How fat mass signals the brain
Leptin is a hormone secreted largely by fat tissue, and its level roughly tracks the amount of stored body fat. The hypothalamus reads circulating leptin as a signal of energy reserves. When fat mass falls during a diet, leptin drops, and that fall is interpreted by the brain as a warning that energy stores are shrinking.
The drop in leptin tends to be proportionally larger than the loss of fat itself in the early phase of a deficit, which is part of why hunger can intensify quickly even after only modest fat loss.
The defensive cascade against a deficit
Falling leptin is associated with a cluster of responses that defend body weight: appetite and food-seeking rise, spontaneous non-exercise activity (NEAT) often declines, and thyroid and sympathetic signaling can down-regulate, lowering energy expenditure. Together these blunt the rate of further loss. This is described as the body's adaptive response, not a fault, and the magnitude varies between individuals.
This is general educational information, not medical advice. Leptin physiology is complex and still under study, and decisions about dieting or any hormone-related concern should involve a qualified professional.