Dr. Rossi, gut-health researcher and founder of probiotic brand Smart Strains and high fiber food brand Bio&Me, spoke to fitness personality Joe Wicks in a new episode of his Health or Hype video podcast.
Produced in partnership with Bupa, the episode focused on a personal experiment that saw Wicks test the effects of a low-fiber diet followed by a 24-hour fast. He temporarily removed many of the fiber-rich and carbohydrate-containing foods from his usual diet to observe how the change affected his energy, mood and gut function.
His experiment found that just three days of eating a low-fiber “beige” diet left him hungrier, more bloated and lethargic, with his mood falling and perceived stress rising, which Rossi explained could reflect the reduced supply of fiber available to gut bacteria.
“Gut bacteria have these special enzymes to break down that fiber and produce all these amazing chemicals which then can get into our blood system and communicate with the rest of our body,” Rossi explained. “So, [...] you were essentially starving those microbes.”
Reducing nutrients available to fiber-fermenting bacteria therefore reduces the production of short-chain fatty acids (SCFAs), including acetate, propionate and butyrate, which contribute to colonic health and interact with host metabolic and immune pathways.
Wicks then contrasted the restrictive phase with “fibermaxxing”—increasing fiber and plant diversity in the diet. However, this led to gastrointestinal symptoms such as bloating and discomfort.
Fibermaxxing
Fibermaxxing is a wellness trend that encourages people to increase their fiber intake to support gut health, digestion and satiety. The trend has gained popularity on social media, particularly alongside interest in GLP-1 weight-loss medications. However, experts increasingly emphasize fiber diversity and “smart fibermaxxing”, which encourages people to consume a diverse range of plant-based prebiotic fibers based on scientific evidence rather than relying on a single fiber source.
How the gut microbiome adapts to fiber intake
Rossi explained that after a period of very low fiber intake, a sudden increase in fermentable fiber can increase microbial activity in the colon, potentially leading to excess gas, bloating and gastrointestinal discomfort.
“Our ancestors used to have 100 grams of fiber plus a day [...] So, we know our body can have more fiber. But if you go from little fiber to loads of fiber, you’re going to potentially get some uneasy symptoms, maybe some extra bloating and discomfort in your gut,” she said.
Rossi noted that one thing missing from the conversation is the importance of considering the gut like any other muscle that needs to be trained to become ‘fiber fit’.
“You wouldn’t ever run a marathon without training your leg muscles, right?” she said.
Gradually incorporating whole grains, fruit, vegetables, beans, nuts and seeds into the diet can therefore help the gut microbiome adapt to increased fiber intake by progressively exposing it to a greater diversity of fermentable substrates, she said.
Experts highlight need for gradually increasing fiber intake
A growing number of brands, including Myota, The Gut Stuff and Huel, are placing greater emphasis on dietary fiber, developing products designed to help consumers increase their intake.
As Thomas Gurry, co-founder of Myota, told NutraIngredients, the gradual increase argument applies to fiber supplementation as well as whole foods.
“If someone is new to increasing fiber in their diet, I’d say this is particularly important in the context of fermentable/prebiotic fibers, which will result in microbial activity in the colon and production of SCFAs,” he explained. “[This] will have the side effect of producing CO₂ as part of the chemistry, and to an ‘untrained’ or sensitive gut, this can produce bloating and/or discomfort.”
When fiber intake increases rapidly, these changes may temporarily exceed an individual’s gastrointestinal tolerance, he added.
“These changes, while not intuitively linked to GI symptoms, are part of the ‘training’ involved, and it is advisable for someone new to take it slowly by increasing the dose based on their comfort levels,” Gurry said. “I’d say this is just as—if not more—true for fiber supplements like Myota, where these are highly fermentable and result in microbial activity, as it is for increasing whole foods-based consumption of fiber.”
