The present study aims to investigate the impact of orexin deficiency on the regulation of energy and glucose metabolism using a mouse model depleted for the prepro-orexin gene. Our data reveal that, despite a decrease of food consumption (at least in males), orexin deficiency induces a significant increase in body weight that is associated with an alteration of the body composition, as males and females orexin deficient mice display an increased fat mass compared to the wild-type littermates. Nevertheless, no significant differences of global energy expenditure and locomotor activity were observed in the mutant mice relative to the control. Glucose homeostasis is also impaired in the absence of orexins, since glucose tolerance and insulin secretion are diminished, and insulin sensitivity is slightly reduced. In addition, the livers of the male orexin-KO mice are significantly larger and heavier with more adipose tissue than the wild type mice. Interestingly, orexin-deficient mice present an upregulation of liver enzyme involved in gluconeogenesis and a down-regulation of GCK, an enzyme which promotes glycogen storage, that may participate to the altered glucose metabolism of the orexin mutant mice. To conclude, the present study indicates that orexin deficiency induces profound alteration in the regulation of energy and glucose metabolism, which is more pronounced in males than in females. These findings support the idea that dysfunction of this orexin system may promote obesity and diabetes, and could represent an interesting therapeutic target in the context of 'diabesity'.