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.
