All About Fiber: Friend, Foe, or Both?

Fiber is not one size fits all!

This happens all of the time

A lot of people who come to see me have been told to "just take fiber." In attempts to make fiber accessible and easy to find, there are 2-3 heavily marketed, brightly packaged brands that doctors tend to recommend and are available at any big box store. When we flip the bottle around, though, we see two things: the active ingredient (something like wheat dextrin, methylcellulose, or psyllium husk) and a laundry list of excipients. Artificial dyes, sweeteners, fillers that do nothing for gut health and often make symptoms worse.

That's when I help my clients strip it down: what's the actual fiber, and what's the "extra stuff" their gut doesn't need?

Usually people have been taking these supplements for years. They're scared to stop, or they find that it makes them actually feel worse. They're then frustrated because they are back to square one, feeling like they must be a unique oddball that the usual solution doesn't work for. I had a client once, let's call him Robert, who came in frustrated after months of religiously taking his doctor's prescribed fiber supplement. "I feel worse than when I started," he told me.

We looked at the label together. Wheat dextrin was the main ingredient. I often test food sensitivities alongside gut testing, and when Robert's results came back, it all made sense. He already had a sensitivity to wheat. His daily fiber supplement was literally fueling the fire in his body. And there was indeed an inflammatory fire - not only were his symptoms screaming this, but his blood chemistry and GI-MAP highlighted it too. Elevated calprotectin, homocysteine, uric acid, ferritin to name a few. Every morning, with the best intentions, he was pouring gasoline on that fire.

His story isn't unique. In fact, it's the story I hear most days.

What Is Fiber, Really?

Fiber is simply the part of plants we can't digest, but our microbes can. And not all fibers act the same.

Soluble fiber dissolves in water and forms a gel. It slows digestion, balances blood sugar, and supports softer stools. It can help with diarrhea and constipation depending on context. Sources: oats, apples, beans, acacia, psyllium

Insoluble fiber adds bulk and speeds up transit. Helpful for sluggish bowels, but if used without enough water, can backfire. Sources: veggies, nuts, seeds, fruit skins

Resistant starch skips digestion in the small intestine and ferments in the colon, feeding microbes that produce short-chain fatty acids. Sources: green bananas, cooled potatoes, lentils

Specialty fibers:

• Acacia fiber: gentle, prebiotic, best fiber for IBS and loose stools

• Partially hydrolyzed guar gum: soluble, well tolerated, doesn't cause much gas

• Beta-glucans from oats and mushrooms: lower cholesterol and modulate immunity

  
  

Not Every Fiber Works for Every Gut

This is where most people get tripped up. Fiber isn't one-size-fits-all.

Some people tolerate psyllium beautifully, while others bloat like a balloon. Insoluble fibers can speed things up for a constipated person, but for someone with IBS, they can be abrasive and painful. Soluble fibers are usually gentler, but if you don't have enough water or digestive capacity, they can also sit heavy.

The key is understanding that different fibers trigger different responses in different guts. This shouldn't be surprising, but somehow it is.

Where FODMAPs Fit In

You've probably heard of the low FODMAP diet, often used for IBS. FODMAPs are fermentable carbohydrates. Some are sugars like lactose and fructose, while others are actually fibers like certain oligosaccharides.

High FODMAP fibers include things like inulin, chicory root, and some legumes. They can be very fermentable, meaning they produce lots of gas when gut microbes feed on them. For some people, that's a good thing: fuel for microbes. For others, it's misery: bloating, cramping, urgency.

This is why you'll hear people say "fiber supplements cause bloating" or "fiber makes me worse." Usually they mean the type of fiber and how their gut is set up to handle it.

What I See in Practice

Constipation: Soluble fibers like psyllium or acacia can soften stool and promote easier movements. Insoluble fibers add bulk, but need enough water and minerals to work well.

Diarrhea: Soluble fibers absorb excess water and slow transit. Acacia and partially hydrolyzed guar gum are especially gentle here.

The microbiome connection: Fiber is food for your microbes. In turn, they produce short-chain fatty acids (SCFAs) - these are basically healing compounds your gut bacteria make when they digest fiber:

• Butyrate: fuels the cells lining your colon, reduces inflammation

• Propionate: helps regulate cholesterol and satiety

• Acetate: supports gut-brain communication and appetite regulation

  
  

When I review stool tests, people with low SCFAs almost always have low microbial diversity. Adding the right fiber often changes that picture.

Why Wheat Dextrin Supplements Cause Problems

Remember Robert, the client who was prescribed a fiber supplement with wheat dextrin as the active ingredient. Around the same time, we ran a food sensitivity test that measures your blood cells' inflammatory response to foods and chemicals.

His results? A clear sensitivity to wheat.

Now, we can't know for certain whether his daily use of wheat dextrin caused that reaction or whether it was there before. But here's what we do know: if wheat dextrin is slipping through a leaky gut lining, it could very well contribute to ongoing inflammation. Continuing to take a product based on wheat dextrin was certainly not helping. In fact, it was hurting him.

This is the exact kind of nuance that gets missed when people are told to "just take fiber." The type of fiber matters.

GMO Crops and Glyphosate Residues

There's a bigger layer too. Many fiber supplements are made from conventionally grown wheat or corn. That raises two concerns:

GMO crops: corn-derived fibers and additives are often GMO. Wheat itself isn't commercially GMO in the U.S., but contamination and processing overlap happen.

Glyphosate exposure: wheat, even non-GMO, is often sprayed with glyphosate before harvest. Glyphosate is linked to gut dysbiosis, reduced microbial diversity, and increased intestinal permeability.

So when someone with gut issues is taking a daily dose of wheat-derived fiber, they're potentially getting a steady drip of glyphosate residues that further irritate the gut and disrupt microbial balance.

This is why I steer clients toward cleaner fiber sources: whole foods, or supplements like acacia or PHGG that are less likely to carry this baggage.

The Marketing vs. the Reality

Highly marketed over-the-counter fibers sound simple and possibly tasty-er than plain ole fiber, but here's what's inside when you check labels:

Wheat dextrin-based products are marketed as gluten-free, but wheat-sensitive clients often react. And some varieties of the most popular brand include the following:

• Powders / Stick packs:

  • Often just wheat dextrin (labeled as 100% in some versions)

  • Flavored versions may add natural/artificial flavors, citric acid, sweeteners (stevia or sucralose), maltodextrin, and food coloring.

• Chewables / Gummies:

  • Sweeteners: sorbitol, sucralose, isomalt, maltitol syrup

  • Binders / fillers: microcrystalline cellulose, maltodextrin

  • Acids / stabilizers: citric acid, sodium citrate

  • Colors & flavors: natural/artificial flavors, fruit/vegetable juice color, sometimes FD&C dyes

  • Anti-caking / lubricants: magnesium stearate, silicon dioxide

Commercial Psyllium husk powders are effective, but often mixed with dyes, flavorings, or sweeteners. Here are the excipients in the most popular brand:

Powdered forms (orange, sugar-free, etc.)

• Flavorings (natural and artificial)

• Citric acid (for tartness/preservation)

• Aspartame or sucralose (in sugar-free versions)

• Maltodextrin (used as a carrier or bulking agent)

• Food dyes (e.g., FD&C Yellow 6 in orange powders)

Capsule/tablet forms

• Gelatin or plant cellulose (capsule shell)

• Magnesium stearate (flow agent)

• Silicon dioxide (anti-caking)

Methylcellulose products are non-fermentable and may relieve constipation, but don't nourish microbes. and the excipients in the most popular brand include:

• Sucrose or aspartame (depending on sugar vs. sugar-free versions, for sweetness)

• Orange flavoring (natural and/or artificial flavors)

• Citric acid (for tartness and stability)

• FD&C Yellow #6 (in the orange-flavored version, as coloring)

The issue isn't only the fiber itself. It's the additives. Artificial colors, aspartame, or maltodextrin can aggravate sensitive guts. I encourage people to read the label, find the actual fiber source, and avoid the extras.

When Fiber Backfires

Too much, too fast leads to bloating, gas, cramping. Too little water makes constipation worse. The wrong type for your condition (insoluble fiber during a flare of IBS or colitis, for example) can be painful. FODMAP sensitivity means fermentable fibers can feel like dynamite in the wrong gut.

Food First: Best Fiber Sources

Rather than powders and pills, I prefer whole foods. Vegetables like leafy greens, carrots, broccoli. Fruits like apples, pears, berries with skin. Legumes like lentils, chickpeas, beans. Nuts and seeds like chia, flax, almonds.

Whole-food snacks work too. My current favorite is dehydrated mushrooms. Crunchy, seasoned, rich in beta-glucans, and they "work" every time.... (for me).

Simple Stewed Apple Peels

A gentle, natural way to get both soluble and insoluble fiber.

Ingredients:

• Peels from 4–6 organic apples

• 1 cup water

• 1 cinnamon stick (optional)

• 1 tsp lemon juice

Instructions: Place apple peels, water, cinnamon, and lemon juice in a small pot. Bring to a simmer, then cover and cook for 15–20 minutes until soft. Enjoy warm by the spoonful, or stir into yogurt.

This delivers pectin (a soluble fiber), plus the insoluble fiber in the skins. Many clients find it helps with regularity without the harshness of powders.

The Takeaway

Fiber isn't good or bad in itself. Finding what your body can handle, starting low and slow, and choosing clean, whole-food sources whenever possible is what matters.

Most people don't get enough fiber in their diets, and their gut microbes (which thrive on fiber) are starving. That's why diversity matters.

If you're curious about your own microbial diversity, short-chain fatty acids, or fiber tolerance, that's where functional stool testing comes in. I help people map their guts every day using comprehensive stool analysis, and functional stool testing is one of the most useful tools to personalize your path forward.

So yes, get some fiber in your diet. Just make sure it's the kind that works for you.

This information is for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. Always consult with a qualified healthcare professional before making changes to your diet or supplement routine, especially if you have existing health conditions or are taking medications.

Kelly Greenway FNTP, MRWP