Set-point theory proposes that the brain — primarily the hypothalamus — maintains body weight within a defended range, adjusting hunger, satiety hormones (leptin, ghrelin), and metabolic rate to resist deviations in either direction.
The evidence for a defended weight range is real: studies of calorie restriction consistently show metabolic adaptation (reduced basal metabolic rate beyond what body composition changes alone explain) and a persistent increase in appetite hormones after weight loss. The CALERIE trial and the long-term follow-up of The Biggest Loser participants demonstrate that these adaptations endure for years, not weeks.
The mechanism, however, is debated. The strict “thermostat” metaphor is probably too simple; the defended range appears wider and more influenceable than a fixed setpoint. Sustained lifestyle changes — particularly regular physical activity, high protein intake, and adequate sleep — appear to shift the defended range over time in ways that short-term dieting does not.
The practical implication is that slower, sustainable approaches to weight management tend to produce better long-term outcomes than aggressive restriction. See calorie deficit, weight cycling, and energy balance.