Members of the cell death-inducing DFF45-like effector (CIDE) gene family have been shown to regulate lipid metabolism. In this article, we report that the third member of the human CIDE family, CIDEC, is down-regulated in response to a reduced caloric intake. The down-regulation was demonstrated by microarray and real-time polymerase chain reaction analysis of subcutaneous adipose tissue in 2 independent studies on obese patients undergoing treatment with a very low calorie diet. By analysis of CIDEC expression in 65 human tissues, we conclude that human CIDEC is predominantly expressed in subcutaneous adipocytes. Together, these observations led us to investigate the effect of decreased CIDEC expression in cultured 3T3-L1 adipocytes. Small interfering RNA-mediated knockdown of CIDEC resulted in an increased basal release of nonesterified fatty acids, decreased responsiveness to adrenergic stimulation of lipolysis, and increased oxidation of endogenous fatty acids. Thus, we suggest that CIDEC is a regulator of adipocyte lipid metabolism and may be important for the adipocyte to adapt to changes in energy availability.