Respiratory Quotient

 Respiratory Quotient

Definition

It is the ratio of the volume of CO₂ produced by the volume of O₂ consumed (CO₂/O₂) during a given time.

RQ is simply a ratio not related to actual amount of gases or gaseous exchange. Normal RQ for a mixed diet is 0.85. It may differ with the change of diet.

Factors affecting RQ-

1.     Diet-

A.   Carbohydrate- In case of carbohydrate the RQ is 1.0. It is because in oxidation of Carbohydrate the amount of O2 consumed is same as CO2 produced as is evident from following equation-

C₆H₁₂O₆+6O₂=6CO₂+6H₂O

                                       (Glucose)

i.e.  R.Q. =6CO₂/6O₂= 1

B.   Proteins- In case of proteins RQ will be 0.8. as seen from the following equation-

2C₃H₇O₂N+6O₂=(NH₂)2CO+5CO₂+5H₂O

                           (Alanin)

i.e. R.Q. =5CO₂/6O₂= 0.8

C.   Fats- In case of fats the RQ will be 0.7 and the lowest of all three molecules because fats is an Oxygen poor compound and it uses external O2 to oxidize C and H. Peruse the following reaction-

2C₃H₅(C₁₈H₃₅O₂)₃+163O₂=114CO₂+110H₂O

                           (Tristearin)

i.e. R.Q. =114CO₂/163O₂= 0.7

2.     Energy Balance- When carbohydrates are converted into fats in the body, RQ will rise because an oxygen poor compound will form from an oxygen rich compound. So more oxygen will be available for oxidation and oxygen from outside will not be needed. Opposite will be the case when Fats are converted into carbohydrate. Thus energy balance will be maintained.

3.     Acidosis- During acidosis CO2 output is greater than O2utilization so the RQ is increased.

4.     Alkalosis- When respiration is depressed less CO2 is produced and retained in the body, hence RQ will fall.

5.     Body Temperature- If body temperature is increased as in fever, rate of respiration will increase and more CO2 will be washed out of bodyresulting in increase in RQ.

6.     Ketosis- It is the condition where little carbohydrate is available in the body for metabolism as in ketogenic diet, starvation or advanced stage of diabetes mellitus. Then body is forced to use its fat reserves for energy supply hence RQ will fall.

7.     Hyperpnoea- Whether voluntary or due to a disease Hyperpnoea, meaning increased rate of breathing, can wash out excess CO2 without change in O2 utilized in oxidation. Hence RQ will above 1.

8.     Exercise- No change will be seen with moderate exercise but with heavy exercise lactic acid is produced and causes temporary acidosis resulting in hyperpnoea and RQ above 1.

Significance of RQ-

1.     It signifies the nature of metabolism and type of food being oxidized.

2.     It is helpful to determine metabolic rate at a given time.

3.     It is helpful to diagnose the pathological conditions with acidosis or alkalosis.