Energy is generated in the human body by the burning of food components. Depending on their nature, food components yield a specific energy content (calorific value), which is determined empirically. Different substrates yield different values, even within one substance group. The energy content is therefore expressed as a mean value in order to simplify matters by rounding figures.
Energy Content of Food Components
The energy content of these substances is expressed in units of kilocalories (kcal) or kiloJoules (kJ). The conversion factor from kcal to the SI unit kJ is 4.1868.
For the assessment of the energy requirement of the hospitalized patient, several methods are available. These methods are either based on the technological assessment of the energy requirement by direct or indirect calorimetry or on empirical approximate equations.
Direct Calorimetry
The human body converts about 45% of the energy content of the food intake into work and about 55% into heat. As far as work is concerned, a distinction is drawn between internal work (respiration, cell pumps, synthetic processes, etc.) and external work (muscular work in moving). Since internal work is finally also converted to heat, one can estimate the body´s energy requirement by measuring the heat given off at rest. Therefore, one measures the heat production by the temperature change produced in a medium.
This method of estimation is called direct calorimetry and is extraordinarily complicated. The equipment required for the measurements is large, immobile and expensive; during the measurements, the patient is placed in a chamber and is not readily accessible. For these reasons, direct calorimetry is rarely used in the clinical setting.
Indirect Calorimetry
Energy expenditure may also be determined by indirect calorimetry. Indirect calorimetry measures gas exchange via oxygen consumption and carbondioxide production. It allows the heat produced by oxidative processes to be determined and relies on the fact that oxidation of particular food components is associated with a specific oxygen consumption and carbondioxide production; urinary nitrogen excretion is determined as a function of protein degradation.
Besides thermogenesis and activity, indirect calorimetry takes into account all the factors of the patient´s illness, including temperature, sepsis and catabolic processes.
In the open-circuit method, the expired air is collected for volumetric measurements and is analysed for its oxygen and carbondioxide content and corrected to standard conditions. These figures are then used to calculate oxygen consumption and carbondioxide production. In the closed-circuit method, the patient is isolated from the outside air and breathes from a reservoir that contains pure oxygen. As gas is expired by the patient, carbondioxide is removed. The decrease in gas volume is directly related to the rate of oxygen consumption and therefore can be used to calculate the metabolic rate. The values obtained by indirect calorimetry reflect the actual energy requirements and the utilisation of the individual substrates.
From a technical point of view it is simpler than direct calorimetry. Indirect refers to the heat production calculation from oxygen consumption and carbondioxide production, rather than from direct measurements. If the two figures are related for a given time period, one can come to a factor named the respiratory coefficient (RQ).
RQ = VCO2 / VO2
VCO2 = expired CO2 volume per unit of time
VO2 = consumed O2 volume per unit of time
For the assessment of the energy requirement of the hospitalized patient, several methods are available. These methods are either based on the technological assessment of the energy requirement by direct or indirect calorimetry or on empirical approximate equations.
Direct Calorimetry
The human body converts about 45% of the energy content of the food intake into work and about 55% into heat. As far as work is concerned, a distinction is drawn between internal work (respiration, cell pumps, synthetic processes, etc.) and external work (muscular work in moving). Since internal work is finally also converted to heat, one can estimate the body´s energy requirement by measuring the heat given off at rest. Therefore, one measures the heat production by the temperature change produced in a medium.
This method of estimation is called direct calorimetry and is extraordinarily complicated. The equipment required for the measurements is large, immobile and expensive; during the measurements, the patient is placed in a chamber and is not readily accessible. For these reasons, direct calorimetry is rarely used in the clinical setting.
Indirect Calorimetry
Energy expenditure may also be determined by indirect calorimetry. Indirect calorimetry measures gas exchange via oxygen consumption and carbondioxide production. It allows the heat produced by oxidative processes to be determined and relies on the fact that oxidation of particular food components is associated with a specific oxygen consumption and carbondioxide production; urinary nitrogen excretion is determined as a function of protein degradation.
Besides thermogenesis and activity, indirect calorimetry takes into account all the factors of the patient´s illness, including temperature, sepsis and catabolic processes.
In the open-circuit method, the expired air is collected for volumetric measurements and is analysed for its oxygen and carbondioxide content and corrected to standard conditions. These figures are then used to calculate oxygen consumption and carbondioxide production. In the closed-circuit method, the patient is isolated from the outside air and breathes from a reservoir that contains pure oxygen. As gas is expired by the patient, carbondioxide is removed. The decrease in gas volume is directly related to the rate of oxygen consumption and therefore can be used to calculate the metabolic rate. The values obtained by indirect calorimetry reflect the actual energy requirements and the utilisation of the individual substrates.
From a technical point of view it is simpler than direct calorimetry. Indirect refers to the heat production calculation from oxygen consumption and carbondioxide production, rather than from direct measurements. If the two figures are related for a given time period, one can come to a factor named the respiratory coefficient (RQ).
RQ = VCO2 / VO2
VCO2 = expired CO2 volume per unit of time
VO2 = consumed O2 volume per unit of time
O2 Consumption, CO2 Production, Respiratory Energy Equivalent and RQ of Various Food Components
Starting from the measured O2 consumption and CO2 production, the metabolic rate (MR) can be calculated from various equations. The simplest is to assign a mean value to the respiratory energy equivalent for oxygen consumption of 4.83 kcal/ l 02.
MR (kcal/h) = 4.83 x VO2
The resulting equation has a maximum error of 8%.
If CO2 production is also considered, the equation becomes:
MR (kcal/h) = 3.9 x VO2 + 1.1 x VCO2
The oxidation of protein is associated with the excretion of nitrogen in urine. One gram of nitrogen in urine is equivalent to an oxygen consumption of 5.94 l and a CO2 production of 4.76 l. If one measures the total urinary nitrogen (UN) per unit of time, the metabolism can be corrected for the fraction of oxidised protein. In Weir´s equation this (v. Weir, 1949)fraction is estimated at 12.5%:
MR (kcal/h) = 3.941 x VO2 + 1.106 x VCO2 - 2.17 x UN
The fraction of the various foodstuffs used as an energy source can be calculated from the total urinary nitrogen (UN in g), the oxygen consumption (VO2 in l) and CO2 production (VCO2 in l) by the following equations (Wilmore, 1977):
Protein oxidation (g) = 6.25 x UN
Carbohydrate oxidation (g) = -(2.56 x UN) - (2.91 ( VO2) + (4.12 x VCO2)
Fat oxidation (g) = -(1.94 x UN) + (1.69 ( VO2) - (1.65 x VCO2).
The fraction for each foodstuff depends on the metabolic situation and on the diet. Average values are 15 - 17% for protein, 50 - 55% for carbohydrate and 30 - 35% for fat oxidation.
The measured metabolic rate at rest depends on circumstantial factors. A minimum results with measurement after 12 - 14 h fasting under conditions of complete bodily rest, mental relaxation and in a thermoneutral environment. 24 hours' energy consumption under these circumstances is called basal metabolic rate (BMR). In hospitals less standardized conditions prevail and energy consumption at rest is approximately 10% higher and referred to as resting energy expenditure (REE).
REE = BMR x 1.1
Empirical Approximate Equations
Basal metabolic rate (BMR) is defined as the quantity of energy required to satisfy the requirements of the body at rest. Basal metabolic rate can be estimated from empirical approximate equations.
The most used formula for the calculation of the basal metabolic rate, based on indirect calorimetry, is the Harris & Benedict formula (Harris and Benedict, 1919):
BMR = 66 + (13.7 x BW) + (5 x ( H) - (6.8 x A) (male)
BMR = 655 + (9.6 x BW) + (1.73( H) - (4.7 x A) (female)
in which BW = body weight in kg, H = height in cm, A = age in years.
The BMR can also be estimated from the Fleisch standard metabolic rates (Fleisch, 1951), based on the body surface. The body surface can be estimated from standard normograms on the relation between body surface area, weight and height.
MR (kcal/h) = 4.83 x VO2
The resulting equation has a maximum error of 8%.
If CO2 production is also considered, the equation becomes:
MR (kcal/h) = 3.9 x VO2 + 1.1 x VCO2
The oxidation of protein is associated with the excretion of nitrogen in urine. One gram of nitrogen in urine is equivalent to an oxygen consumption of 5.94 l and a CO2 production of 4.76 l. If one measures the total urinary nitrogen (UN) per unit of time, the metabolism can be corrected for the fraction of oxidised protein. In Weir´s equation this (v. Weir, 1949)fraction is estimated at 12.5%:
MR (kcal/h) = 3.941 x VO2 + 1.106 x VCO2 - 2.17 x UN
The fraction of the various foodstuffs used as an energy source can be calculated from the total urinary nitrogen (UN in g), the oxygen consumption (VO2 in l) and CO2 production (VCO2 in l) by the following equations (Wilmore, 1977):
Protein oxidation (g) = 6.25 x UN
Carbohydrate oxidation (g) = -(2.56 x UN) - (2.91 ( VO2) + (4.12 x VCO2)
Fat oxidation (g) = -(1.94 x UN) + (1.69 ( VO2) - (1.65 x VCO2).
The fraction for each foodstuff depends on the metabolic situation and on the diet. Average values are 15 - 17% for protein, 50 - 55% for carbohydrate and 30 - 35% for fat oxidation.
The measured metabolic rate at rest depends on circumstantial factors. A minimum results with measurement after 12 - 14 h fasting under conditions of complete bodily rest, mental relaxation and in a thermoneutral environment. 24 hours' energy consumption under these circumstances is called basal metabolic rate (BMR). In hospitals less standardized conditions prevail and energy consumption at rest is approximately 10% higher and referred to as resting energy expenditure (REE).
REE = BMR x 1.1
Empirical Approximate Equations
Basal metabolic rate (BMR) is defined as the quantity of energy required to satisfy the requirements of the body at rest. Basal metabolic rate can be estimated from empirical approximate equations.
The most used formula for the calculation of the basal metabolic rate, based on indirect calorimetry, is the Harris & Benedict formula (Harris and Benedict, 1919):
BMR = 66 + (13.7 x BW) + (5 x ( H) - (6.8 x A) (male)
BMR = 655 + (9.6 x BW) + (1.73( H) - (4.7 x A) (female)
in which BW = body weight in kg, H = height in cm, A = age in years.
The BMR can also be estimated from the Fleisch standard metabolic rates (Fleisch, 1951), based on the body surface. The body surface can be estimated from standard normograms on the relation between body surface area, weight and height.
Standard Metabolic Rates
The actual energy expenditure (AEE) will normally be higher than the BMR). Only in prolonged fasting is energy expenditure reduced by 10 - 15%. Energy requirement increases for example with fever (12% per 10° C), with all kinds of stress (+5% to +100%), with food intake (specific dynamic action: +12% for protein, +6% for carbohydrate, +2% for fat, +6% for mixed diets) and with physiological activity. AEE can therefore be estimated using calculated BMR and factors to correct for increases in energy requirements. Despite the fact that estimating the AEE from calculation of the BMR is very widely used and represents a simple and quick method, it is not always accurate because of the large range of thermogenic responses to injury, trauma and infection.
Equations and Factors to Estimate Actual Energy Expenditure (AEE)
Examples of Calculation of the Actual Energy Expenditure (AEE)
Male person, aged 20, 170 cm tall, 60 kg of body weight, confined to bed, having multiple trauma and 38° C body temperature.
BMR = 66 + (13.7 x 60) + (5 x 170) - (6.8 x 20) = 1600 kcal
AEE = BMR x AF x TF x IF = 2900 kcal
Female person, aged 56, 163 cm tall, 54 kg of body weight, mobile, in a postoperative state and 38° C body temperature.
BMR = 665 + (9.6 x 54) + (1.7 x 163) - (4.7 x 56) = 1200 kcal
AEE = BMR x AF x IF x TF = 1880 kcal
Male person, aged 39, 189 cm tall, 91 kg of body weigth, confined to bed having burns of 40% of body surface and a body temperature of 39° C.
BMR = 66 + (13.7 x 91) + (5 x 189) - (6.8 x 39) = 1990 kcal
AEE = BMR x AF x IF x TF = 4470 kcal
