It can be assessed indirectly by recording the uptake of oxygen that is necessary for the oxidation of nutrients. When I litre of oxygen is consumed in the human body there is, on average, a turnover of 20W (5 kcal) of energy. Fortunately, this single conversion factor applies whether carbohydrates, fats, proteins or alcohol provide the original chemical energy. If one wants to know which is actually combusted, one must also measure the volume of carbondioxide in the exhaled air. The ratio between the volumes of co2 and o2 is called the respiratory quotient (RO). The RO is as follows: