

That’s not fearmongering—it’s been shown in both human and mouse models. A 2016 study published in Cell showed that a low-fiber diet can wipe out microbial diversity in just a few generations. Once those species are gone, it’s hard to get them back.

So Yes, anti-nutrients exist. But calling them “bad” without context misses the point.

You’re Not Just Feeding You

Only about 43% of the cells in your body are human. The rest are microbial—bacteria, viruses, fungi, and other microorganisms that live mostly in your gut.

That’s right—you’re more microbe than human, at least in terms of cell count.

And those microbial cells don’t eat protein or fat. They eat fiber.

When we starve our gut bugs by removing fiber-rich foods, the diversity of the microbiome shrinks. Long term, this can weaken the gut lining, compromise immunity, and increase inflammation—exactly the opposite of what most people are trying to fix with restrictive diets.

Your body is an ecosystem. You have to feed all the species for your overall health to thrive.

Types of Fiber (And Why Variety Matters)

Not all fiber is the same—and your gut thrives on diversity.

1. Soluble fiber
   • Dissolves in water, forms a gel
   • Helps regulate blood sugar & cholesterol
   • Sources: oats, chia seeds, apples, beans, citrus
2. Insoluble fiber
   • Adds bulk to stool, helps with regularity
   • Sources: whole grains, vegetables, nuts, seeds
3. Prebiotic fiber
   • Fermented by gut bacteria
   • Promotes beneficial bacteria growth
   • Sources: garlic, onions, leeks, asparagus, bananas, artichokes
4. Resistant starch
   • A type of prebiotic that resists digestion
   • Sources: green bananas, cooked and cooled potatoes or rice, lentils

The best approach? Eat a variety of plant foods to get a mix of fibers and phytochemicals. Studies show that people who eat 30+ different plant foods a week have more diverse, resilient microbiomes.

How Much Fiber Do We Actually Need?

According to the Dietary Guidelines for Americans:

• Women: 25 grams per day
• Men: 38 grams per day

But guess what? The average American is only getting around 10–15 grams daily. That’s less than half of what we need—and it’s a missed opportunity for gut repair, immune resilience, and long-term disease prevention.

Top Fiber-Rich Foods to Add to Your Day

• Flaxseeds and chia seeds (great in smoothies or oats)
• Berries (low in sugar, high in antioxidants and fiber)
• Lentils, chickpeas, black beans
• Avocados
• Brussels sprouts, broccoli, kale
• Quinoa and oats
• Apples, pears, and bananas
• Cooked and cooled potatoes or rice for resistant starch

Bottom Line

Fiber isn’t just “roughage”—it’s communication between you and your microbial co-pilots. It’s nourishment for the cells that protect your gut lining, train your immune system, and influence your mood, metabolism, and even your hormones.

Avoiding fiber to dodge anti-nutrients is like avoiding exercise to prevent sore muscles—it ignores the bigger picture of adaptation and resilience.

So… eat the plants!

References:

Tanes C, Bittinger K, Gao Y, Friedman ES, Nessel L, Paladhi UR, Chau L, Panfen E, Fischbach MA, Braun J, Xavier RJ, Clish CB, Li H, Bushman FD, Lewis JD, Wu GD. Role of dietary fiber in the recovery of the human gut microbiome and its metabolome. Cell Host Microbe. 2021 Mar 10;29(3):394-407.e5. doi: 10.1016/j.chom.2020.12.012. Epub 2021 Jan 12. PMID: 33440171; PMCID: PMC8022197.

Cronin P, Joyce SA, O’Toole PW, O’Connor EM. Dietary Fibre Modulates the Gut Microbiota. Nutrients. 2021 May 13;13(5):1655. doi: 10.3390/nu13051655. PMID: 34068353; PMCID: PMC8153313.

Abreu y Abreu AT, Milke-García MP, Argüello-Arévalo GA, et al. Dietary fiber and the microbiota: A narrative review by a group of experts from the Asociación Mexicana de Gastroenterología. Revista de Gastroenterología de México (English Edition). 2021;86(3):287-304. doi:10.1016/j.rgmxen.2021.02.002 

Goldman B. Low-fiber diet may cause irreversible depletion of gut bacteria over generations. News Center. January 13, 2016. Accessed May 5, 2025. https://med.stanford.edu/news/all-news/2016/01/low-fiber-diet-may-cause-irreversible-depletion-of-gut-bacteria.html. 

Desai MS, Seekatz AM, Koropatkin NM, et al. A dietary fiber-deprived gut microbiota degrades the colonic mucus barrier and enhances pathogen susceptibility. Cell. 2016;167(5). doi:10.1016/j.cell.2016.10.043