Dietary Fibre

What’s the deal with dietary fiber? Or is it fibre?

Great question on the spelling — both are correct. “Fiber” is American English, “fibre” is British/Canadian/Australian English. Same thing either way.

As for the substance itself: dietary fiber is the part of plant foods that your body can’t digest or absorb. Unlike fats, proteins, or carbs, fiber passes through your stomach, small intestine, and colon relatively intact. That might sound useless, but it’s actually doing a lot of important work along the way.

There are two main types. Soluble fiber dissolves in water to form a gel-like substance. It can help lower blood cholesterol and glucose levels. You find it in oats, beans, apples, citrus fruits, and barley. Insoluble fiber doesn’t dissolve — it adds bulk to stool and helps food move through your digestive system. Think whole wheat flour, nuts, vegetables, and bran.

The benefits are pretty wide-ranging: it helps maintain bowel health, lowers cholesterol, helps control blood sugar, aids in weight management (fiber-rich foods tend to be more filling), and is associated with reduced risk of cardiovascular disease and type 2 diabetes. There’s also growing interest in how fiber feeds beneficial gut bacteria, which ties into broader health effects that researchers are still working to understand.

Most adults in Western countries don’t get enough. General recommendations are around 25–30 grams per day, but average intake tends to be closer to 15. The usual advice is to increase it gradually (to avoid gas and bloating) and drink plenty of water alongside it.

You said “two main types”, what are the other types? Is fibre a specific molecule, a type of molecule, or a whole group of molecules, or something else?

This is a really good question that gets at something most nutrition advice glosses over.

“Dietary fiber” isn’t a single molecule or even a single type of molecule — it’s more of a functional category. It’s essentially an umbrella term for carbohydrates and related compounds that resist digestion by human enzymes. The soluble/insoluble split is a useful simplification, but the reality is much messier.

Under that umbrella you’ll find a whole zoo of different substances:

Cellulose is probably the most familiar — it’s the structural component of plant cell walls, a long chain of glucose molecules bonded in a way our enzymes simply can’t break. Chemically it’s very similar to starch, but that one difference in bond geometry makes it indigestible to us.

Hemicellulose is a group of polysaccharides that hang out alongside cellulose in cell walls but have more varied and branching structures.

Pectin is the stuff that makes jam set — a soluble fiber found in fruits.

Beta-glucans are found in oats and barley and are particularly associated with cholesterol-lowering effects.

Inulin and fructo-oligosaccharides are chains of fructose molecules. These are heavily fermented by gut bacteria, which is why they’re often called “prebiotics.”

Resistant starch is an interesting case — it’s literally starch that resists digestion. It can form when you cook and then cool starchy foods like potatoes or rice. Some people debate whether it should “count” as fiber.

Lignin is the odd one out because it’s not even a carbohydrate — it’s a complex phenolic polymer that gives wood its rigidity. It’s found in small amounts in things like root vegetables and wheat bran.

There’s also gums (like guar gum), mucilages, and psyllium (the active ingredient in Metamucil).

So the soluble/insoluble distinction is really just one way of slicing a very diverse group. Researchers increasingly think a more useful way to categorize fibers is by their fermentability — how readily gut bacteria can break them down — and their viscosity — whether they form gels. These properties seem to drive different health effects more than the simple soluble/insoluble label does.

So, if I buy a fibre supplement like Metamucil, I’m getting more soluble fibre? Does it have the same benefits as non-soluble fibre?

Yes, Metamucil is psyllium husk, which is primarily soluble fiber. It’s a bit of a special case though, because psyllium is one of those fibers that blurs the categories — it’s soluble (it forms a gel when it absorbs water) but it’s also only partially fermented by gut bacteria, and it has significant bulking effects. So it actually mimics some properties of insoluble fiber too, which is part of why it’s so popular.

That said, taking a single-source fiber supplement is not really the same as getting a diverse mix of fibers from whole foods. A few reasons for that:

Different fibers do different things. Psyllium is great for regularity and has good evidence for lowering cholesterol and moderating blood sugar spikes. But it’s not particularly good at feeding the diverse ecosystem of gut bacteria the way something like inulin or the mix of fibers in legumes and whole grains would be. Your gut microbiome has hundreds of species, and different bacteria thrive on different substrates. A varied fiber intake supports a more diverse microbial community, which is generally associated with better health outcomes.

Whole foods come with extras. When you eat lentils or broccoli or oats, you’re not just getting fiber — you’re getting vitamins, minerals, polyphenols, and other bioactive compounds that may work synergistically with the fiber. A supplement strips all of that away.

The physical structure matters too. Intact plant cell walls, for instance, can slow down digestion in ways that a powdered supplement can’t replicate. This is part of why eating a whole apple has different metabolic effects than drinking apple juice, even if you added fiber back in.

So Metamucil isn’t a bad thing — if you’re struggling with regularity or your doctor has recommended it for cholesterol, it’s well-supported. But thinking of it as a substitute for eating a variety of high-fiber foods would be a mistake. It’s more like a targeted tool than a complete solution.