Energy Balance and Metabolism
How your body uses calories, and what that means for your goals
Metabolism is simply how your body turns food into energy, running every function from breathing to training. Understanding it takes the mystery out of weight change and helps you set realistic goals.
What your metabolism does
Your metabolism never stops, even at rest. It powers breathing, circulation, digestion, cell repair, hormones and temperature control. Food provides the fuel, which is stored and released as energy (think of ATP as your body’s battery).
Energy balance, simply
| If you are in | That means | The result |
|---|---|---|
| Balance | Energy in equals energy out | Weight stays the same |
| A surplus | Energy in is greater than energy out | Weight gain |
| A deficit | Energy in is less than energy out | Weight loss |
Energy comes from carbohydrate (4 kcal/g), protein (4 kcal/g), fat (9 kcal/g) and alcohol (7 kcal/g).
Where your calories go
Your Total Daily Energy Expenditure (TDEE) is what you burn in a day. Most of it, around 70%, is your Basal Metabolic Rate: the energy just to keep you alive. The rest comes from digesting food, everyday movement and exercise.
What affects your metabolic rate
- Body size: larger bodies burn more.
- Muscle: more lean tissue raises your rate.
- Sex: men often have higher rates, mainly from more muscle.
- Age: your rate drifts down roughly 2 to 3% a decade after 20, mostly from lost muscle.
- Plus activity, climate, hormones, illness and genetics.
Working out your needs
Equations like Schofield or Harris-Benedict estimate your rate from your age, sex and weight, then multiply by an activity factor for your TDEE. They are a useful starting point, but can be off by up to 20%, so treat the number as a guide and adjust based on real results.
This leaflet is general nutritional guidance for education only. It is not medical advice, diagnosis or treatment, and it does not replace advice from your GP or a registered dietitian. Always check with a qualified professional before making significant changes, particularly if you have a health condition, are pregnant, or take medication.