Energy is defined as ‘the capacity to do work’. Without food and water we would literally die and one of the reasons for this is because without enough energy, our body cells literally don’t have the energy to do their work (and their work is essentially, to keep us alive). (Mahan and Raymond, 2017).Each macronutrient provides a person with a different amount of energy. Both carbohydrates and protein provide the body with 4kCal/g while fats, which are very energy dense, provide 9kCal/g.A calorie is a measurement of energy. So the abbreviation of calorie is kCal and the abbreviation for kilojoule is kJ. They are essentially the same measurement, but with different names. Just like kilometre and miles and both a measurement of distance and kilogram and pound are both a measurement of weight. Just as you would have conversation for these measurements, 1 kCal = 4.18kJ. If you want to change calories to kJ multiply by 4.18 and if you want to convert kJ to calories divide by 4.18 (Mahan and Raymond, 2017).