What is metabolism?

In one line

Metabolism is the name for all the chemical changes by which the body breaks down what it eats to get energy and uses that energy to build what it needs.

What is metabolism?
The word behind 'fast metabolism' and 'basal metabolic rate'
You often hear things like 'I gain weight easily because my metabolism is slow' or 'my basal metabolic rate is this much'. But metabolism does not only mean burning calories. Here is, step by step, what metabolism actually means.
In a word
All the chemical changes inside the body
Metabolism is the name for all the chemical changes that happen inside a living body. Breaking down food to get energy, and using that energy to build what the body needs, are both metabolism. In Korean, two other words for metabolism are used with the same meaning.
The breaking side: breakdown
Splitting big things into small ones to get energy
One side of metabolism is 'breakdown'. Breakdown means splitting something large and complex into small pieces. As the body splits the carbohydrates, fats and proteins in food into small pieces, it takes out the energy they hold. This side is also called 'catabolism'.
The building side: synthesis
Using energy to build what the body needs
The other side is 'synthesis'. Synthesis means linking small pieces together to build something large and complex. Linking 'amino acids', the small pieces that make up protein, into muscle protein is one example. This side is also called 'anabolism'.
ATP links the two
It carries the energy from breakdown
The energy from breakdown is stored in a small molecule called 'ATP' and used for building and for movement. ATP works like a charged battery that cells can use right away. Only about 40% of the energy from breakdown ends up in ATP; the other roughly 60% becomes heat that keeps the body warm.
When there is extra or too little
Extra is stored, shortfalls are drawn from storage
When breakdown yields more energy than the body uses, the body turns the extra into fat and stores it. When there is too little, it draws on the stored energy. Energy is stored in the liver, the muscles and body fat.
Basal metabolic rate
The energy used even when you are still
Basal metabolic rate is the amount of energy the body uses even while doing nothing and resting. This energy keeps the heart beating, the lungs breathing and the body warm. It is measured while awake, lying still, and without having eaten.
Three parts of daily energy
The biggest share is basal metabolism
The energy used in a day falls into three main parts. Basal metabolism takes the biggest share, more than half, and digesting and processing food takes about 10%. The rest goes to moving the body.
A confusing term: metabolic syndrome
It does not mean a slow metabolism
'Metabolic syndrome' does not mean a slow metabolism. It is the state of having at least three of five warning signs that raise the risk of heart disease and diabetes, such as a large waist, high blood pressure, and abnormal blood sugar and blood fat levels.
A common misconception
Do people with obesity have a slow metabolism?
No. People with obesity actually used more energy per day, and had a higher basal metabolic rate, than lean people. When the difference in lean mass (muscle and everything except fat) was taken into account, even that gap disappeared. The idea that 'a slow metabolism keeps the body heavy' did not match the research.
Summary
All the breaking and building chemistry inside the body
Metabolism is the name for all the chemical changes by which the body breaks down what it eats to get energy (breakdown) and uses that energy to build what it needs (synthesis). People with obesity did not have a slow metabolism. Related terms: ATP, basal metabolic rate, muscle protein synthesis.