An enzyme secreted by the body's fat tissue controls energy levels in the brain, according to new research. The findings, in mice, underscore a role for the body's fat tissue in controlling the brain's response to food scarcity, and suggest there is an optimal amount of body fat for maximizing health and longevity. The findings may help explain the many studies that show a survival benefit to having a body mass index toward the low end of what is considered overweight.