|Activity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)|
|Energy requirements in normal infants and children|
Whilst we may choose the breast-fed infant as a model for energy requirement in early life, it has been notoriously difficult to define the breast-fed infant's intake of some nutrients, especially those which vary in their concentration in milk during the course of a feed. The measurement of milk fat, and therefore energy intake, has been particularly fraught with methodological difficulties.
Traditionally, energy consumed by breast-fed infants has been assessed by matching data on human milk composition, obtained by manual or mechanical expression of the breast, with daily milk intake measured by test-weighing or the use of stable isotope tracers (e.g., deuterium oxide) given to the baby (COWARD et al., 1979; LUCAS et al., 1987a) or mother (COWARD et al., 1982).
The technique chosen for the estimation of milk volume intake requires careful consideration. Test-weighing is clearly unsuitable for mothers who feed frequently. In some cultures, breast-feeding may take place on up to 20 occasions per day and may be virtually continuous over some periods. Test-weighing under these circumstances is socially and biologically invasive, and unidirectional weighing errors at each feed can result in serious cumulative error. In the industrialized countries, with infrequent feeds and with mothers who are motivated to do their own test-weighing, the technique can be satisfactory. Some concerns have been expressed over the 'dose-to-the-baby' isotope-kinetic method, though, as discussed below, satisfactory validation has been achieved in formula-fed babies (LUCAS et al., 1987a).
All available methods for measuring milk volume intake are subject to errors, but these are likely to be small if a method appropriate to the experimental circumstances is selected (COWARD, 1984). The principal difficulty, however, has been one of obtaining representative breast-milk samples. Two problems exist. First, milk fat, a major determinant of energy content, varies considerably among individuals and changes diurnally and throughout lactation. Moreover, both milk fat concentration and milk flow rate may change continuously during the course of a feed from each breast (LUCAS, LUCAS and BAUM, 1980). Therefore it is difficult to calculate energy intake from isolated samples of 'foremilk' or 'hindmilk'. Second, milk that has been obtained by manual or mechanical expression (EBM) of the whole breast may not have the same composition as milk obtained during normal suckling (so-called suckled breast milk (SBM); see below). For example, since fat concentration rises as milk is ejected, energy content may be overestimated if more milk is expressed than the infant would have consumed spontaneously. A possible mismatch between EBM and SBM may be especially marked for the second breast, if the infant normally takes a relatively small volume, leaving a significant residue of high-fat hindmilk that could be removed by expression. Thus, the energy content of EBM obtained by complete expression of the first breast, as reported by the DHSS (1977), may not reflect the average energy content of SBM from both breasts. Entire 24-hour collections of EBM may be analysed, but this approach is subject to the objection that during such a prolonged 'experimental' period, the normal biology of lactation (e.g., the hormonal milieu) could be altered, perhaps affecting milk flow or even composition; in addition, this procedure is generally unacceptably invasive. Other techniques have included foremilk and hindmilk sampling (PRENTICE et al., 1986; SAINT, MAGGIORE and HARTMANN, 1986) or midmilk sampling (NEVILLE et al., 1984) and analytic data have been linked to an equation to derive average milk fat. However, in our experience the pattern of changes in milk fat and flow rate are very variable between individuals, making such techniques difficult to validate.
In an attempt to resolve some of these difficulties, SBM has been studied directly (LUCAS, LUCAS and BAUM, 1980). A thin nipple shield, worn during a feed, can be used to collect sequential SBM samples. For population studies, changes in milk composition during a feed can be matched with cross-sectional data on the pattern of milk flow from mother to infant, estimated by a test-weighing protocol that avoids repeated interruption of the feed. Using this approach, LUCAS and colleagues (1980) showed that, early in lactation, SBM had a lower fat and energy content than had been reported previously in EBM. These workers also studied SBM in individual mother-infant pairs: a microminiaturized ultrasonic flowmeter was incorporated into the tip of the nipple shield (How et al., 1979) allowing changes in milk flow and composition to be measured simultaneously throughout the feed. Although these methods have considerable potential for investigating the physiology of lactation, they impede milk flow and may interfere with normal feeding behaviour due to the presence of a device and an investigator.
The advent of the doubly-labelled water method has made it possible to study energy expenditure, energy intake and the energy content of human milk without recourse to milk sampling. The impact of this technology is discussed in the following sections.