Food has been an ultimate source of energy. Food energy is chemical energy that animals (including humans) derive from food through the process of cellular respiration. The total energy content of a food can be found by burning it and measuring how much heat is released. Eating transfers the stored energy from the food to the animal that eats it. All parts of the body (muscles, brain, heart, and liver) need energy to work. This energy comes from the food we eat. Energy is measured in units of kilocalories (kcal) or kilojoules (kJ).Foods with fewer calories per gram such as fruits, vegetables, low fat soups, lean protein and fibre-rich foods have a relatively low energy density. The energy value of a particular food is calculated from a knowledge of macronutrient composition and the modified Atwater factors for these macronutrients. The energy values shown in the chart have been obtained with this approach. Different kinds of foods are converted to energy at different rates, some — such as candy and other simple sugars — can give you a quick lift, while others — such as whole grains and healthy unsaturated fats. Where energy is the issue, it's better to eat small meals and snacks every few hours than three large meals a day. Foods rich in complex carbs and protein are the best picks for all-day energy,