Energy stores are defended by hormonal and metabolic adaptation to weight loss
Expansion
Body weight is actively defended. After deliberate weight loss:
- Leptin falls disproportionately, signalling starvation
- Ghrelin rises, increasing hunger
- Peptide YY and GLP-1 fall, reducing satiety
- Adaptive thermogenesis: resting energy expenditure falls below that predicted for the new, smaller body, and this persists for years
- Thyroid hormone and sympathetic activity fall
The combined effect is increased hunger and reduced expenditure, which explains why weight regain is the norm rather than a failure of willpower.
Adipose tissue is an endocrine organ, secreting leptin, adiponectin, resistin and inflammatory cytokines. Visceral fat is metabolically more active and drains to the portal vein, delivering free fatty acids and cytokines directly to the liver, which is why central obesity carries greater metabolic risk than total fat mass.
This physiology explains why interventions that alter the hormonal signals, notably GLP-1 receptor agonists and bariatric surgery, produce more durable weight loss than dietary restriction alone.