Respiratory Quotient

When foodstuffs are oxidized, oxygen is consumed and carbon dioxide is produced. The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed is called the respiratory quotient (R.Q.); its numerical value varies according to the type of food substance which is being oxidized. Thus, in the oxidation of glucose, represented by the reaction C6H12o6 + 6O2 → 6CO2 + 6H2O, 6 molecules of oxygen are consumed and 6 molecules of carbon dioxide produced, for each molecule of glucose burned. In this case, the ratio =1.00. A similar type of calculation yields values of 0.71 for the R.Q. of fat and of 0.80 for protein. In practice, the expired air of the subject is collected for a given period and the percentage of oxygen and of carbon dioxide determined by analysis. Knowing the composition of the inspired air and the volume and composition of the expired air, the volumes of oxygen consumed and of carbon dioxide produced (and hence the R.Q.) are easily calculated. The grams of protein oxidized are calculated from the urinary nitrogen (1 gram of urinary nitrogen reprethe metabolism of 6.25 grams of protein). The total R.Q. is corrected for protein metabolism by subtracting the oxygen consumed and carbon dioxide produced in the oxidation of protein from the total oxygen consumed and carbon dioxide produced. The resulting ratio of is called the "non-protein" R.Q.; from it the relative proportions of fat and of carbohydrate oxidized are obtained from standard tables. Under resting conditions the R.Q. gives a fairly reliable indication of the relative proportions of carbohydrate, fat, and protein which are being oxidized, although any considerable degree of interconversion of the major foodstuffs will seriously affect the ratio. The respiratory quotient is a reliable index of the fuel used in exercise only when the entire recovery period is included in the study. The reason for this is found in certain of the chemical reactions which accompany exercise and recovery. During moderate to strenuous exercise, more lactic acid is formed in the contracting muscles than can be neutralized by the muscle buffers. Much of the excess lactic acid diffuses into the blood stream where it is buffered by the various blood buffers of which the most abundant is sodium bicarbonate (NaHCO3) according to the reactions:

HL + NaHCO3 → NaL + H2CO3 (L represents the lactate ion)

H2CO3 → C02 + H20

As a result of this reaction, a large amount of carbon dioxide is eliminated in the expired air which did not result directly from oxidation and which, therefore, did not involve a corresponding consumption of oxygen. The ratio will be elevated and is frequently greater than 1.0 (R.Q.'s as high as 2.0 may be obtained during very strenuous exercise).

During recovery the lactate produced during exercise is gradually removed by oxidation, urinary excretion, and reconversion to glycogen. As a result the alkali with which this lactate had been combined is available to combine with carbon dioxide once more, so that much of the carbon dioxide produced by oxidations during the recovery period will be retained in the blood instead of being eliminated in the expired air. Consequently, the ratio will be depressed, and R.Q.'s as low as 0.5 may be obtained. This ratio returns to normal when recovery from exercise is complete. If, however, the expired air is collected from the beginning of exercise to the end of tile recovery period, the excess CO2 elimination during exercise is balanced by a corresponding amount of CO: retention during recovery and the correct R.Q. may be calculated. In practice it is often difficult to determine accurately when recovery has been completed, since the oxygen consumption may remain above the preexercise level for some hours. The adoption of an arbitrary time period of recovery is sometimes necessary, and probably results in only minor inaccuracies in the final calculations.