|Activity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)|
|Methods to assess physical activity and the energy expended for it by infants and children|
|2. Methods of acquiring information on the physical activity of infants and children|
2.4.1. Indirect calorimetry
The classical technique of measuring energy expenditure by indirect calorimetry requires either the use of a Douglas bag to obtain samples of expired air for analysis or some form of respirometer. Neither method is ideal, especially in the case of small children. Moreover, since there is really no satisfactory respirometer on the market, the method has very considerable restrictions.
Neverthess, the Douglas bag technique can be utilized to provide the actual energy cost of a particular activity, and if measurements are carried out on the variety of activities of any individual child, together with information on the duration of these separate activities, then the total energy expended during any appropriate period of time can be calculated. If it is decided to use this technique, useful descriptions of the methodology and of problems incurred in a field study are given by SARIS (1982) in his account of his studies on children in Holland.
It has already been stated earlier in this article that there may be occasions when actual energy expenditure values are necessary in the assessment of the importance and implications of physical activity in infants and children. Nevertheless, these are rare situations, and the resources needed to carry out the measurements on anything other than a very small and statistically unsatisfactory scale, make it almost a prohibitive procedure. Fortunately, adequate information can generally be obtained without it on any relevant aspect of activity. It is usually possible to extract from published tables of energy expenditure, a value for a particular activity which might be reasonably appropriate for a given individual child.
2.4.2. Stable isotopes
The mean energy expenditure during a period of several days can be measured by an indirect estimate of CO2 production after giving a dose of the stable isotopes deuterium and 18O to the infant or child. The required number of days during which samples of body fluid have to be collected for the isotope analyses is still a matter of some uncertainty, but may be at least 7-10 days.
On the assumption that these measurements provide a valid estimate of total energy expenditure over these 7-10 days or more, is this technique of value in the present context? If there were only minor problems entailed in this procedure, we might well feel that such measurements provided some useful information. However, at least for the present, there are considerable technical difficulties with the mass spectrometers used to measure the isotopes, and there are also large financial costs for both the isotopes and the instrument. More importantly, the information obtained may be no more than would be available from an accurate measurement of energy intake, since no details about levels of physical activity can be provided. Estimates of total energy expenditure in a group of infants or children may well conceal important differences in the type, intensity, and duration of physical activity and therefore, in the present context, may have rather limited value.