As we measure money in rupees, weight in kilograms, energy is measured in calories. Therefore, a calorie is nothing but unit of energy. Any work we do, we need calories. If we talk for a minute we’ll need about 2 calories. If we run we’ll be requiring about 10 calories in a minute. For any work we need calories. Even when we sleep the body has to keep on beating the heart; breathing and these will require about half a calorie every 1 minute. A calorie in Medical Science is called kilocalories (kcal).
Scientifically speaking, “Calorie” is ‘unit’ of energy, i.e., ‘the amount of heat necessary to raise the temperature of one kilogram of water by one-degree centigrade from 14.5 ˚C to 15.5 ˚C. In layman’s language, we can say ‘as petrol to the car the same as calorie to the body’, that means calorie is fuel for the body, which comes from the food we eat. For each and every activity body requires its fuel that is ‘Calories’.

If we do compare a human body/ machine with car. Then result will:

Car and Human Body/ Machine

Car Human Body/Machine
Runs on Fuel Runs on fuel
Fuel can be Petrol; Diesel Fuel can be any food
Measured in Liters Measured in calories

When we eat food, whatever way they are prepared, whatever be the composition, we start digesting it. The digestion process starts in the mouth cavity – the saliva starts breaking down these food items. By our teeth we tear and grind our food into small pieces and turn them into a paste form. This food now reaches stomach through our food pipe called esophagus. Liquid food like milk, juices do not need to stop in the mouth cavity and directly reach the stomach.

The stomach now further digests the food by the acids and enzymes of the gastric juice. It keeps on mixing the food and keeps them in it for about half an hour to two hours. After this the food is further digested in the first part of the small intestine by the juices coming from pancreas and liver. Once this food is broken down into smallest possible parts, the intestine absorbs these molecular food particles into blood.
The blood now carries these food particles to each and every cell of the body – of which there are about 10000 billion in the whole body. These food particles are now digested inside each of the cells and allow them to live, contributing to their function.

The total energy that is supplied to the body by the foods that we eat gets routed through the mitochondria present in the cells spread over the body. This energy can be called calories – as we try to estimate them when we walk the mitochondria present in the muscles of the legs become more active and demand more blood and food. They produce more calories foe the working muscles. For every activity our body needs some calories. It is calculated that the body of a sedentary person needs about 1600 calories