Special Report: Fiber Isn't Clinically Essential. Here's Why That Argument Misses the Point Entirely.

Is Fiber Really Essential?

Let’s start by saying something you don’t see very often: we’re going to agree with the people who are going to disagree with this piece.

The carnivore guys, the low-carb enthusiasts, the influencers who've built their audiences on the argument that fiber is overrated, we’re going to admit something: technically, they have a point. A very specific, defensible point.

If you read APC, you know we’re agnostic about how you want to eat. That’s a personal choice. We simply care about clarifying the points that go crazy in social media algorithms, but create more confusion, stress, and frustration.

There is no such thing as “fiber deficiency disease.” You will not develop scurvy from skipping legumes. Your gums won't bleed if you stop eating oats. Unlike vitamin C, unlike protein, unlike iron, fiber doesn't have a named acute deficiency condition in the clinical literature. The body doesn't require it the way it requires certain other nutrients.

So, anyone who makes that argument, give them a point. They've earned it.

But it misses the point entirely.

The point about “fiber deficiency” is almost completely irrelevant to the question you should actually be asking if you’re truly focused on your health, well-being, or longevity. 

And that is: what does your body need to thrive, avoid disease, and function as it was designed?

The Seatbelt Analogy (And Why It Will Change How You Understand Nutrition)

The argument against fiber — the real one, the sophisticated version — goes like this: fiber isn't essential by clinical definition, therefore the obsession with hitting 25 to 38 grams per day is nutritional establishment overcorrection, probably influenced by grain industry funding, and people who eliminate or dramatically reduce fiber do just fine.

That argument uses a framework built to diagnose sailors on long ocean voyages in the 1700s. The deficiency model of nutrition — remove X, get disease Y — was designed to identify what the body needs to not fall apart. It solved a real problem brilliantly and gave us everything from vitamin D supplementation to iodized salt.

But it was never designed to answer the question we're actually facing today: what does chronic insufficiency of a given nutrient cost you over a lifetime?

Outcomes are what matter in human health. You can spin any story, make it sound “sciency,” but at the end of the day, you want to know, “What will this actually do for my body?” 

And that's what the deficiency framework can't see.

If you need to understand this better, consider the seatbelt. 

Nobody in an emergency room is being treated for “seatbelt deficiency syndrome.” There's no clinical name for the condition caused by not wearing one. 

If you apply the anti-fiber framework to seatbelts, you conclude that seatbelts are optional because the body doesn't require them. 

Let that process for a second. 

But no one who has actually examined the outcome data argues that seatbelts don't matter. We measure their value differently. We look at what happens to people who wear them versus those who don't, across thousands of accidents and decades of data. We look at outcomes.

That's how we have to look at fiber.

When you stop asking "does the body technically require fiber to survive?" and start asking "What happens, across large populations, over decades, when fiber intake is chronically high versus chronically low?" — the picture that emerges is not ambiguous. It's not preliminary. It's some of the most consistent data in nutritional science.

You can decide for yourself, but here’s what the evidence actually says happens when you do or don’t prioritize fiber.

What Happens to Your Heart

When you eat soluble fiber — psyllium being the most studied — it forms a gel in your digestive tract. That gel traps bile acids, the compounds your liver needs to complete digestion. When those bile acids get swept out in stool rather than reabsorbed, the liver has to manufacture more. And to do that, it pulls cholesterol from your bloodstream.

This is not magic. It's chemistry you can trace step-by-step. Which is why the FDA, one of the more conservative bodies when it comes to health claims on food and supplements, issued an approved health claim linking psyllium fiber to a reduced risk of heart disease. 

Clearing that bar requires a level of evidence that most nutrients never reach.

Psyllium is the foundation of Fiber+ for exactly this reason. We wanted the cardiovascular mechanism built on the most evidence-backed fiber that exists.

A meta-analysis of 28 randomized controlled trials found that roughly 10 grams of psyllium per day reduces LDL cholesterol by somewhere between 5 and 10 percent from baseline. Across nearly 2,000 participants, the finding held whether or not they already had high cholesterol going in. 

A 5 to 10 percent reduction in LDL is, in clinical terms, meaningful. It's the kind of effect a cardiologist would notice and discuss at a follow-up appointment.

The larger picture is even more compelling. 

A systematic review and meta-analysis following more than 1.2 million people found that each 7-gram increase in daily fiber intake was associated with a 9 percent lower risk of cardiovascular disease. 

Every 7 grams. That's roughly one serving of lentils, or two apples, or a large serving of broccoli with dinner.

The dose-response relationship was linear in the range where most Americans actually sit, meaning incremental improvements deliver proportional protection. You don't have to eat perfectly to move the needle.

If a drug produced a 9 percent reduction in cardiovascular disease risk with no meaningful side effects, it would be prescribed universally. Fiber does it through your food.

What Happens to Your Blood Sugar (And Why You Feel Like That At 3 pm)

I want to resist referencing "type 2 diabetes" for a moment, because it tends to make people think the discussion doesn't apply to them. Let me start somewhere closer to your actual week.

The afternoon crash. The irritability before meals. The craving for something sweet around 3 pm that you attribute to willpower failure, but that actually has a fairly simple biochemical explanation. These are blood sugar events. They're not character flaws.

Fiber slows gastric emptying, which means glucose enters your bloodstream gradually rather than in a spike. The spike triggers an exaggerated insulin response. The insulin overshoots, blood sugar drops quickly, and your body reads that drop as an emergency and demands sugar. You're not weak. You're responding to a physiological signal. The question is whether your diet is creating that signal unnecessarily.

An umbrella review of 16 meta-analyses — a review of reviews, which gives it substantial weight — found that people with the highest fiber intake had a 15 to 19 percent lower risk of developing type 2 diabetes compared to those with the lowest intake. That's a population-level finding. But the mechanism behind it operates at the level of every individual meal.

There's also a longer-term adaptation worth knowing about. 

Resistant starch — found in legumes, cooked-and-cooled potatoes and rice, and green bananas — goes largely undigested in the small intestine and gets fermented in the colon. Clinical trials have found that consistent intake of resistant starch improves insulin sensitivity within three to six weeks. Your cells become more responsive to insulin even between meals. The benefit compounds.

Fiber helps keep blood sugar stable, which in turn provides stable energy. Fewer cravings. Less reactive eating. Better decision-making through the second half of the day. None of this requires you to have pre-diabetes to be relevant. It applies to anyone who has ever felt like they're fighting their appetite rather than directing it.

What Happens in Your Gut (And Why It’s About Far More Than Reducing Bloating)

Here's the fertilizer metaphor, because it's the most accurate one I've found.

Your gut contains somewhere between 38 and 100 trillion bacteria. Most of them are beneficial, and they need to eat. Their preferred food is fermentable fiber, particularly resistant starch and certain soluble fibers. When those bacteria eat fiber, they produce a short-chain fatty acid (called butyrate).

Butyrate is the primary fuel for the cells lining your colon. When those cells are well-fed, the gut wall stays tight and selective, letting in nutrients, keeping out bacteria and inflammatory compounds. When they're chronically starved of fiber, gaps form in that lining. Inflammation follows. And it doesn't stay local.

The World Cancer Research Fund — the body that sets global standards for cancer prevention evidence — rates fiber's protective effect against colorectal cancer as "convincing." That's their highest classification. They reserve it for findings so consistent across methodologies, populations, and study designs that challenging them requires extraordinary evidence.

A systematic review of 25 prospective studies (including a dose-response meta-analysis of 16 of those studies) found a 10 percent reduction in colorectal cancer risk for every 10-gram increase in daily fiber intake.

And the findings didn't vary meaningfully based on population, country, or methodology. That’s very rare in research. So, in other words, add 10 grams of fiber. Reduce your colorectal cancer risk by 10 percent. The math doesn't get more clear than that.

But the gut story doesn't stay in the gut. And this is where things get interesting.

What Happens Over a Lifetime

I want to take you to the highest-altitude view for a moment, because sometimes a single number does more than paragraphs of explanation.

A 2024 systematic review and meta-analysis pulled together 64 prospective cohort studies covering 3.5 million people globally. People who consumed the most dietary fiber had a 23 percent lower risk of dying from any cause compared to those who consumed the least.

Twenty-three percent. All-cause mortality. Sixty-four independent studies. 3.5 million people.

That's not a marginal effect. 

The researchers noted that, in some analyses, it’s comparable in scale to the benefit seen from adding regular exercise or reducing processed food consumption. The relationship held across cardiovascular disease, cancer, metabolic disease, and inflammation simultaneously. Not because fiber is some miraculous single intervention, but because those conditions are all downstream of the same biological environment that fiber helps maintain.

I've spent a long time around researchers, clinicians, and coaches who are genuinely skeptical of nutrition science — and reasonably so — given how much of it doesn't replicate or gets reversed in five years. 

Nobody argues with numbers like these. 

They might argue about mechanism, about dose, about source. But the directionality of the data on fiber and longevity is about as settled as it gets in nutritional epidemiology.

The Connection Nobody Saw Coming

Everything I've covered so far is, in some sense, what you already vaguely knew fiber was supposed to do. Good for the heart. Good for blood sugar. Good for digestion. The standard story.

This is where the standard story ends.

When people ask me what I'm paying attention to in brain health research right now, I give them the usual answers: sleep quality, stress management, exercise, social connection, and cognitive challenges. 

But the thing underappreciated by almost everyone, including people who care deeply about cognitive longevity, is the gut-brain connection. And specifically: what happens to your brain when you chronically underfeed the bacteria that produce butyrate?

The gut-brain axis isn't metaphorical. It's physical. There is a direct communication highway — the vagus nerve — running between your digestive system and your brain. There's also a second channel: the bloodstream, through which compounds your gut bacteria produce travel to and through the brain. Butyrate is one of those compounds. And what researchers have found about what it does when it gets there is hard to dismiss.

Excuse us, because we tend to avoid the super sciency, but let us nerd out for a moment.

First, butyrate acts as an HDAC inhibitor. HDAC stands for histone deacetylase — an enzyme involved in gene expression. When butyrate inhibits it, specific genes associated with inflammatory signaling in the brain get dialed down. 

Neuroinflammation isn't just a symptom of neurodegenerative disease in this growing body of research. It appears to be a driver of it. 

Butyrate produced from fiber fermentation has a direct, documented mechanism for reducing it. 

That's why researchers studying Alzheimer's prevention are increasingly paying attention to fiber. Not because fiber prevents Alzheimer's, but because neuroinflammation is part of the pathway, and butyrate is part of what modulates that pathway.

And then there's the cognitive aging data. A 2025 cross-sectional study found that older adults with higher dietary butyrate intake — a proxy for fermentable fiber consumption — performed significantly better on cognitive function measures. The research is still observational and can't establish causation. I'm not going to oversell it. But this is the first human data point moving the butyrate-cognition connection out of animal models and into older adults, which is exactly the population the question matters most for.

And then there's this: 95 percent of the serotonin in your body is produced in your gut, not your brain. The enteroendocrine cells in your gut lining synthesize serotonin in response to signals from gut bacteria and the compounds they produce during fermentation. The pathway between fiber and serotonin production isn't clean or linear enough to say "fiber increases serotonin." But the connection between gut microbiome health and mood, anxiety, and emotional baseline is one of the most active areas in neuroscience right now.

A randomized, placebo-controlled trial found that colon-delivered short-chain fatty acids — produced by bacterial fermentation of dietary fiber — significantly reduced the cortisol response to acute psychological stress in 66 healthy men over one week. It's worth noting that a subsequent study testing butyrate alone did not replicate the cortisol-reducing effect, which is why this area of research is best understood as directionally promising rather than settled.

But if the findings are repeated, your mood — your baseline level of calm, your emotional resilience, your capacity to handle a hard day without reaching for something to numb it — is partly downstream of what's happening in your digestive system. And what's happening in your digestive system is largely determined by what your gut bacteria have to eat. They eat fiber. Most of us have been told gut health is about bloating and regularity. It's also about how you feel walking around in your own head.

What the Other Side Gets Right

None of this means the carnivore or low-carb critics are simply wrong about everything. They aren't.

The strongest version of their argument isn't really a defense of zero fiber. It's a critique of processed food. They're right that the food industry has responded to fiber messaging by selling a lot of fortified junk; low-nutrient, heavily processed products that slap "good source of X" on the label and deliver very little of the whole-food benefits the research actually measured. 

They're also right that for specific GI conditions — certain presentations of inflammatory bowel disease, acute IBS flares — a temporary reduction in fiber, under medical guidance, can be appropriate. The body is not a case study. Individual variation exists. And for some people, fiber can be disruptive.

But these corrections don't rescue the underlying argument. The question was never whether zero fiber can work for some people under specific circumstances. 

The question is whether the population-level evidence supports deprioritizing fiber for the average person. Across 3.5 million people and 64 studies, the answer is no.

Fix Your Fiber Gap. Improve Your Health. (Take The Fiber Quiz)

The average American consumes about 10 to 15 grams of fiber per day. The recommended intake for adults ranges from 25 to 38 grams, depending on age and sex. That's a gap of 15 to 25 grams, and it's not a personal failure. It's structural.

If you’re curious about how much fiber you’re consuming, we created a quiz for you. 

The entire APC team took the Fiber Quiz, and it was eye-opening. 

Modern food processing systematically removes fiber from grains, vegetables, and legumes and replaces it with refined calories that digest quickly and provide almost no fermentable substrate for the gut bacteria, your cardiovascular system, immune system, and brain, all of which depend on it. The food environment was engineered to under-deliver on exactly the thing the research says matters most. You're fighting a structural problem with individual willpower. That's a hard fight.

Good news: the dose-response relationship for fiber is relatively linear at the low end of intake. 

Going from 10 grams to 20 grams per day captures some cardiovascular and metabolic benefit. You don't have to reach 38 grams by next Tuesday. You have to be meaningfully better than whatever you're doing now.

Some of you asked us how to get more fiber from food, without the Fiber+ supplement we helped develop.

Here’s a quick guide:

Lentils:~16g per cup (cooked)
Berries: ~8g per cup
Vegetables (broccoli): ~4-5g per cup
Whole grains (oats, barley, etc.): ~4-6g per serving
Cooked and cooled potatoes: ~4g per medium potato, with a meaningful resistant starch boost
Cooked and cooled rice: ~2-3g per cup, also higher in resistant starch once chilled​​​​​​​​​​​​​​​​
Green bananas: ~4-5g fiber per medium banana, plus ~5-6g resistant starch (must be genuinely green)

All of us who run the Pump Club have nutrition habits built over decades, eating mostly whole foods with lots of fruit, vegetables, and grains, and all of us have days where we do not hit the number. We’ve had vegetarian members of the team take the test and find that even they are falling short.

Fiber+ isn’t us trying to get you to pay for something you don’t need (we’ve never done that and we never will); it’s us developing something we all take that can help 95% of people who fall short with their fiber intake, no matter how hard they try.

To start, let’s find your baseline.

Most people have no idea what their actual daily intake looks like because they've never tracked it. Run the exercise once — just once — to find out where you're starting from.

For most people living a normal modern life, getting to 35-plus grams from food alone on a consistent basis is genuinely difficult. 

Supplementation isn't a replacement for whole foods, but it's a reasonable gap-filler when the real-world logistics of eating patterns collide with what the research recommends. It's why we created Fiber+ (more on that in a moment).

The mechanism matters here, too. Not all fiber supplements work the same way. Psyllium addresses cardiovascular outcomes. Resistant starch reaches the colon intact and feeds butyrate production. Single-source fibers solve one problem. The evidence increasingly suggests multiple mechanisms are worth supporting simultaneously.

That's why we built Fiber+.

Most fiber supplements are single-source. Psyllium only. Inulin only. One mechanism, one outcome. Given everything the research says about how fiber works — cardiovascular protection through bile acid binding, blood sugar stabilization through gastric slowing, and butyrate production through colonic fermentation — a single-fiber product leaves most of the benefit on the table.

Fiber+ was designed around the multi-mechanism reality. Psyllium for the cardiovascular pathway the FDA recognized. Resistant starch to reach the colon intact and feed butyrate production. A blend built to address what the evidence actually says, not what's cheapest to put in a capsule.

One serving could be all it takes to help you cover the gap with multiple mechanisms working simultaneously.

It mixes clean and is something where you’ll see and feel the difference quickly. The cinnamon is delicious and hits the spot, and the unflavored doesn't affect taste, and fits into whatever eating pattern you already have. No diet change required. No food philosophy required. Just the fiber your gut bacteria, your cardiovascular system, and your brain all depend on — consistently, without having to think about it.

APC readers get 35% off their first-purchase subscription or 14% off one time purchase. Make sure you use the code “PUMPCLUB,” but by using this link, no code is needed (the discount applies automatically at checkout).

The Myth Wasn't About Fiber

The carnivore guys got the definition right. Fiber is not essential by clinical definition. You will not develop a named deficiency disease by eliminating it.

They got the implication completely wrong.

Essential versus optimal is not the same question. Acute versus chronic are not the same timeframe. Surviving without something is not evidence that the something doesn't matter.

The myth was never that fiber is essential. The myth was that "essential" is the right standard for evaluating what you should put in your body across a lifetime.

For sixty years of research involving tens of millions of people, across cardiovascular disease, metabolic disease, cancer, and cognitive aging and all-cause mortality, the picture is consistent in a way that very little nutritional science manages to be. The data kept piling up. At some point, you stop waiting for more evidence, and you start acting on what you know.

The argument against fiber was clever. It was technically accurate in one narrow framing.

It was also wrong about what actually matters. So you need to ask yourself: do you want to win an argument on a technicality, or just be healthier?

No matter how you get it, make fiber a priority (assuming you don’t have a health condition), and your body will thank you in all the ways that really matter. 

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Publisher: Arnold Schwarzenegger​

Editors-in-chief: Adam Bornstein and Daniel Ketchell