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General values for EeqCO2-body should not be used in subjects with major nutrient or energy imbalance. An individual who is starving or on a very low calorie diet will derive most of his energy from fat, especially after the glycogen stores are depleted. Protein will make a small contribution to energy expenditure 9. The EeqCO2-body under these circumstances may therefore be close to 27 kJ/l CO2, which is about 15% higher than in weight-stable subjects who maintain themselves in nutrient balance whilst ingesting a typical 'western' diet (Table 10.4).
The extent of this difference can be calculated using Equation 5, which requires knowledge of the fuels used for oxidation. If it is assumed that the energy deficit is largely accounted for by oxidation of fat, then it can be shown 2 that the energy intake (western type diet) has to be reduced by as much as 50% below energy expenditure to increase the EeqCO2-body by as much as 6%. This confirms the worked examples previously published by Black et al 1.
In contrast, overfeeding (with a western type diet) to the extent of 50% above energy expenditure, will reduce the EeqCO2-body by only about 6% ². This degree of overfeeding is greater than that which occurs in the human neonate during the first four months of life, a period of rapid growth. During this period the metabolisable energy intake provided in milk (EeqCO2-milk, 24.3 kJ/l) is about 34% higher than energy expenditure 10. This extra energy is deposited predominantly as fat (1.5 kg) and to a lesser extent protein (0.4 kg) 10. Calculations based on these changes suggest that the EeqCO2-body will differ from the EeqCO2-milk by only about 3.2%. Therefore, it is clear that nutrient imbalance has to be substantial to change the EeqCO2-body by more than 5%. Again these estimates confirm the figures published by Black et al 1.
Large changes in EeqCO2-body may also occur during rapid repletion in depleted subjects receiving hypercaloric regimens rich in carbohydrate. When the energy intake from carbohydrate is greater than energy expenditure the RQ of the body may rise to values that are persistently greater than 1.0 11, and the EeqCO2-body may fall to values that are below 21.1 kJ/l CO2. This differs by more than 15% from subjects maintaining constant body composition whilst ingesting a western type diet.
Although the assessment of EeqCO2-body
in states of nutrient imbalance is more difficult than under normal circumstances,
reasonable estimates of EeqCO2-body can be made by taking into account the
clinical or physiological state under investigation, and the associated changes in body
weight. Estimates of body composition may also be useful under certain circumstances, but
in practice it is often found that the difficulty of accurately estimating changes in body
composition over the short periods used for DLW measurements are such that little
advantage is gained over the use of weight changes alone.
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AI, Silverg PA, Milic-Emili J & Kinney JM (1980) Respiratory changes induced by the
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