Fish skin gelatin hydrolysates as dipeptidyl peptidase IV inhibitors and glucagon-like peptide-1 stimulators improve glycaemic control in diabetic rats: A comparison between warm- and cold-water fish

•Warm-water fish skin gelatin contains high amounts of imino acids.•Warm-water fish skin gelatin hydrolysates have great DPP-IV inhibitory activity.•Six isolated peptides comprised Pro as the second N-terminal residue.•HSGH and TSGH inhibited plasma DPP-IV activity and induced GLP-1 secretion.•TSGH improved the glycaemic control better than HSGH in diabetic rats.•The protein with high imino acid content had the potential to improve diabetes. Various warm- and cold-water fish skins were used to prepare gelatin hydrolysates and compare their in vitro dipeptidyl peptidase IV (DPP-IV) inhibitory activity. The DPP-IV inhibitory activity of gelatin hydrolysates from warm-water fish was greater than that from cold-water fish. The