A composition for lowering a glycemic index (GI value) of a food or feed, comprising a Cyamopsis tetragonolobus bean protein and a degraded galactomannan; a low-glycemic index food comprising polygalactosyl mannose and gliadin and glutenin in a specified ratio; a low-glycemic index food comprising polygalactose and/or a polygalactose derivative and gliadin and glutenin in a specified ratio; a low-glycemic index food comprising polygalactosyl mannose and amylose and amylopectin in a specified ratio; a low-glycemic index food comprising specific polygalactose and/or a polygalactose derivative and amylose and amylopectin in a specified ratio.