Difference Between Direct Calorimetry and Indirect Calorimetry (With Table)

Calculation of heat produced or involved by a body are essential parameters to study calorimetry. The heat produced could be through reactions or changes which can be physical as well as chemical. Under calorimetry, there are major two ways to calculate heat – through direct calorimetry or indirect calorimetry.

Direct Calorimetry vs Indirect Calorimetry

The main difference between direct calorimetry and indirect calorimetry is direct calorimetry is a method to measure heat by quantifying metabolic rates under a controlled environment while indirect calorimetry is a method to measure heat by measuring metabolic fuels that are combusted.

Direct calorimetry is a traditional method for calculating heat in calorimetry. It was used since the 19th century. It is an expensive technique. It requires several appropriate and controlled conditions. The observation of direct calorimetry takes place inside a calorimeter. The subject needs to be present during the study.

While indirect calorimetry was lead by the combination of direct calorimetry. It produces precise and accurate results. The principle behind the working of the indirect calorimetry is that it measures the values of the rate of respiratory gases that is oxygen and carbon dioxide during inspiration and expiration. It is a cost-effective process and does not require controlled conditions.

Comparison Table Between Direct Calorimetry and Indirect Calorimetry

Parameters of ComparisonDirect CalorimetryDirect Calorimetry
Aim To measure the actual heat produced from a body under stipulated conditions To measure values of inspiration and expiration
ConditionsThe heat produced is observed under thermally sealed chambersThe heat produced is observed through respiratory gases under open conditions
AccuracyGives approximate results Gives accurate results as the calculation procedures are precise
Price It is an expensive techniqueIt is less expensive than Direct Calorimetry
ApplicationTo calculate the basal metabolic rate (BMR) to evaluate organs like brain, kidney, liver, and heart Design nutritional regimens, hypermetabolic stress, evaluate states of organ failure, in surgical suites, and determine burn unit

What is Direct Calorimetry?

Direct calorimetry is the procedure followed to measure the amount of heat generated in the process, in the body undergoing activity. The energy expenditure is calculated in the form of heat. The subject or body under measurement must be placed in a particular environment. These specific confined environments are called whole-room calorimeters.

Direct calorimetry is a widely used technique for measuring the heat produced in the body. The thermal unit Kcal (kilocalorie) is used. The direct calorimetry method is used to calculate heat flow in humans, animals especially mammals, fishes and even to determine the energy content of food items. Direct calorimetry applies the law of conservation of energy and has paved the way for the development of indirect calorimetry.

Direct calorimetry is an expensive process to calculate heat energy and estimate energy production. It includes methods like complex engineering and appropriate conditions like air-tight and insulated chambers. The technique does not provide knowledge about the nature of substrate oxidized. There are even several operational difficulties.

Direct calorimetry has formed the basis of metabolism in health and diseases. It was the standard technique to measure the human metabolic rate. Direct calorimetry helps in the quantification of aerobic and anaerobic metabolism produced by heat energy during an exchange between the body and the environment.

What is Indirect Calorimetry?

Indirect calorimetry is a technique to calculate heat energy produced by living organisms by evaluating the production of carbon dioxide and nitrogen waste or consumption of oxygen. It is an efficient method for the estimation of the type of substrate, energy metabolism, and the rate of the substrate. It is a reliable tool for assessing energy expenditure.

The indirect calorimetry technique is used to determine and evaluate aspects like thermogenesis, the pathogenesis of the metabolic disease, assimilation of nutrients, and the energetics during physical exercise. It is used in disciplines like measuring physical fitness, metabolic requirements, in medical and surgical suites and units, and nutrition therapy.

There are four major techniques for gas collection under indirect calorimetry – Douglas bag, canopy, face mask, and interface with a ventilator. It has two energy expenditure units – Resting Energy Expenditure (REE) and Respiratory Quotient (RQ). Indirect calorimetry is an inexpensive technique and requires minimum instrumentation.

Indirect calorimetry is also used in several tests like diuretics, urinary output, and capillary leak syndrome. Indirect calorimetry works on the principle of considering the entire organism as an ideal system or compartment of living cells, which is floating on an extracellular fluid and has constant blood renewal. The data obtained from indirect calorimetry is detailed and has practical application.

Main Differences Between Direct Calorimetry and Indirect Calorimetry

  1. Direct calorimetry does not provide information about the type and rate of a substrate while indirect calorimetry provides information about the type and rate of the substrate.
  2. Direct calorimetry is an expensive method and requires numerous instruments while the indirect method is less costly and involves less number of instruments.
  3. Direct calorimetry uses direct observation of a calorimeter while indirect calorimetry measures consumption of oxygen and production of carbon dioxide.
  4. Direct calorimetry provides approximate answers with long procedures while indirect calorimetry provides accurate and precise results under feasible conditions.
  5. Direct calorimetry measures within insulated chambers while indirect calorimetry measures in open chambers.

Conclusion

Calorimetry is a crucial field for the study of biological analysis. Calorimetry can measure precise small energy changes. The main role of calorimetry is to measure chemical and biochemical reactions and changes. It gives insights into the estimation of metabolic rate. The principles of calorimetry are useful in various fields and domains. Calorimetry is the basis of thermodynamics.

Calorimetry has controlled conditions for observation to make sure no heat is lost during the reaction. The slightest loss could lead to drastic variations and alterations in the result. The calibrated instrument used in calorimetry is called a calorimeter.

The two main methods of heat evaluation are direct calorimetry and indirect calorimetry. Both methods have distinct aims, procedures, and applications. Depending upon the need and requirement of the field or experiment, the following methods are applied.

References

  1. https://www.sciencedirect.com/science/article/pii/004060319401963H
  2. https://academic.oup.com/ajcn/article-abstract/33/6/1287/4692699
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