Why the body spends less energy at rest
Basal metabolic rate is the energy the body uses to keep basic functions going—such as breathing, circulation, and maintaining body temperature—while fully at rest. During prolonged calorie restriction, this rate can fall for two reasons. First, weight loss leaves less body tissue to maintain, and smaller bodies generally require less energy. Second, the body may make an additional adjustment: it uses somewhat less energy than expected for its new size and composition. This adjustment is called adaptive thermogenesis.
The adjustment is more than a smaller body
The body’s energy budget is not fixed. When food is scarce, changes in hormones and other signals can shift how much energy it uses. A person may also move less without noticing—fidgeting less, for example—which lowers daily energy use beyond the change in resting metabolism. These effects vary between people and are not all captured by basal metabolic rate alone.
Why weight management can feel harder
If energy needs decline, the calorie intake that once produced weight loss may eventually bring weight loss to a halt. After weight loss, maintaining the lower weight can therefore require fewer calories than someone might expect from their previous needs. The practical implication is that a plateau does not necessarily mean nothing has changed: the body may now be spending less energy, both because it is smaller and because it has adapted to prolonged restriction.
