Not all fiber is created equal — doctors share which kinds truly support longevity

Fiber is known to be a crucial part of a healthy diet, promoting fullness and gut health.

As trends like “fibermaxxing” make high-fiber eating more popular, understanding the best sources is increasingly important for individual health, according to nutritionists.

Longevity expert Peter Attia, a Stanford University physician based in Austin, Texas, spoke about how fiber aligns with a healthier lifestyle in a sneak peek of his newest “Ask Me Anything” episode of “The Peter Attia Drive” podcast.

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People who engage in a beneficial habit, such as eating a high-fiber diet, tend to do “many other healthy things,” according to the doctor.

“They might be exercising more, they’re much less likely to smoke, they’re probably getting more sleep,” he said. “You are very likely to be capturing other healthy habits when you’re trying to simply measure one thing.”

For this reason, it can be difficult to “disentangle” fiber’s specific effects, Attia said, since people who eat more fiber also tend to consume other beneficial plant nutrients and make generally healthier lifestyle choices.

Different types of fiber

Different types of fiber are known to function differently in the body, according to Attia.

“There are lots of things that are classified as fiber, [and] a wide range of physical properties that a particular fiber might have, but these different properties produce different effects on the body,” he said.

“Some fibers provide primarily microbiome support because they’re actually fermented by gut microbiomes. Some fibers can improve blood sugar … and others simply bulk up stool.”

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Soluble fiber dissolves in water, while insoluble fiber does not — and each type plays a different role in digestion.

Insoluble fiber includes vegetables, the skin of fruits, whole grains and bran, according to Attia. It will remain “largely intact” as it moves through the digestive system, adding stool bulk, reducing constipation and supporting regular bowel movements.

Insoluble fibers are not fermented by gut bacteria, but they stimulate the gut lining to release water and mucus, which “dilutes toxins in the colon” and speeds up “intestinal transit,” the doctor said.

Most soluble, or viscous, fiber can absorb water and create a gel in the gut, which can lead to slower gastric emptying, reduce blood sugar spikes and potentially lower cholesterol levels, according to Attia.

Some examples of soluble fiber include pectin (found in fruits), beta-glucan (found in whole oats) and psyllium husk, which is a plant most often consumed as a supplement.

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Some soluble fibers belong to a subgroup called fermentable, or prebiotic, fibers. These fibers are broken down by gut bacteria to produce short-chain fatty acids (SCFAs) — compounds such as butyrate that help support gut health, regulate pH, improve metabolism and reduce inflammation, according to various studies.

“My mantra is ‘baby steps’ whenever I introduce fiber … a little at a time goes a long way.”

Fermentable fibers are found in foods like beans, pectin and inulin, as well as in prebiotic sources such as onions, garlic, asparagus and chicory root. They’re also present in certain resistant starches, including green bananas, legumes, and cooked, starchy foods like potatoes, rice and pasta.

Resistant starches, while fermentable, are not soluble, Attia noted. They are categorized into several types:

• RS1: Found in whole grains, seeds and legumes
• RS2: Found in raw potato starch, unripe bananas and maize (corn) starch (commonly used in fiber supplements)

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In most diets, a mix of fiber types is consumed. Plant-based foods — especially those with skins, along with oats, beans and legumes — tend to be the richest sources, Attia shared.

Benefits of high-fiber diets

Some key benefits of a high-fiber diet include satiety and weight management, glycemic control, cardiovascular health and colorectal cancer prevention, Attia said. 

Robin DeCicco, a certified holistic nutritionist in New York, shared with Fox News Digital in a previous interview that increasing fiber intake can be “quite beneficial.”

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“Everything from gastrointestinal health to cardiovascular health, weight management, prevention of diabetes and certain cancers, to feeling fuller throughout the day and minimizing cravings for sugars and starches, are all major benefits of fiber intake,” she said.

As health is individualized, DeCicco warned that those with pre-existing gastrointestinal conditions could experience complications by introducing too much fiber at once.

“If your system is not used to fiber, and you start to overload, you can easily become bloated, get cramps and have constipation,” she said.

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It’s “easier on the stomach” to spread it out, since fiber can bulk up in the stomach and cause a back-up, according to DeCicco.

“This is why it’s especially important to incorporate fiber slowly,” she said. “My mantra is ‘baby steps’ whenever I introduce fiber to any client. A little at a time goes a long way toward long-term health.”