A nutritionist reveals the fiber mistake most people don’t realize they’re making—and an easy way to fix it

When people decide to increase their fiber intake, the advice is standard: eat more wholegrains, increase intake gradually, and drink plenty of water.

However, there’s a bigger issue that often gets overlooked, according to Natalie Rouse, who is a registered nutritionist (RNutr) and head of nutrition at supplements brand Free Soul.

“One of the biggest mistakes I see is treating fiber as one single thing, when different types of fiber have very different effects and actions,” Rouse tells Fit&Well.

Thinking of fiber as a single nutrient and relying on a few high-fiber foods could mean you’re missing out on all of the benefits available—even if you’re hitting your daily target.

“With the current fibermaxxing trend, someone could technically hit a high fiber target but still have a fairly narrow range of plant foods,” explains Rouse.

In the U.S., it is recommended that adults consume 30g of fiber a day. Quantity is important, but getting different types of fiber matters too. “Fiber is much more complex than simply soluble and insoluble fiber, with different types having different properties and effects, so dietary variety is key,” says Rouse.

I asked Rouse to talk through the types of fiber she thinks readers should know about, starting with soluble and insoluble fiber.

Soluble fiber

“Soluble fiber is found in foods such as oats, barley, beans, lentils, chickpeas, apples, citrus fruits, chia seeds, flaxseeds and psyllium,” Rouse says.

This type of fiber forms a gel-like substance in the digestive tract, which slows down digestion and reduces the rise in blood glucose after a meal.

“It’s an important part of a healthy diet and has several well-established roles in digestive and metabolic health,” explains Rouse.

“Some soluble fibers can also help reduce low-density lipoprotein (LDL), which is often referred to as ‘bad cholesterol’. We need some LDL, but don’t want it too high so fiber can help with this,” adds Rouse.

Insoluble fiber

Insoluble fiber works differently to soluble fiber.

“Found in wholegrains, nuts, seeds and vegetables, insoluble fiber supports normal bowel function,” explains Rouse. “It adds bulk to stools to help promote regular bowel movements and prevent constipation.”

Fermentable fiber

Not to be confused with fermented foods, fermentable fiber is a type of soluble fiber, but it’s notable because Rouse says it feeds some of the beneficial bacteria in the gut.

“When these bacteria break down fermentable fibre, they produce short-chain fatty acids, which play an important role in gut health,” Rouse says. “For example, butyrate provides an important energy source for the cells lining the colon and helps maintain the gut barrier.”

Root vegetables and bulbs like chicory and garlic are good sources of fermentable fiber, as are legumes, which means beans, lentils and chickpeas are all on the menu.

Resistant starch

“Resistant starch is particularly interesting because, although it is a type of carbohydrate, it behaves more like fibre,” Rouse says, and a soluble one at that.

“It passes through the small intestine largely undigested and reaches the colon, where it can be fermented by gut bacteria. This means it can support the gut microbiome while also helping to moderate blood sugar responses after eating.”

Good sources of resistant starch include oats that have been cooked and cooled (as in these homemade granola bars), potatoes that have also been cooked and cooled, wholegrains and green bananas.

How to incorporate all of these types of fiber into your diet

Rather than trying to keep track of a long shopping list, Rouse has a few rules of thumb that will help you add variety into your diet and get the full benefits of fiber.

“Start the day with porridge oats, add beans or lentils to lunch or dinner, include vegetables and fruit with all meals and snacks, and opt for wholegrain bread, rice and pasta,” she suggests.

“These foods naturally provide different types and characteristics of fiber, including soluble, insoluble and fermentable fiber, as well as resistant starch.”

If your current intake is low, Rouse recommends increasing the amount of fiber you consume every day gradually to reduce the likelihood of bloating and abdominal discomfort.