Metabolism is commonly invoked to explain why two people eating similarly carry different amounts of body weight. Measured energy expenditure gives a narrower account than the explanation implies.
Resting expenditure dominates the daily total
Most energy used across a day supports basic maintenance: circulation, breathing, ion gradients across cell membranes, protein turnover and temperature regulation.
Even in physically active people, this resting component typically accounts for the largest share of daily expenditure, with activity and digestion making up the remainder.
Because it dominates, differences in resting expenditure between individuals matter more for total energy use than differences in exercise habits alone.
Lean tissue accounts for most of the variation
Organs and muscle consume energy continuously, while adipose tissue consumes considerably less per unit of mass. Resting expenditure therefore tracks lean mass closely.
When researchers adjust measured expenditure for lean mass, the differences between individuals shrink substantially. Most of what looked like metabolic variation was body composition.
Organ mass contributes disproportionately. Brain, liver, heart and kidneys are metabolically expensive relative to their size, which is why scaling is not simply proportional to weight.
Residual differences exist but are modest
After accounting for lean mass, age and sex, a genuine spread remains between people, reflecting genetics, thyroid function and other physiological factors.
That residual spread is real, and for an individual it is not trivial. It is nonetheless much smaller than the differences implied by everyday descriptions of fast and slow metabolism.
Measuring it requires indirect calorimetry under controlled conditions, since prediction equations describe population averages and carry meaningful error for any one person.
Expenditure changes with body mass
A body carrying more mass expends more energy at rest and more during movement, because there is more tissue to maintain and more weight to move.
When body mass falls, expenditure falls with it. Some studies find the decline exceeds what the loss of tissue alone predicts, a phenomenon described as adaptive thermogenesis.
Its magnitude and persistence remain debated, but the direction is consistent and it explains why maintenance after loss differs from the period before it.
Where medical assessment belongs
Thyroid disorders and certain medications genuinely alter energy expenditure, and those are diagnosed by testing rather than inferred from difficulty managing weight.
Body weight is governed by many factors beyond energy expenditure, including appetite regulation, medications, sleep and medical conditions that require clinical evaluation.
The value of the measurement literature is mainly in correcting the assumption that unexplained metabolic differences account for most of what people observe.