Energy expenditure
The total calories burned in a day, the sum of basal metabolic rate, the thermic effect of food, spontaneous activity and deliberate exercise.
A day’s total energy expenditure breaks down into four components of very unequal weight, and it is this split that makes most reasoning about « burning calories » misleading.
The basal metabolic rate accounts for 60 to 70% of the total: it is what the body uses at complete rest to keep its vital functions going. The thermic effect of food, the energy spent on digesting and absorbing, accounts for about 10%, more for protein, of which 20 to 30% of the energy goes on its digestion alone.
The most neglected component is not sport, it is NEAT, non-exercise activity: walking, climbing stairs, standing, fidgeting while talking. In someone who does little sport, it far exceeds the expenditure of a gym session, and it varies enormously from one person to another, which explains a good part of the differences in « metabolism » seen at the same weight.
One last thing to know before any diet: expenditure adapts downwards when you cut intake. The body economises, notably by reducing spontaneous activity, often without you noticing. This is one of the mechanisms that make weight loss slower and slower, and regain easier.