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 eat and grind our food into small pieces and turn them into a paste form. This food now reaches the stomach through our food pipe called oesophagus. Liquid foods 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 the smallest possible parts, the intestine absorbs these molecular food particles into the blood.
The blood now carries these food particles to 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 by an organ called mitochondrion – which supplies energy to 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 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 for 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.