The Bioenergetics of the Immune System Nutrient

conservation at times OF starvation is an important survival trait for any organism. This principle is elegantly demonstrated in the Report entitled "Survival for immunity: The price of immune system activation for bumblebee workers " by Yannick Moret and Paul Schmid-Hempel (10 Nov., p. 1166), in which the survival of bumblebees was significantly impaired by their inability to perform this function. Activation of the innate immune system of these insects caused increased mortality only when their nutrient intake was limited. The most likely cause of this increased mortality was the energy demands imposed by the immune response in the context of a paucity of energy substrate. In mammals, it has been shown that the adipocyte-derived hormone leptin is a regulator of metabolism and bodyweight (1). An important physiological role of leptin is as a signal of starvation, in that a falling serum leptin concentration leads to neurohumoral and behavioral changes that seek to preserve limited energy reserves for immediately vital functions (2). It has been proposed that reduced leptin levels during conditions of starvation lead to impaired reproductive, thermogenic (2), an