Food as fuel for the body machine:

Body requires food for internal as well as external activities and for growth. Another way of  looking at this is that the body requires energy for both  its internal and external activities. In  this sense, the body can be compared with a machine and food is the fuel which is required to run the body. Energy is produced  in  the body from food by a process called metabolism which is comparable to combustion or burning of a fuel. Oxygen is used up in this 'combustion'  and carbon dioxide is produced. If  the body does not get at least a minimum of energy,  it will not be able to carry on  its normal  internal functions or undertake external work. However, a major difference between a machine and the human body  is that the latter is combsed of  the same fuel which it uses to provide energy. A machine cannot use itself as fuel to run. But our body cah. Thus, during fast the body can bum  its own fats  to get energy, resulting in  loss of weight. If  a body  is supplied with more energy than  is required to run it, it is stored in the body in the form of fat for future use. Energy requirements of  the body are measured in terns of "calories"  or kilo-calories (1000 calories), usually written as Calories, with a capital C. For example, a tea spoon of  sugar (5 gm), when burnt, produces 20 Calories of heat. Scientists  have studied energy requirements of  infants, girls, boys and adults  of different ages and weights to find out how many Caloties they use in doing different things. You would be curious to know how much food is required to keep our body healthy and active. In order to calculate this, we need to know  : i)  energy value of different nutrients and ii) factors influencing the energy requirement of an  individual.