What determines whether excess calories are turned into fat or are burned off? The key lies in a process known as diet-induced thermogenesis, an intricate system of communications masterminded by the brain which literally means “heat production,” according to the senior author of a new study, Bradford B. Lowell, of Beth Israel Deaconess Medical Center’s department of endocrinology. “The body requires a certain number of calories in order to function and maintain good health,” explains Lowell, who is also an associate professor of medicine at Harvard Medical School. “When the body takes in more calories than it needs, it either converts the extra into stored fat, which sticks around and can lead to obesity, or it converts them into heat, which is subsequently dissipated.” In a study with mice, Lowell and his collaborators discovered that mice burn off extra calories when a group of molecules called beta adrenergic receptors are “turned on” by neurotransmitters released from sympathetic nerve endings. The findings, which could ultimately aid researchers in identifying mutations that cause obesity as well as in developing anti-obesity drugs.