Just as an engine needs fuels like coal, petrol etc. to run, similarly our body also requires fuel which provides us energy to work with. The substances which act as fuel or energy for our body are fats, carbohydrates, proteins etc. — the substances which our food consists of. Whatever food we take it gets burnt with the help of oxygen which we get from in-breathing, thereby releasing energy by breaking into these constituents. This energy thus produced is used in the performance of all of our voluntary and involuntary body functions. For instance, protein is required for growth and tissue repair while vitamins and the minerals are necessary for the proper functioning of various body organs.
So, we need to supply these substances regularly to our body for its efficient functioning. That is why we take food regularly.
Out of these substances taken as food our body absorbs and utilizes useful ones and rejects those which are not needed, in the form of waste material. We excrete daily about 2 kg of urine and stool. Carbon-dioxide also comes out during out-breathing as a waste material. About 400 gm of water comes out through the process of sweating and out-breathing.
Human body temperature remains constant between 98°F and 99°F. This is a wonderful equilibrium between the production and consumption of heat in the body so that the body temperature remains constant. Food is measured in units of energy, called Calories.
Our body is a wonderful machine which works continuously without any break. Even when we are in sleep many processes such as respiration, digestion, heart beating etc. go on incessantly. These processes also consume fuel continuously.