The present disclosure is directed to the treatment of diseases or conditions characterized by the buildup of fatty tissue, or hyperphagia, such as Prader-Willi syndrome, obesity, metabolic syndrome, type II diabetes, etc. A composition containing a monoclonal antibody directed against gastric inhibitory polypeptide is administered. This results in a reduced rate of weight gain, weight loss, and/or reduction in fatty tissue, and a marked decrease in lipid synthesis and accumulation.