Direct and Indirect Calorimetry

Direct calorimetry measures the amount of body heat released by a person. The subject is put into an insulated chamber, often the size of a small bedroom, and body heat released raises the temperature of a layer of water surrounding the chamber. A kcal, as you recall, is related to the amount of heat available to raise the temperature of the water. By measuring the water temperature in the direct calorimeter before and after the body releases heat, scientists can determine the energy expended. This method resembles the bomb calorimeter method for measuring the energy content in food.

Direct calorimetry works because almost all the energy used by the body eventually leaves as heat. However, few studies use direct calorimetry, mostly because of its expense and complexity.

For indirect calorimetry, instead of measuring heat output, the most commonly used method measures the amount of oxygen a person uses. A predictable relationship exists between the body’s use of energy and oxygen. For example, when metabolizing a mixed diet of carbohydrate, fat, and protein-a typical blend of nutrients the human body needs 1 litter of oxygen to yield about 4.95 kcal of energy.

Instruments used to measure oxygen consumption for indirect calorimetry have great versatility. They can be mounted on carts and rolled up to a hospital bed or carried in backpacks while a person plays tennis or jogs. There are even new hand held instruments. Tables showing energy demands of exercises rely on information gained from indirect calorimetry studies.

Another approach to indirect calorimetry uses stable isotopes of oxygen and hydrogen. In this method, a person consumes isotopically labeled water. A technician measures the 2H2o and the H2 18O later that arises in body fluids, such as urine. Using the difference between the decline in the amount of 2H2O compared to H2 18O over a week or so and some mathematical formulas, total carbon dioxide (CO2) output per day can be estimated. This method works because 2H diffuses throughout the body’s water and the 18O diffuses throughout both the body water and bicarbonate (HCO3) stores. 2H is then only eliminated from the body via water production, while the 18O is eliminated both as water and carbon dioxide originally associated with bicarbonate. This ultimate estimate of CO2 output is used to calculate energy expenditure, just as id done with oxygen use in indirect calorimetry. 2H and 18O are stable isotopes of hydrogen and oxygen (therefore, they are nonradioactive); special instruments can measure them in body-fluids. This stable isotope method is quite accurate but also very expensive. It is the basis for setting energy needs for humans.

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