Metabolism
The sum of chemical processes by which the body produces energy and builds or breaks down substances.
What it means
Metabolism is the totality of chemical reactions sustaining life, conventionally divided into catabolism — breaking substances down to release energy — and anabolism — building complex molecules from simpler ones.
The basal metabolic rate is the energy used at complete rest, and accounts for the majority of daily energy expenditure. It is influenced by muscle mass, age, sex and thyroid function.
In pharmacology the word has a narrower meaning: how the body chemically alters a medicine, largely in the liver, in order to eliminate it.
In practice
"Metabolic syndrome" describes the clustering of abdominal obesity, raised blood pressure, raised blood glucose and abnormal lipids.
Drug interactions frequently arise because one medicine alters the metabolism of another — the mechanism behind grapefruit juice affecting statins, and behind enzyme-inducing drugs reducing contraceptive efficacy.
Why it matters
Metabolic rate is often blamed for weight change, but measured differences between individuals are smaller than commonly assumed and are largely explained by body composition.
An unexplained change in weight, appetite or temperature tolerance is worth investigating — thyroid disease and diabetes both present this way.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026