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10.2 Estimating Eeq_CO2 for different oxidation mixtures

The Eeq_CO2 for an oxidation mixture depends both on the proportion of energy derived from individual fuels and on the end products of metabolism ^{2, 3}. The variability of these two factors in different circumstances therefore requires some discussion. However it is first necessary to briefly consider the methods available for estimating the energy equivalent of CO₂ for a subject (Eeq_CO2-body) and for a mixed diet (Eeq_CO2-diet).

10.2.1 Estimation of the Eeq_CO2-body

The Eeq_CO2 for an oxidation mixture in a subject is simply given by:

.........1

The total CO₂ released is equal to the quantity of CO₂ produced from the oxidation of individual fuels:

Total CO₂ = CO₂ from protein oxidation (CO₂-prot) + CO₂ from fat oxidation (CO₂-fat) + CO₂ from carbohydrate oxidation (CO₂-carb) + CO₂ from alcohol oxidation (CO₂-alc)

.........2

Substituting Equation 2 into Equation 1 gives:

.........3

The volume of CO₂ released during the oxidation of an individual fuel is given by:

.........4

Therefore the Eeq_CO2-body for a fuel mixture can also be expressed as:

.........5

where p, f, c and a represent the percentage of energy derived from the oxidation of protein, fat, carbohydrate and alcohol respectively, and 23.33, 27.46, 21.12 and 30.49 are the associated values of Eeq_CO2 for these fuels in kJ/l (Table 10.1).

Table 10.1. The energy equivalent of O₂ and CO₂ and the respiratory quotient of fat, protein, carbobydrate and alcohol

* End products assumed to be urea, ammonia and creatinine in the nitrogenous ratio 90:5:5.

** Glucose polysaccharide.

From Livesey & Elia ⁵.

In practice it is not often that information is available about the exact proportion of energy derived from the oxidation of individual fuels especially in human studies carried out over extended periods of time. Therefore to calculate Eeq_CO2-body it is necessary to estimate either the percentage of energy derived from individual fuels, or the overall RQ of the oxidation mixture which is given by:

.........6

where O₂prot, O₂fat, O₂carb and O₂alc represent the O₂ utilised during the oxidation of protein, fat, carbohydrate and alcohol respectively, and 0.835, 0.71, 1.00 and 0.667 are the respective respiratory quotients of these fuels ⁴.

The volume of O2 consumed during the oxidation of individual fuels is given by Equation 7 which can be substituted into Equation 6:

.........7

where Eeq_CO2 is the energy equivalent of O₂ (Table 10.1).

The Eeq_CO2 of a carbohydrate/fat oxidation mixture is related to the RQ according to Equation 8 (see Elia and Livesey ⁴ for derivation):

.........8

The RQ of the fat/carbohydrate oxidation mixture is almost linearly related to the proportion of energy derived from carbohydrate oxidation (c) (Equation 9) and inversely related to the proportion derived from fat oxidation:

.........9

The variation in Eeq_CO2 with the RQ of a carbohydrate/fat oxidation mixture is shown in Figure 10.1. Values of RQ below 1.0 refer to the net oxidation of fat and carbohydrate, and those above 1.0 to the oxidation and conversion of carbohydrate to lipid ⁴.

In practice fat and carbohydrate are never oxidised in isolation and although they are the primary determinants of RQ it is necessary to consider the effects of protein and alcohol. The oxidation of protein to different nitrogenous end products is associated with various values of Eeq_CO2-prot (Table 10.2), which would cause a slight deviation below the curve generated by a pure fat/carbohydrate oxidation mixture (Fig 10.1). The more reduced the nitrogenous end product, the greater the deviation of the value from the fat/carbohydrate curve. In contrast alcohol oxidation would cause a slight deviation above the curve. However, all these values are so close to the fat/carbohydrate curve in Fig 10.1 (within 5-6%) that if the overall respiratory quotient of a fuel mixture were known, the associated Eeq_CO2 of that mixture can be closely predicted ¹. A general formula for calculating the Eeq_CO2 of a carbohydrate/fat/protein oxidation mixture from the RQ of the oxidation mixture is given by Equation 10 (see also Equation 5), which assumes that about 12% of energy expenditure is derived from protein oxidation and that urinary N is distributed in urea, creatinine and ammonia in the ratio 90:5:5 ^{4, 5}.

.........10

This relationship is also illustrated in Table 10.3.

The relationship between the energy equivalent of CO₂ and the respiratory quotient of a carbohydrate fuel mixture (see curve). The energy equivalent of CO₂ and the respiratory quotient associated with the oxidation of individual fuels are also indicated: carbohydrate (Carb); fat (Fat); alcohol (Alc) and protein (Protein). Four values are indicated for protein to take into account its conversion to urea, ammonia, allantoin and uric acid. See text for details.

Table 10.2. The respiratory quotient and energy equivalent of O₂ and CO₂ associated with the oxidation of Kleiber's standard protein to different nitrogenous end products.

Kleiber's standard protein = C₁₀₀H₁₅₉N₂₆O₃₂S_0.7

From Elia ².

Table 10.3. Relationship between the respiratory quotient and the energy equivalent of CO₂ for a carbohydrate/fat/protein oxidation mixture

Derived from Equation 10.

As indicated above, there is an increasing error in the overall Eeq_CO2 as the proportion of energy derived from protein oxidation increases (the maximum error is 3.2% when all the energy is derived from protein). A small error will also occur when the equation is used to calculate the overall Eeq_CO2 for an oxidation mixture that includes alcohol (maximum error 5.8% when all the energy is derived from alcohol). Therefore the direct approach suggested by Equations 3 and 5, is theoretically more sound than the indirect approach which involves calculation of the RQ.

10.2.2 Estimation of the FQ and Eeq_CO2-diet

The Eeq_CO2 can be calculated by equations analogous to Equations 3 and 5 using the metabolisable energy of fuels ^{1, 2}. In this respect it should be noted that the energy derived from the oxidation of given quantities of endogenous nutrients such as fat or protein, is greater than the metabolisable energy derived from the same quantity of dietary nutrients. This is because digestibility does not have to be considered when endogenous fuels are mobilised for oxidation (eg the heat of combustion of fat is about 39.3 kJ/g, whereas the metabolisable energy is about 37.7 kJ/g). For subjects in nutrient balance the metabolisable energy intake of individual fuels is expected to equal the energy derived from the oxidation of these fuels by the body. Under these circumstances the Eeq_CO2-diet should equal the Eeq_CO2-body. Similarly, (under these circumstances of nutrient balance), the food quotient will equal the respiratory quotient. However, differences between Eeq_CO2-body and Eeq_CO2-diet (or FQ and RQ) will occur when there is energy or nutrient imbalance (see below). Therefore the value of Eeq_CO2-body, which is of primary importance in the estimation of energy expenditure by the doubly-labelled water technique, depends both on the extent of nutrient imbalances as well as on the composition of the diet.

10.2.2.1 Variation in Eeq_CO2-diet and FQ

The Eeq_CO2-diet for groups of individuals in Britain is found to be remarkably constant (at about 23.85 kJ/l) irrespective of whether it is calculated from data of food intake obtained from dietary recall or from measurements of weighed food intake obtained over an extended period of time ². Furthermore, even if the gross changes in dietary intake recommended by some modern dietary goals were fully implemented, they would make little difference to the FQ and Eeq_CO2-diet (Table 10.4). However, in many Third World countries, where the proportion of dietary energy derived from carbohydrate is high, the Eeq_CO2-diet may be up to 10% lower than that of a typical western diet (Table 10.4).

Further examples of circumstances in which Eeq_CO2-diet and FQ may deviate quite substantially from those of a typical Western diet are given below:

a) Ethnic minorities living in 'western societies' ¹.

b) Certain groups of athletes (US college wrestlers and 'track and field athletes' whose FQ may be close to or over 0.90 and Eeq_CO2-diet as low as 22.5 kJ/l CO₂). This however is unusual since the values for Eeq_CO2 generally range from 23.0-23.7 kJ/l CO₂ (see Elia ² for further analysis).

c) Other societies. An analysis of food available for consumption in 146 countries around the world (Food and Agricultural Organisation) shows the (alcohol free) FQ to vary from about 0.85 to 0.95 and the Eeq_CO2-diet from about 21.8 to 23.9 kJ/l CO₂ (11% range) ². Individual dietary surveys in various countries, show similar variations in FQ and Eeq_CO2-diet ^{1, 2}.

d) Hospitalised patients receiving unusual combinations of nutrients in artificial feeds. For example, the FQ of 56 commercial enteral feeds was found to range from 0.80 to 0.98, and the Eeq_CO2-diet from 21.4 to more than 24.5 kJ/l CO₂ (14% range) ².

e) Infants fed exclusively on certain formulae.

Table 10.4. The composition of different diets and their associated food quotient (FQ) and energy equivalent of CO₂ Eeq_CO2-diet

NACNE: National Advisory Committee on Nutrition Education 1983.

High CHO diet: as typically eaten in parts of India, Africa and other Third World countries (see ref ²).

10.2.2.2 Individual variation in Eeq_CO2-diet

The use of a general value for Eeq_CO2-diet of about 23.85 kJ/l CO₂, for the 'western' type diet, was found to predict to within ± 5% the Eeq_CO2-diet of 63 randomly-selected individuals, whose dietary intake was assessed by weighing the food eaten over a one week period (Bingham et al ⁶ and personal communication). However, it is important to note: a) that the reported alcohol intake, which affects the Eeq_CO2-diet was low in this group of subjects (4% of the dietary intake for men and 1.2% for women); and b) that some of the subjects especially women were dieting at the time of the study and were therefore in nutrient imbalance. During dieting the Eeq_CO2-body is likely to be higher than the Eeq_CO2-diet to an extent which depends both on the degree of dietary restriction and on the composition of the diet (see below).

10.2.2.3 The effect of alcohol

Alcohol can significantly affect the Eeq_CO2-diet and FQ of the diet because its Eeq_CO2 is greater than those for fat, protein and carbohydrate, since its FQ is lower. However, the extent to which Eeq_CO2-diet is affected by the presence of alcohol in the diet depends not only on the amount consumed, but also on the composition of the rest of the diet ². For example alcohol produces a greater change in the Eeq_CO2-diet when it is added to a diet rich in carbohydrate (ie a diet with a high FQ and low Eeq_CO2-diet) than when it is added to a diet rich in fat. This effect is illustrated in Table 10.5, for values of alcohol intake ranging from 10-50% of energy intake.

Table 10.5. Effect of increasing amounts of alcohol on the dietary equivalent of CO₂ (Eeq_CO2-diet) and food quotient (FQ) of four different diets

Alcohol intake of course varies considerably in people of different nationalities ⁷. It accounts for about 6% of energy intake of adults in Britain ⁸, but this varies considerably depending on the geographical area as well as on age, sex and socioeconomic background of the subject. Alcohol intake is particularly low in Moslem countries, and high in other countries such as France, where the mean intake per person is two-fold greater than in the U.K. ⁷. These averaged differences according to region or nationality mask much larger differences between individuals or groups of individuals, especially men, who tend to drink considerably more alcohol than women. Indeed, various dietary surveys suggest that the mean alcohol intake of selected populations ranges from 0-50% of total energy ².

A particular problem to be borne in mind in relation to adjusting the Eeq_CO2-diet for alcohol consumption is that it may often be seriously under-reported.

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