Gastric inhibitory peptide 1 (GIP-1) (3-42), a fragment of the incretin hormone GIP and antagonist to the GIP receptor, is generated through the action of serum dipeptidyl peptidase 4 (DDP-4). When administered at 25 nmol/kg to an ob/ob mouse diabetes model, GIP-1 (3-42) elevates plasma glucose levels and lowers plasma insulin levels, demonstrating its effect on reducing insulin secretion from BRIN-BD11 pancreatic cells at a concentration of 100 nM.