The present invention provides a method for reducing the weight of a subject, preventing weight gain in a subject, lowering blood glucose, or treating hepatic steatosis, comprising administering to a subject in need thereof a therapeutically effective amount of a pharmaceutical composition comprising interleukin-25 or IL-25-activated macrophages. In addition, the present invention provides a method of treating cachexia or promoting weight gain in a mammal in need of such treatment, comprising administering to the mammal a therapeutically effective amount of a pharmaceutical composition comprising an inhibitor or antagonist of interleukin-25 in a mammal in the treatment of cachexia.