All metabolic processes in the body require energy supply (food) but depend on the use of oxygen (O₂) for extraction of energy which ultimately results in heat production.

There are two approaches to quantify human energy expenditure during rest and physical activity. They are:

1. direct calorimetry and
2. indirect calorimetry.

Direct calorimetry is the method of directly quantifying the body’s rate of biologic work (energy expenditure/metabolism) by measuring body’s heat production.

Indirect calorimetry is the method of indirectly quantifying the body’s rate of biologic work by measuring body’s O₂ use.

When we say ‘burn the calories’, what do we really mean? Studies show that when we burn a food mixture (varying combinations of proteins, fats, carbohydrates) with one litre of O₂ in a bomb calorimeter, there is a release of approximately 4.82 kCal.

The steady rate conditions between O₂ consumption and energy release while burning varying mixture of food constituents provide us with a reliable method (measuring O₂ uptake) to quantify biologic work during rest and physical activity.

There are two methods to perform indirect calorimetry:

1. closed circuit spirometry and
2. open circuit spirometry.

In closed circuit spirometry, the subject rebreathes from a spirometer filled with 100% O₂. When the subject continues breathing (in and out from the spirometer) for a pre-determined time interval, the amount of O₂ in the spirometer reduces as some O₂ is used by the subject’s body. The total oxygen consumption can be calculated by subtracting the initial amount of O₂ (100%) in the spirometer to the final amount of O₂ at the end of the pre-determined time interval.

In open circuit spirometry, the subject breathes ambient air with a constant composition. This method involves measuring the total volume of air breathed during a specified time interval, and measuring the O₂ / CO₂ concentrations in the expired air during rest or physical activity for the same time interval. The total O₂ consumption can be calculated by determining the changes in O₂ / CO₂ concentrations in expired air compared to ambient air.

The three common techniques of measuring O₂ consumption using open circuit spirometry method are:

1. classic bag technique,
2. portable breath by breath gas analysis technique and
3. portable gas analysis technique.

Respiratory Quotient: When we burn food mixtures with varying combinations of proteins, carbohydrates and fats using one litre of O₂, there is a release of approximately 4.82 kCal. However, this calorific value of O₂ varies between 2 to 4% with large variations in the food constituents. We must account for this variability while determining body’s energy expenditure (heat production) through indirect calorimetry. This requires the measurement of CO₂ produced per unit of O₂ consumed with all three types of substrates (protein, carbohydrates, fats). It can be expressed as a ratio called respiratory quotient (RQ) for all three substrates.

The respiratory quotient is defined as the ratio of CO₂ produced to O₂ consumed while food is processed in the body:

RQ = CO₂ produced ÷ O₂ consumed

1. Carbohydrate metabolism:

The chemical equation for the oxidation of glucose:

C6H12O6 + 6 O2 → 6 CO2+ 6 H2O

RQ = 6 CO2 / 6 O2 = 1.0

2. Fat metabolism:

The chemical equation for the oxidation of palmitic acid is:

C16H32O2 + 23 O2 → 16 CO2 + 16 H2O

RQ = 16 CO2 / 23 O2 = 0.696

3. Protein metabolism:

The chemical equation for the oxidation of albumin is:

C72H112N18O22S + 77 O2 → 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2

RQ = 63 CO2 / 77O2 = 0.8

4. Non-protein metabolism:

RQ = 2.78 Litres of CO2 / 3.22 Litres of O2 = 0.86

As the food mixtures always have a combination of all three substrates, none of the above RQ can be assigned to the protein metabolism in the diet. A commonly used estimate is 0.8 as it is a metabolism of a mixture of 40% carbohydrates and 60% fats. We can thus apply the caloric equivalent of 4.825 kCal of O₂ for energy transformations.

References:

1. Péronnet, F. and Massicotte, D., 1991. Table of nonprotein respiratory quotient: an update. Can J Sport Sci, 16(1), pp.23-29.
2. Zuntz, N. and Schumburg, W.A.E.F., 1901. Studien zu einer Physiologie des Marsches (Vol. 6). Hirschwald.
3. Lusk, G., 1924. Animal calorimetry. Analysis of the oxidation of mixtures of carbohydrate and fat. A correction. J. Bio. Chemi., 59:41-42.
4. Richardson, H.B., 1929. The respiratory quotient. Physiological Reviews, 9(1), pp.61-125.
5. American College of Sports Medicine, 2013. ACSM’s guidelines for exercise testing and prescription. Lippincott Williams & Wilkins.