Metabolism refers to the countless chemical processes going on continuously inside the body that allow life and normal functioning. These processes require energy (kilojoules) from food. The amount of kilojoules your body burns at any given time is regulated by your metabolism.

The total amount of energy your body uses each day is divided into three main categories:

1. Resting metabolic rate (RMR) also called basal metabolic rate
2. Energy used during physical activity
3. Thermic effect of food (TEF)

RMR is the energy burned by the body at rest to maintain the systems of the body and to regulate body temperature. For example your body still needs to burn energy when you are asleep to ensure you continue to breath and your heart continues to pump blood etc. RMR accounts for approximately 60 - 80% of the total daily energy expenditure in most adults.

This is the kilojoules burned during movement and physical activity and depends on how active you are. Your muscles need energy to work.  This means you use energy to get out of bed, to vacuum the house, to do the washing up and play with the kids. The amount of energy you use depends on how active you are each day and how hard the physical activity is, for example someone gardening for an hour will probably use less energy than someone running for an hour. The energy used during physical activity and exercise is the only form of energy expenditure that you have control over.

Your metabolism rises after you eat as you use up energy to eat, digest and metabolise the food you have just eaten. The TEF can last for several hours after a meal and depends on the energy and composition of the meal. Generally the TEF of a mixed meal (a meal containing carbohydrate, protein and fat) is about 6 - 10% of total daily energy expenditure.

How do we put a number on our metabolism or work out the amount of energy to give our bodies?

Without the use of laboratory facilities it is possible to assess your estimated energy requirements (EER) using a prediction equation.

First you need to estimate your RMR and then adjust it to take into account the amount of energy you need or use during physical activity and exercise.

A PAL or ‘physical activity level’ is used to make this adjustment. Your PAL depends on how active you are each day.

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