The respiratory quotient value indicates which macronutrients are being metabolized, as different energy pathways are used for fats, carbohydrates, and proteins.
Measuring this ratio is equal to RQ only at rest or during mild to moderate aerobic exercise without the accumulation of lactate.
For oxidation of a fatty acid molecule, namely palmitic acid:[6] A RQ near 0.7 indicates that fat is the predominant fuel source, a value of 1.0 is indicative of carbohydrate being the predominant fuel source, and a value between 0.7 and 1.0 suggests a mix of both fat and carbohydrate.
In general, molecules that are more oxidized (e.g., glucose) require less oxygen to be fully metabolized and, therefore, have higher respiratory quotients.
Conversely, molecules that are less oxidized (e.g., fatty acids) require more oxygen for their complete metabolism and have lower respiratory quotients.
However, 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.
By increasing the proportion of fats in the diet, the respiratory quotient is driven down, causing a relative decrease in the amount of CO2 produced.
Underfeeding, which forces the body to utilize fat stores, will lower the respiratory quotient, while overfeeding, which causes lipogenesis, will increase it.
In patients with liver cirrhosis, non-protein respiratory quotient (npRQ) values act as good indicators in the prediction of overall survival rate.
Experimental studies with natural bacterioplankton using different single substrates suggested that RQ is linked to the elemental composition of the respired compounds.
[12] Based on the stoichiometry of the different metabolized substrates, the scientists can predict that dissolved oxygen (O2) and carbon dioxide (CO2) in aquatic ecosystems should covary inversely due to the processing of photosynthesis and respiration.
[13] Using this quotient we could shed light on the metabolic behavior and the simultaneous roles of chemical and physical forcing that shape the biogeochemistry of aquatic ecosystems.
Field measurements of the concurrent consumption of oxygen (-ΔO2) and production of carbon dioxide (ΔCO2) can be used to derive an apparent respiratory quotient (ARQ).