The wellness industry loves a hero ingredient. 2025 was the year protein dominated grocery aisles, appearing in everything from popcorn to soda and even water. Now, fibermaxxing is coming for its glory, with the humble nutrient promising not only smoother, more satisfying poops but also benefits for metabolic health, heart health, and then some. But where do probiotics—another gut-health darling that had its own heyday as the supplement du jour a few years back—come into play?
If you’re trying to optimize your gut health and digestion, is it fiber or probiotics that’ll do most of the heavy lifting? Below, experts help us unpack these two gut health all-stars and share which one reigns supreme. But first: a quick refresher on both.
The benefits of fiber
Found in plant foods, fiber is a type of carbohydrate that your body can’t digest—but your gut bacteria can. “When diverse dietary fibers reach your colon, your bacteria ferment them and produce short-chain fatty acids (SCFAs),” says Will Bulsiewicz, MD, a board-certified gastroenterologist and the author of Plant Powered Plus. Butyrate is one important SCFA that helps to not only strengthen your gut barrier and protect your body against pathogens. Beyond the colon, SFCAs also “regulate blood sugar, help manage cholesterol, and trigger the release of appetite-suppressing hormones like GLP-1 and PYY,” says Dr. Bulsiewicz. “They even optimize your immune system and reduce inflammation.” Beyond SCFA production, fiber helps beneficial bacteria thrive and supports the transit and absorption of nutrients in the gut, says Federica Amati, PhD, MPH, RNutr, head nutritionist at ZOE.
And of course, most of fiber’s fame derives from its benefits for digestion, no matter if your issue is too many or too few trips to the toilet…or just a generally uncomfortable experience once you’re there. “Fiber improves bowel irregularity and makes stools bulkier and softer, helping things move through the digestive tract more smoothly,” Felice Schnoll-Sussman, MD, MSc, a board-certified gastroenterologist at Weill Cornell Medicine and NewYork-Presbyterian, tells SELF.
Depending on your digestive woes, it can also help to get specific about the type of fiber you’re getting. That’s right: There are two kinds—soluble and insoluble fiber—and each serves a different purpose. The former dissolves in water and can help slow digestion, which Dr. Schnoll-Sussman says is helpful for diarrhea and managing blood sugar. (“It can also lower cholesterol by binding to bile acids, which can be associated with a reduced risk of cardiovascular disease and obesity,” she adds.) Insoluble fiber, meanwhile, doesn’t dissolve in water and can help boost transit in the bowel, thus addressing constipation.
