Respiratory Quotient, commonly abbreviated as R.Q, is a physiological index used to describe the relationship between oxygen consumption and carbon dioxide production during metabolism. It provides important insight into which type of nutrient (carbohydrate, fat, or protein) is being oxidized for energy in the body.

Respiratory quotient is denoted by R.Q. It is the ratio of volume of CO₂ produced (expired) to the volume of O₂ consumed (inspired); this ratio can be represented as below:

RQ = Volume of CO₂ produced / Volume of O₂ consumed

In more modern physiological terms, this is often interpreted as:

RQ = VCO₂ / VO₂

where VCO₂ is the rate of carbon dioxide production and VO₂ is the rate of oxygen consumption.

Under steady metabolic conditions, R.Q is independent of the total metabolic rate and depends mainly on the type of substrate being oxidized. This means that whether a person is resting or exercising at a constant intensity, the RQ primarily reflects whether carbohydrates, fats, or proteins are being used as the main energy source rather than how much total energy is being expended.

  RQ of Major Nutrients

1. Carbohydrates

For complete oxidation of carbohydrate, R.Q is 1 because in the oxidation of glucose, which is represented by the equation:

C₆H₁₂O₆ + 6O₂ ----------------> 6CO₂ + 6H₂O

The CO₂ produced is equal to O₂ utilized. Therefore, R.Q of glucose combustion is 1.0.

From a physiological perspective, this indicates a one-to-one exchange between oxygen uptake and carbon dioxide release, which is characteristic of pure carbohydrate metabolism.

2. Fats

For fats, e.g. triolein, the oxidation is represented by the following equation:

C₅₇H₁₀₄O₆ + 80 O₂ --------------------> 57 CO₂ + 52 H₂O

R.Q for this fat is therefore:

57 / 80 = 0.71

Other fats also have R.Q near 0.70.

This lower value reflects the fact that fat metabolism consumes relatively more oxygen compared to the amount of carbon dioxide produced, due to the highly reduced nature of fatty acids.

3. Alcohol

Alcohol sometimes is responsible for providing energy to man. Its R.Q is 0.67 as is clear from the following equation:

C₂H₅OH + 3O₂ --------------------------> 2CO₂ + 3H₂O

Here, oxygen consumption is proportionally higher relative to carbon dioxide production, resulting in a lower RQ value.

4. Proteins

The determination of R.Q of protein is more difficult because protein contains N and S in addition to C, H, and O. However, using specialized methods, the R.Q of protein has been found to be 0.80.

This intermediate value reflects the mixed metabolic pathways involved in amino acid oxidation and the additional biochemical steps required for nitrogen disposal.

  Mixed Diet and Physiological Variations in RQ

Under normal circumstances an organism does not utilize only one type of foodstuff as its source of energy but obtains its energy from combustion of all three foodstuffs, i.e. carbohydrates, fats, and proteins. Thus the R.Q of an intact animal will be the average of the R.Q of all these 3 types of foodstuffs.

It has been found that on an ordinary mixed diet, the R.Q is 0.85.

In the post-absorptive state, i.e. when the person has not ingested any food article in the last 14 hours, the R.Q is usually 0.82. This reflects a relative shift toward fat utilization once readily available dietary glucose has been used.

An excess of carbohydrates in the diet tends to raise the R.Q, as carbohydrate oxidation predominates.

In contrast, greater fat oxidation (for example, during high-fat intake or starvation) leads to a fall in the R.Q.

  Pathological and Metabolic Deviations

Under pathological conditions, R.Q may exceed 1.0, particularly in states of excessive lipogenesis (conversion of carbohydrates into fats), because more CO₂ is produced relative to O₂ consumption.

In prolonged starvation or uncontrolled diabetes mellitus, R.Q may fall closer to 0.7 due to predominant fat utilization. In such conditions, the body relies heavily on fat stores and ketone metabolism, which lowers CO₂ output relative to oxygen consumption.

  Methods of Determining R.Q in Human Being

There are two methods of determining R.Q in man, which are the open circuit method and closed circuit method. In both of these methods, the amounts of O₂ consumed and CO₂ evolved by a person over a given length of time are to be found out first.

Oxygen consumption (VO₂) and carbon dioxide production (VCO₂) are commonly measured per unit time and are used to assess metabolic fuel utilization in resting and exercising states. These measurements form the basis of indirect calorimetry.

1. Open Circuit Method

In the open circuit method, the subject is made to breathe into a Douglas bag for a few minutes. The volume of air breathed is measured in a gas meter and a sample is analyzed for the O₂ and CO₂ concentrations.

For this purpose, Haldane gas analysis apparatus is used. In this apparatus, a known volume of the gas sample is first treated with KOH solution. CO₂ is absorbed by KOH, resulting in a corresponding decrease in the original volume of the gas sample being analyzed. From this, the concentration of CO₂ in the expired air is found out.

Later, the remaining gas is made to react with alkaline pyrogallate (pyrogallic acid in KOH), which absorbs O₂; from this, the concentration of O₂ in the expired air is found out.

Because the subject inhales atmospheric air whose composition is known (practically constant), it is a quite easy way to find out the amount of O₂ used and CO₂ given off in the experiment.

From these values, R.Q is calculated by the formula:

R.Q = Volume of CO₂ exhaled / Volume of O₂ utilized

This method is widely used in indirect calorimetry studies for measuring energy expenditure in humans, particularly in physiological and clinical research settings.

2. Closed Circuit Method

In the closed circuit method, a spirometer is filled with O₂ for inhalation by the subject. The subject inhales from and then exhales into the same apparatus.

The expired gases are made to pass over a concentrated solution of NaOH, which absorbs all CO₂ present in these gases. As the subject continues breathing from and into the spirometer, the amount of O₂ in the spirometer falls, resulting in a fall in the spirometer volume.

The decrease in the volume of gas in the spirometer is automatically recorded on a calibrated paper which is wound on a drum rotating at a prefixed speed. The fall in the volume of the spirometer gives the volume of O₂ consumed.

For the determination of CO₂, it is liberated from the absorbent by adding H₂SO₄ to it. The reactions which take place are given below:

2NaOH + CO₂ -------------------------> Na₂CO₃ + H₂O (Absorption of CO₂)
Na₂CO₃ + H₂SO₄ --------------------> Na₂SO₄ + H₂O + CO₂ (Release of CO₂)

The CO₂ released from the absorbent is made to enter the spirometer, which shows an increase in its volume. This increase in the volume of the spirometer gives the volume of CO₂ exhaled by the subject.

From the values of O₂ consumed and CO₂ liberated, and putting these values in the formula of R.Q:

R.Q = Volume of CO₂ exhaled / Volume of O₂ utilized

R.Q can then be determined accurately.

This method is generally more suitable for controlled laboratory measurements and was historically used in early respiratory physiology studies. In modern practice, it has largely been replaced by electronic metabolic carts, which provide continuous, highly precise measurements of respiratory gases with improved accuracy and convenience.