Dietary fiber supplementation alters the gut microbiome in a subset of prediabetic individuals with specific metabolic profiles, resulting in increased abundance of Parabacteroides distasonis, Bacteroides uniformis, and Lactobacillus mucosae, which are associated with improved glycemic control.

In some people with prediabetes, undigested fiber reaches the gut and feeds specific bacteria that produce short-chain fatty acids. These fatty acids signal the gut to release a hormone that tells the pancreas to make more insulin, which lowers blood sugar. This only happens in people whose gut bacteria are already set up to respond this way — others’ bacteria don’t change, so their blood sugar doesn’t improve.