Rosenbaum et al. (2015) in Metabolism investigated how changes in body weight, particularly weight loss, affect energy metabolism and hormonal regulation. The article builds upon previous work showing that the body actively “defends” against weight loss. When people lose weight, energy expenditure and certain hormone levels drop more sharply than one would expect based on lower body weight alone. This phenomenon is often referred to as “adaptive thermogenesis.”

The authors describe how the body becomes more efficient after weight loss: resting metabolism, spontaneous activity, and thermic response to food decrease. At the same time, hormones involved in hunger and satiety change. Leptin decreases, which can intensify feelings of hunger and further reduce energy expenditure. Thyroid hormones and sympathetic nervous activity can also change, causing the body to consume fewer calories. Together, these adaptations increase the likelihood of weight gain after a diet. The article also discusses the role of the brain, particularly hypothalamic circuits that regulate energy intake and expenditure. The body appears to have a “setpoint” for weight that it attempts to maintain via hormonal and neural signals. After weight loss, this system often remains set to the previous, higher weight, making relapse biologically more likely.

Importantly, these reactions can persist for a long time. Even when someone successfully loses weight, the metabolic and hormonal adaptations persist and make weight maintenance difficult. The authors conclude that obesity and weight regulation are not only behavioral issues but are also strongly biologically driven. Effective treatment must therefore take these adaptive counter-reactions of the body into account.