Isolated omentin polypeptides that selectively express in omental fat tissue and methods for the use of the polypeptides. The polypeptides can be used in a method for modifying insulin action and/or glucose metabolism in an animal. The polypeptides can be used to promote glucose uptake by animal adipocytes and other cells, tissues, and/or organs. The polypeptides can also used to provide a therapeutic treatment for diseases of or related to glucose metabolism and adipose tissues. The polypeptides are also incorporated into diagnostic tests and testing kits for diagnosing or detecting a disease or condition involving animal tissue that contains, uses, or expresses the polypeptide in an animal suspected of having the disease or condition.