Units of Energy


Energy Sources Introduction

There are three sources of energy for the body fats, carbohydrates and proteins.The carbohydrates and fats are the main source while protein is secondary. Also carbohydrates are stored in the form of glycogen stores in muscles and fats are stored around the body. The energy received from these sources is measured in calories and because these units are very small. The values for each source differs with 1 gram of fat = 9.0 kcal = 38 kJ, 1 gram of carbohydrate = 4.0 kcal = 17kJ and 1 gram of protein = 4.0 kcal = 17 kJ.

Energy Expenditure

  1. Energy is needed by the body to stay alive, grow, keep warm and move around.
  2. Energy is provided by food and drink. It comes from the fat, carbohydrate, protein and alcohol the diet contains.
  3. Energy requirements vary from one individual to the next, depending on factors such as age, sex, body composition and physical activity level.
  4. Energy expenditure is the sum of the basal metabolic rate (the amount of energy expended while at complete rest), the thermic effect of food (TEF, the energy required to digest and absorb food) and the energy expended in physical activity.
  5. To maintain body weight, it is necessary to balance the energy derived from food with that expended in physical activity. To lose weight, energy expenditure must exceed intake, and to gain weight, energy intake must exceed expenditure.

Energy Expenditure Measurements

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.

Basal metalobism is the energy expended in the cellular processes necessary to the maintenance of life. The basal metabolic rate 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) + 1.85(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)

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.

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.