Brain pathways controlling food intake and body weight.

Evidence has existed for more than 50 years in support of the hypothesis that body energy stored in the form of fat is homeostatically regulated. Implicit in this concept is the existence of a biological system that operates dynamically over time to match cumulative energy intake to energy expenditure. For example, to compensate for weight loss induced by energy restriction, animals must enter a period of positive energy balance (i.e., energy intake greater than energy expenditure) that is sustained for as long as it takes to correct the deficit in body fat stores. Having reached this point, the animal must return to a state of neutral energy balance if stable fat mass is to be maintained. The identification of neuronal circuits in the hypothalamus that, when activated, exert potent, unidirectional effects on energy balance provides a cornerstone of support for this model. The additional finding that these central effector pathways are regulated by humoral signals generated in proportion to body fat stores, including the hormones insulin and leptin, helps to round out the picture of how energy homeostasis is achieved. The goal of this overview is to highlight the evidence that specific subsets of hypothalamic neurons containing specific signaling molecules participate in this dynamic regulatory process, and to put these observations in the larger context of a biological system that controls body adiposity.