The aim of this study was to evaluate the effect of the colonic fermentation of unavailable carbohydrates from unripe banana (mass - UBM - and starch - UBS) over parameters related to glucose and insulin response in rats. Wistar male rats were fed either a control diet, a UBM diet (5 % resistant starch - RS) or a UBS diet (10 % RS) for 28 days. In vivo (oral glucose tolerance test) and in vitro (cecum fecal fermentation, pancreatic islet insulin secretion) analyses were performed. The consumption of UBM and UBS diets by Wistar rats for 28 days improved insulin/glucose ratio. Also, pancreatic islets isolated from the test groups presented significant lower insulin secretion compared to the control group, when the same in vitro glucose stimulation was done. Total short chain fatty acids produced were higher in both experimental groups in relation to the control group. These findings suggest that UBM and UBS diets promote colonic fermentation and can influence glycemic control, improving insulin sensitivity in rats.