Respiratory quotient: Carbohydrates, fats and protein
The respiratory quotient (or RQ or respiratory coefficient), is a dimensionless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. It is calculated from the ratio of carbon dioxide produced by the body to oxygen consumed by the body. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry. It is measured using a respirometer. The Respiratory Quotient value indicates which macronutrients are being metabolized, as different energy pathways are used for fats, carbohydrates, and proteins. A value of 0.7 indicates that lipids are being metabolized, 0.8 for proteins, and 1.0 for carbohydrates. The approximate respiratory quotient of a mixed diet is 0.8. Some of the other factors that may affect the respiratory quotient are energy balance, circulating insulin, and insulin sensitivity.
Carbohydrates: The respiratory quotient for carbohydrate metabolism can be demonstrated by the chemical equation for oxidation of glucose:
C6H12O6 + 6 O2 → 6 CO2+ 6 H2O
Because the gas exchange in this reaction is equal, the respiratory quotient for carbohydrates is: RQ = 6 CO2 / 6 O2 = 1.0
Fats: The chemical composition of fats differs from that of carbohydrates in that fats contain considerably fewer oxygen atoms in proportion to atoms of carbon and hydrogen. The substrate utilization of palmitic acid is:
C16H32O2 + 23 O2 → 16 CO2 + 16 H2O
Thus, the RQ for palmitic acid is approximately 0.7. RQ = 16 CO2 / 23 O2 = 0.696
Proteins: The respiratory quotient for protein metabolism can be demonstrated by the chemical equation for oxidation of albumin:
C72H112N18O22S + 77 O2 → 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2
The RQ for protein is approximately 0.8. RQ = 63 CO2/ 77O2 = 0.8
Due to the complexity of the various ways in which different amino acids can be metabolized, no single RQ can be assigned to the oxidation of protein in the diet; however, 0.8 is a frequently utilized estimate.