A person's energy requirement is the amount of energy he or she needs to maintain health, growth, and a level of activity. A person's energy expenditure is the amount of energy he or she uses, and the measure of this can be used to determine the person's energy requirements. Energy expenditure can be calculated either with an equation or by direct measurement.

Calculation of Energy

Basal metabolism is the energy expended in the cellular processes necessary to the maintenance of life. The basal metabolic rate (BMR) is measured in Calories and is primarily accounted for by the activity of the brain, heart, liver, and kidneys. The Harris-Benedict equations are commonly used for calculation of the BMR in adults.

BMR for men (kcal) = 66 + 13.7(weight in kg) + 5(height in cm) - 6.8(age in years)

BMR for women (kcal) = 655 + 9.6(weight in kg) + 1.85(height in cm) - 4.7(age in years)

Measurement of Energy Expenditure

Direct Calorimetry

Direct calorimetry measures the heat production of an individual, in calories, when placed in an insulated chamber where the heat is transferred to surrounding water. This is a very accurate method of measuring energy expenditure.

Indirect Calorimetry

Indirect calorimetry measures respiratory gas exchange from which energy expenditure can be estimated. Oxygen consumption (VO2) and carbon dioxide (CO2) production occur during the oxidation of carbohydrate, protein, and fat. Heat production can be calculated from a measurement of VO2 and/or CO2.

Doubly-Labeled Water

Doubly-labeled water contains isotopes of oxygen and of hydrogen. The technique involves the consumption by the patient of the doubly-labeled water then the measurement of the amount of isotope in the person after a washout period of 7- 14 days. Most of the hydrogen and oxygen isotopes are lost through excretion and evaporation, but some of the oxygen isotope equilibrates with carbon dioxide and is expired in air. The amount of CO2 produced can be used to figure heat production.

Heart-Rate Monitoring

Measurement of heart rate can be used to estimate energy expenditure because there is a strong relationship between heart rate and oxygen consumption during activity. The relationship differs depending on whether a person is at rest or active. The relationship is calibrated for each individual.