[Cytokines and nutritional disorders]

During an infection, a decrease in food intake together with elevated energy expenditure appears. Anorexia is one of the most common signs of illness and is often considered as an undesirable manifestation of sickness. However, compelling data demonstrate that anorexia constitutes an adaptative strategy systematically organised for pathogens elimination. Microbial products stimulate the production by immunocompetent cells of cytokines, which orchestrate the immune response. Since the administration of cytokines reduces food intake, it has been suggested that these agents play a key role in mediating anorexia during infection. This review details the mechanisms of cytokine-induced anorexia, focusing on the role of endogenously produced brain cytokines and more particularly interleukin-1 (IL-1). De novo synthesis of IL-1 occurs in the brain during peripheral infection mimicked by the administration of bacterial endotoxin lipopolysaccharide (LPS). Centrally produced IL-1 acts on its receptors to mediate anorexia as demonstrated by the use of knockout mice and specific IL-1 receptor antagonist. Functional neuroanatomy demonstrates further that LPS or IL-1 specifically activates the hypothalamic neurons that control food intake. Leptin is tightly regulated by IL-1, suggesting the involvement of this hormone in the anorexia of infection. The mechanisms by which hypothalamic arcuate nucleus neuropeptides, which are regulated by IL-1 and leptin, could mediate anorexia during infection are discussed.