Types of Fiber

The types of fiber science comes down to where each fiber works: gel-forming fibers act in the stomach and small intestine, fermentable fibers feed bacteria in the colon, and unfermented fibers add bulk. In one study, each person’s starting microbiome explained about 78% of the variation in how their gut community looked.

What is soluble fiber? It’s plant fiber that dissolves in water, and the kinds that thicken into a gel do most of the work. Across 28 trials, gel-forming psyllium lowered LDL cholesterol by an average of 0.33 mmol/L, while thin soluble fibers such as inulin don’t deliver those gel-dependent benefits.

Soluble vs insoluble fiber isn’t a contest: your body needs both, and most high fiber plant foods contain a mix. Soluble fiber lowers LDL cholesterol by about 5.6 mg/dL per extra 5 grams a day, while insoluble fiber adds bulk and, in a study of 43,881 men, was tied to a 37% lower diverticular disease risk.

The types of fiber you eat behave very differently, because each one gels, ferments or bulks to a different degree. In a 2020 trial, two resistant starches that differed only in a small chemical tweak raised different gut bacteria, one boosting butyrate and the other propionate, in every participant who took them.