Metabolism stays broadly steady through midlife
The short answer is: around age 60, not in the thirties or forties. In large-scale measurements of daily energy use, metabolism—the energy the body spends on its work—was broadly stable from about age 20 to 60. After 60, average energy use began a gradual decline, rather than dropping suddenly on a birthday.
That measure includes more than the calories burned while resting. It also reflects the energy needed to move, digest food, and keep organs functioning. Researchers can estimate total daily energy expenditure by tracking how the body processes harmless forms of hydrogen and oxygen in water over time.
Why the change is gradual
The body’s energy needs depend partly on its size and composition. Muscle and organs require energy to maintain; as people age, changes in lean tissue and in how the body functions can reduce that demand. The decline after 60 is an average pattern across people, not a precise timetable for any one person.
Daily calorie needs can still change earlier for other reasons. A shift in activity, body weight, or muscle mass can alter how much energy someone uses without meaning that the body’s underlying metabolism has suddenly “slowed.”
What the age figure does—and doesn’t—mean
The finding challenges the familiar idea that metabolism inevitably plunges in early adulthood. It does not mean everyone burns the same number of calories until 60, or that energy needs fall by the same amount afterward. Age marks a population-level turning point, not a personal metabolic switch.
