When your body has enough energy, it sends a hormone called leptin to your brain to say 'we're good.' But when you're starving and leptin drops, your brain slows down things like reproduction and metabolism to save energy—and if you give leptin back, those functions start working again.

This study explains that leptin is a hormone made by fat cells that tells the brain how much energy the body has. When leptin drops during starvation, the brain slows down metabolism and reproduction—this study backs that idea by showing leptin was discovered as the key signal for this system.