Determining Metabolic Rate

It is the output of energy by a person which is expressed in Calories Per square meter body surface area per hour. The body obtains it energy by the oxidation of food. Because the oxidation of food results in the production of heat, the energy output. In other words metabolic rate of an individual may be found out by measuring his heat production over a known period of time.

   Determination of Metabolic Rate

There are two methods to determine the metabolic rate of a person:

Direct Calorimetry

The subject is placed in an insulated chamber called Atwater-Rosa-Benedict Calorimeter. He is instructed to do the type of activity for which the metabolic rate has to be measure. Water is allowed to flow through the chamber at a certain rate and the heat lost by the subject is used to raise the temperature of this water. The temperature of water on entering as well as leaving the chamber is noted. In this way the heat given off by the subject during a known period of time can be found out. It is also obvious that the subject loses heat also by evaporation of water from skin and respiratory tract. These water vapors are taken up by a suitable chemical absorbent and their mass is measure at the end of the experiment. Because one gram of evaporated water represents a loss of 0.58 Cal, the total amount of heat lost by this route can be found out from the mass of water vapors given off by the subject. The number of Cals thus obtained is added to the amount of heat calculated from the rise in the temperature of the water flowing through the chamber. From the total of Cals lost by the subject over the known period of time one can easily find out the number of Cals lost per hour. If the surface area of the individual is known, then metabolic rate, i.e. Cals given off per square meter surface area per hour can be determined. The value thus obtained will represent metabolic rate for the particular activity performed by the subject during the experiment.

Indirect Calorimetry

In this method, the respiratory quotient and the rate O2 utilization of a subject is determined first. The energy output can be easily calculated if these two values are known. The energy expenditure of a person 70 Kg during various forms of activity in one hour is given below.
Sitting at rest                         --------------------                  100 Cals
Typewriting                             --------------------                  140 Cals
Walking slowly                       --------------------                 200 Cals
Walking moderately fast  -------------------                   300 Cals
Swimming                                  -------------------                   500 Cals
Walking very fast                  --------------------                  650 Cals
Walking Upstairs                  --------------------                  1100 Cal

Example: A person utilized 60 liters of O2 in one hour while he was walking. His Respiratory quotient during this period was 0.85. Find out his energy expenditure.
It is known that at Respiratory quotient of 0.85, 1 liter of O2 liberates 4.862 Cals. Therefore 60 liters of O2 will liberate 60 x 4.862 = 291.72 Cals. Thus it can be said that this person utilized 291.72 Cals in 1 hour while he was walking.