The Respiratory Quotient (RQ) is a unitless number which is a ratio of the carbon dioxide produced (CO ₂ pro) during tissue metabolism to the oxygen consumed (O ₂ con).  RQ reflects net substrate oxidation and can also be determined by indirect calorimetry.

 
RQ = CO ₂ pro/O ₂ con

The units for CO ₂ pro and O ₂ con should be identical and thus cancel each other out.  Typical units are ml/min.  For example, if 182 mL/min of CO ₂ is produced and 220 mL/min O ₂ is consumed, the RQ=182/220=0.827.

Clinically, RQ has been used to examine the mixture of energy sources being utilized in active metabolism.  Additional findings include:

• In one study, a higher RQ with the patient on their normal diet, may significantly predict weight gain over time in non-obese women
• There is a familial (inherited) nature in which foods are metabolized that is not explained by non-genetic factors
• Late vs. early gestation does not seem to alter normal increases in RQ that occur after consuming a meal 

Results can be between 0.7 and 1.0, which depend upon the substrate being used for energy.  If only carbohydrate is utilized, then RQ=1.0, however if only fats are being oxidized, the RQ=0.7.  For proteins, as different amino acids are being utilized, the average RQ=0.8.  In any given individual the  value will be between 0.7 and 1.0 depending upon the mixture of these 3 energy sources that are actually being utilized.

References

Marra M, et al. Fasting Respiratory Quotient as a Predictor of Long-Term Weight Changes in Non-Obese Women. Ann Nutr Metab. 2004;48:189-92.

Nagy LE, King JC. Postprandial energy expenditure and respiratory quotient during early and late pregnancy.

Toubro S, et al. Twenty-Four-Hour Respiratory Quoteient: The Role of Diet and Familial Resemblance. J Clin Endocrinol Metab. 1998;83(8):2758-64.