A composition in aqueous solution includes insulin and at least one substituted anionic compound chosen from substituted anionic compounds consisting of a backbone formed from a discrete number u of between 1 and 8 (1≤u≤8) of identical or different saccharide units, linked via identical or different glycoside bonds, the saccharide units being chosen from the group consisting of hexoses, in cyclic form or in open reduced form, said compound comprising partially substituted carboxyl functional groups, the unsubstituted carboxyl functional groups being salifiable. A pharmaceutical formulation including the composition is also set forth.