Respiratory quotient: Energy sources

The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed while food is being metabolized:

RQ = CO_{2 eliminated}/O_{2 consumed}

Most energy sources are food containing carbon, hydrogen, and oxygen. Examples include fat, carbohydrates, protein, and ethanol. Their metabolism is represented by the equation:

C_xH_yO_z + (x + y/4 – z/2) O₂ → x CO₂ + (y/2) H₂O

The normal range of respiratory coefficients for organisms in metabolic balance usually ranges from 1.0 – 0.7 and examples are calculated below:

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.