In this review, Abdul Dulloo explains why the body actively defends fat mass—and often regains it preferentially after weight loss. For this paper, Dulloo revisits the Minnesota Starvation Experiment, reanalyzing detailed data from prolonged semistarvation and controlled refeeding in healthy men. The findings show that weight regain is driven by coordinated biological mechanisms: adaptive thermogenesis suppresses energy expenditure, hunger remains elevated through compensatory hyperphagia, and fat mass is restored more rapidly than lean tissue. Importantly, metabolic rate can remain suppressed even as fat is regained, creating conditions for “fat overshoot.” Dulloo frames these responses as evolutionarily conserved famine reactions that regulate body composition, offering a physiological explanation for obesity relapse, weight cycling, and the persistent difficulty of sustaining fat loss.