Metabolism is invoked constantly in discussions of weight, usually as an explanation for differences between people. The reality is narrower.
What it means
The total energy your body uses. It has components: the energy to keep you alive at rest, which is most of it; the energy to digest food; and the energy of movement, both deliberate exercise and everything else.
How much it varies
Less than people assume. Between two people of similar size, age and body composition, resting metabolic rate typically differs by a modest percentage rather than by a factor.
The largest determinant is body size, particularly lean mass. A larger body burns more energy at rest, which is why metabolic rate falls as weight is lost – the body is smaller.
What actually differs
Unconscious movement varies substantially between individuals – fidgeting, posture changes, general restlessness. That component can differ by hundreds of calories daily and is largely not under deliberate control.
What does not work
Foods claimed to boost metabolism. The effects measured are real and tiny, on the order of a few calories.
Eating frequently to keep it running, which has been tested and does not affect total energy expenditure.
What does affect it
Muscle mass, modestly, which is one argument for strength training. Age, which reduces it gradually. And thyroid function, which is a medical matter with a real and measurable effect.
Why it matters less than expected
Because the variation between people is small relative to the variation in what they eat and how much they move. Metabolism is not usually the explanation.
The caveat
General information. Anyone concerned about unexplained weight change should see a doctor, since it can indicate a treatable condition.