In certain aspects, the present invention provides compositions and methods for increasing thermogenic adipocytes (e.g., brown adipocytes or other UCP-1 expressing adipocytes) by administering an antagonist of an ActRIIB signaling pathway. Examples of such antagonists include ActRIIB polypeptides, anti-ActRIIB antibodies, anti-myostatin antibodies, anti-GDF3 antibodies, anti-Noda1, anti-activin, and anti-GDF11 antibodies. A variety of metabolic and other disorders may be treated by causing an increase in thermogenic adipocytes.