Obesity has traditionally been explained as the consequence of a chronic positive energy balance. While this principle remains valid, it does not explain one of the greatest challenges in obesity treatment: why maintaining weight loss over the long term is considerably more difficult than achieving it in the first place. Today, we know that the body responds actively to weight loss through a series of physiological adaptations that promote the recovery of lost weight.
One of the key players in this process is the brain. The hypothalamus integrates hormonal and metabolic signals from adipose tissue, the gastrointestinal tract, and other peripheral organs to regulate appetite, satiety, and energy expenditure. Following weight loss, these neural circuits activate coordinated responses aimed at restoring energy stores: appetite increases, highly palatable foods become more rewarding, and the body attempts to return to its previous weight. From an evolutionary perspective, these responses represent highly conserved survival mechanisms that evolved to protect against starvation rather than obesity.
At the same time, metabolism also adapts. Beyond the expected reduction in energy requirements resulting from a smaller body size, the body actively reduces resting energy expenditure as part of an energy-conserving response to perceived energy scarcity. This adaptive response may persist even after body weight has stabilised, making long-term weight maintenance particularly challenging. In this context, skeletal muscle may become an important ally. As one of the body’s most metabolically active tissues, preserving muscle mass through regular exercise may help attenuate part of this physiological adaptation while maintaining physical function and metabolic health.
Another emerging area of research concerns the concept of “obesity memory.” Experimental studies suggest that obesity may induce persistent molecular and epigenetic changes in adipose tissue, while immune cells within this tissue may also retain a pro-inflammatory phenotype after substantial weight loss. Although these mechanisms remain under investigation, particularly in humans, they raise the possibility that the body retains a physiological memory of its previous obese state, predisposing it to regain weight. This concept complements the set-point theory, which proposes that the brain actively defends a preferred range of body weight through coordinated physiological responses.
Understanding obesity therefore requires moving beyond the idea that weight loss is simply a matter of willpower. Rather than being opposed by a single organ, weight loss is challenged by an integrated physiological response involving the brain, metabolism, adipose tissue, skeletal muscle, and potentially other systems that are only beginning to be understood. Unravelling these adaptations will be essential for developing more effective treatments and reducing the stigma that still surrounds obesity.
PhD candidate in Exercise and Health at the University of Zaragoza (Spain) and a member of the EXER-GENUD Research Group. Her research focuses on obesity, bariatric surgery, body composition, energy metabolism, and exercise interventions aimed at preserving muscle mass and improving health outcomes after weight-loss surgery. Her work combines clinical research, exercise physiology, and public health perspectives to better understand the role of physical activity in obesity management.