ABSTRACT
Body energy balance in humans and laboratory animals is regulated by a set of very complex biological and behavioral signals, mechanisms, and pathways. Molecular (e.g., leptin, insulin) and biophysical (e.g., vagal afferents responding to a change in duodenal glucose concentration) signals in the periphery and brain are linked in a neural network that controls eating behavior and energy intake. Signals generated in the periphery and brain are integrated and interpreted, and the appropriate motor program and resulting behavioral response in terms of energy intake generated in the brain of humans and animals. For example, a person who has been ill for 4 days with influenza has decreased energy intake and has lost 5 kg of weight. As soon as the illness ends, the brain will correctly interpret the fall in leptin and other peripheral and brain signals as weight loss and then increase energy intake for the next several days; the person will regain the 5 kg lost. Suppose that, 3 months later, the same person goes on vacation for a month in France or on a cruise ship, overconsumes energy, and gains 5 kg. When the person
returns to habitual patterns of energy intake and expenditure, the brain will correctly interpret the increased leptin and other signals as weight gain. The brain will decrease energy intake for the next period until the 5 kg are lost and the habitual body mass and composition are restored. These are examples of the usual regulation of energy balance in lean humans and animals.