The respiratory quotient is a very useful parameter to evaluate the metabolic mixture used at rest or during a physical exercise. Due to the chemical differences that characterize them, the complete metabolization of fats, proteins and carbohydrates requires different amounts of oxygen. Consequently the type of oxidized energy substrate will also affect the quantity of carbon dioxide produced.
QR = CO 2 produced / O 2 consumed
Considering that each macronutrient has a specific QR, by evaluating this parameter it is possible to trace the nutrient mixture metabolized at rest or during a specific working activity.
Respiratory quotient of carbohydrates
The generic molecular formula of a carbohydrate is Cn (H 2 O) n. It follows that within a carbohydrate molecule the proportion between the number of hydrogen atoms and those of oxygen is fixed and equal to 2: 1. To oxidize a generic hexose (carbohydrate with six carbon atoms such as glucose) six oxygen molecules will therefore be required, with the consequent formation of 6 molecules of carbon dioxide (C 6 H 12 0 6 + 60 2 → 6H 2 0 + 6C0 2 ) .
The respiratory quotient of carbohydrates will therefore be equal to: 6CO 2 / 6O 2 = 1.00
Respiratory quotient of lipids
Lipids are distinguished from carbohydrates by their lower oxygen content in proportion to the number of hydrogen atoms. Consequently their oxidation requires a higher amount of oxygen.
Taking palmitic acid as an example, we discover that during its oxidation, 16 molecules of carbon dioxide and water are formed for 23 molecules of oxygen consumed. C 16 H 32 O 2 + 23 O 2 → 16 CO 2 + 16 H 2 O
The respiratory quotient will therefore be equal to: 16 CO 2/23 O 2 = 0.696
Normally lipids are attributed a respiratory quotient equal to 0.7, bearing in mind that this value ranges from 0.69 to 0.73 in relation to the length of the carbon chain which characterizes the fatty acid.
Respiratory quotient of proteins
The main difference that distinguishes proteins from fats and carbohydrates is the presence of nitrogen atoms. Because of this chemical difference, protein molecules follow a particular metabolic pathway. The liver must first eliminate nitrogen through a process called deamination. Only then can the remaining part of the amino acid molecule (called ketoacid) oxidize to carbon dioxide and water.
Like lipids, keto acids are also relatively poor in oxygen. Their oxidation will therefore lead to the formation of a quantity of carbon dioxide lower than that of oxygen consumed.
Albumin, the most abundant protein in plasma, oxidizes according to the following reaction:
C 72 H 112 N 2 O 22 S + 77O 2 → 63CO 2 + 38 H 2 O + SO 3 + 9 CO (NH 2 ) 2
The respiratory quotient will therefore be equal to: 63 CO 2/77 O 2 = 0.818
The protein QR is fixed by convention at 0.82 .
Meaning of the respiratory quotient
To meet the body's energy demands, each of us uses different metabolic mixtures in relation to physical effort. The more intense this is, the greater the percentage of oxidized glucose. A large part of the energy produced at rest derives from the metabolization of fatty acids. For this reason it is legitimate to expect a respiratory quotient close to 0.7 at rest and higher during intense exercise.
Performing activities ranging from absolute rest to light aerobic exercise, the respiratory quotient is around 0.82 ± 4%. This data, obtained experimentally, testifies to the organism's oxidation of a mixture composed of 60% fat and 40% carbohydrates (in resting or moderate physical activity the protein's energetic role is negligible, we therefore speak of a non-protein respiratory quotient).
Each value of QR corresponds to a caloric equivalent of the oxygen that represents the number of calories released per liter of O2. Thanks to this data it is possible to trace the energy expenditure of a working activity with great precision. We hypothesize that during a moderate aerobic exercise the respiratory quotient, measured by gas analysis, is equal to 0.86; consulting a specific table we find that the energy equivalent per liter of oxygen consumed is 4, 875 Kcal. At this point, to discover the energy expenditure of the exercise, it will be sufficient to multiply the liters of oxygen consumed by 4.875.
During an intense physical effort the situation changes radically and the respiratory quotient undergoes large variations. Due to the massive production of lactic acid, numerous auxiliary metabolic mechanisms are activated, such as buffer systems and hyperventilation. In both cases there is an increase in the elimination of CO2, independent of the oxidation of the energy substrates. Increasing the data present in the numerator (CO2) and keeping the denominator constant (O2) the respiratory quotient undergoes a surge reaching values higher than unity.
During recovery after intense activity, when a part of carbon dioxide is used to reform bicarbonate reserves, the respiratory quotient, on the other hand, falls below the limit value 0.70.
It is therefore clear that in such situations the respiratory quotient does not reflect exactly what happens at the cellular level during the oxidation of the energy substrates. In these cases respiration physiologists prefer to talk about external respiratory quotient or relationship between respiratory exchanges (R).
