Step-by-step explanation:
Metabollic rate is the amount of energy expended from an organism at a given point in time.
Equation of how glucose (food) is broken down via respiration:
C6H12O6(s) + 6O2(g) --> 6H2O(l) + 6CO2(g)
The amount of energy produced for the types of food is roughly proportional to the amount of oxygen use, so that the metabolic rate can be measured by measuring the rate of oxygen consumption.