A heavier person burns more calories because greater body mass is the single largest determinant of the total energy required simply to exist and to move. This higher caloric demand is a direct consequence of the physical and biological demands associated with maintaining a larger structure. The energy required for basic internal functions and physical activity increases proportionally with body weight.
The Baseline Burn: Weight and Resting Metabolic Rate
The body’s requirement for energy while at rest is the resting metabolic rate (RMR), and a heavier individual exhibits a higher absolute RMR. This baseline burn accounts for most of the total daily calories expended, supporting fundamental biological processes. Increased expenditure is necessary because a larger body contains more metabolically active tissue that constantly demands fuel.
Maintaining a larger mass of internal organs, which are highly active, requires significant energy. Organs such as the liver, brain, heart, and kidneys have a disproportionately high metabolic rate per gram compared to other tissues. These organs scale up to support the greater physiological demands of a larger individual.
A heavier person also possesses a greater total mass of skeletal muscle and adipose (fat) tissue, both contributing to the RMR. Although adipose tissue has a low metabolic rate per pound, its volume still adds to the overall energy requirement. The circulatory system must also work harder to pump blood through a more extensive network of vessels, increasing the heart’s energy cost.
This combination of larger organ mass and increased supportive system work means a heavier body requires more calories to sustain life. This difference in RMR is why a heavier person burns more calories over 24 hours than a lighter person, even if both remain sedentary.
Moving Mass: Calorie Expenditure During Activity
The relationship between body weight and calorie burn becomes more pronounced when physical activity is introduced. Energy expenditure during movement is governed by the principles of physics, where moving a heavier object requires a greater application of force. When a person walks, runs, or lifts weights, their body must expend energy to overcome the inertia and gravitational pull on their entire mass.
A heavier person must exert more force with every step to propel their mass forward and upward. This increased exertion results in a higher calorie expenditure for the same distance traveled or activity duration compared to a lighter person. The greater workload on the muscles demands more fuel to complete the mechanical work.
Two people walking side-by-side for thirty minutes at the same pace will burn different amounts of energy. The individual with the higher body weight performs a greater amount of physical work because they are moving a larger mass across the same distance. The energy cost of physical activity is directly proportional to the body mass being moved.
The Adaptive Response: Metabolism During Weight Maintenance
While a heavier body burns more calories, losing weight changes this dynamic. As significant weight is lost, the total daily energy expenditure (TDEE) decreases because there is less mass for the body to maintain and move.
The body often exhibits metabolic adaptation. After substantial weight loss, the measured RMR is frequently lower than predicted for a person of that new, lighter body weight. The body becomes more efficient at using energy, requiring fewer calories than predicted to sustain the lower weight.
This adaptive reduction in energy expenditure means the body is attempting to conserve energy, acting as if it is in a state of energy deficit. This metabolic shift requires an individual who has lost weight to consume fewer calories to maintain their new weight than someone who was naturally that body weight. A sustained downward adjustment in calorie intake is necessary to prevent weight regain.