Energy has traditionally been expressed as calories or kilocalories. More recently, the units of energy have been changed to kilojoules. There are 4.2 kilojoules in 1 kilocalorie. Someone having 2000 kilocalories each day would be having 8400 kilojoules, also known as 8.4 megajoules.
The energy value of a food indicates its value to the body as a fuel.
After a food is ingested, some of its energy may be ‘lost’ during digestion and metabolism. Although the energy value of some foods has been found by combustion in a bomb calorimeter, more usually the amounts of the macronutrients – fat, protein, carbohydrate and alcohol (ethanol) – in a food are taken into account when assessing the total energy value of the food. The energy value for each macronutrient must be known, and an allowance made for body losses.
Fat is the most energy-dense macronutrient, followed by alcohol, protein and carbohydrate.
Energy requirement can be thought of as the amount needed to maintain the basic processes of life at rest, that is, basal metabolism, plus the amount needed for physical activity under a variety of circumstances.
Body weight is an important factor in determining how much energy we need, since more energy will be needed to sustain and move a greater body mass.