Review: Brain mechanisms treat higher past weight as normal and drive regain after loss

Weight loss is not merely a matter of willpower involving eating less and moving more, according to a scientific review. The human brain evolved complex defenses to protect body fat stores during periods of scarcity, and these systems can treat a previously higher body weight as the new normal after loss (The Conversation).

Researchers Valdemar Brimnes Ingemann Johansen and Christoffer Clemmensen of the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen detail this in a comprehensive review published in the journal Cell in August 2025. The paper outlines how the central nervous system controls energy homeostasis through neuroendocrine signals and neural circuits (Cell).

When weight is lost, the body responds as if facing a survival threat. Hunger hormones increase, food cravings intensify, and energy expenditure decreases. These adaptations optimized energy storage in environments with uncertain food supplies but now contribute to difficulties maintaining weight loss in settings of abundant calorie-dense food and reduced activity needs (The Conversation; Nature reviews).

The brain possesses mechanisms that effectively “remember” a prior higher weight and defend it. Once the body has been heavier, circuits treat that level as the baseline to maintain. This helps account for the frequent regain of lost weight after dieting. The response reflects evolutionary biology rather than a failure of personal discipline (Cell; The Conversation).

Medications such as Wegovy (semaglutide) and Mounjaro (tirzepatide) offer an approach by mimicking gut hormones that signal the brain to reduce appetite. They can produce substantial weight loss in many users. However, not all individuals respond, side effects can limit adherence, and weight commonly returns when treatment ends as biological defenses reassert (The Conversation; PubMed).

The review highlights neuroplasticity and related insights as potential paths for future therapies that might more durably adjust these defensive signals. Broader research also indicates that improvements in sleep, physical activity, nutrition, and mental well-being can enhance metabolic and heart health independently of major changes on the scale (Cell; Nature).

Obesity is framed as a complex biological condition influenced by brain function, genetics, and environment rather than solely individual behavior. Genetic studies show high heritability of body mass index, with many associated variants linked to brain pathways involved in appetite. Environmental factors, including food availability, interact with these predispositions (Cell).

Prevention strategies discussed include healthier school meals, limits on junk-food marketing to children, urban design favoring walking and cycling, and attention to early-life periods from pregnancy to about age seven, when weight-regulation systems appear particularly adaptable. Parental diet, infant feeding, and early habits can influence long-term appetite and fat-storage control (The Conversation).

Sustainable habits such as prioritizing sleep to help regulate appetite and incorporating regular activity remain practical steps for individuals, even as pharmacological and societal approaches evolve. The authors note that science, medicine, and policy are beginning to address the biological drivers more directly (The Conversation).

The review synthesizes decades of evidence on brain control of energy balance and its relevance to anti-obesity treatments, building on established concepts such as set-point regulation while incorporating newer data on circuits, plasticity, and gut-brain signaling.