Two people of similar age can have noticeably different resting energy requirements. The variation is largely explained by measurable physical factors rather than by an unexplained metabolic quality.
Most energy is spent doing nothing in particular
Resting metabolic rate is the energy required to keep the body running while at rest: maintaining ion gradients, synthesising proteins, circulating blood, sustaining temperature.
It accounts for the largest share of daily energy expenditure in most people, well ahead of deliberate exercise.
Digestion and physical activity make up the remainder, with activity being the most variable component between individuals and between days.
Organs and muscle differ in energy cost
Tissues are not equally demanding. Brain, liver, heart and kidneys consume energy at high rates per unit mass despite being a small fraction of body weight.
Skeletal muscle uses less per kilogram at rest but contributes substantially because there is a lot of it.
Fat tissue is metabolically active but at a considerably lower rate, which is why total fat-free mass predicts resting rate better than total weight does.
Size explains most of the between-person variation
Larger bodies require more energy at rest, simply because there is more tissue to maintain and more surface through which heat is lost.
Once fat-free mass is accounted for, the differences between individuals shrink markedly, and much of what remains falls within measurement variation.
This is why predictive equations use weight, height, age and sex: those variables are standing in for body composition.
Age and hormones shift the figure
Resting rate declines across adulthood, driven substantially by the gradual loss of muscle mass and by changes in organ tissue.
Thyroid hormone sets the general pace of cellular metabolism, and both underactivity and overactivity move resting rate measurably.
These are clinical conditions detected by blood testing rather than inferred from how a person feels their metabolism behaves.
Measurement is possible but not casual
Resting rate is measured by indirect calorimetry, analysing oxygen consumption and carbon dioxide production under controlled conditions after fasting and rest.
Consumer devices and equations estimate rather than measure, and their error for an individual can be considerable even when they perform acceptably across a group.
Unexplained changes in weight, temperature tolerance or energy are worth investigating with a doctor, since several treatable conditions affect metabolic rate directly.