|Activity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)|
|The energy requirements of growth and catch-up growth|
Growth is a non-specific term that is used to include a constellation of changes associated with the elaboration of form and function. It is a process that has been characterised as canalised and target-seeking (genetically determined). Although growth is not necessarily a continuous process on a day-to-day basis, under optimal circumstances it appears to be continuous from week to week. There are a range of adverse factors, both dietary and environmental, which may impede, slow down, or even reverse some or all aspects of growth. Following the removal of the adverse circumstance, a process of repair and recovery takes place enabling the individual to regain the original growth channel. This process of recovery has been called catch-up growth (TANNER, 1978). Superficially, the process of catch-up growth may have the appearance of an intensification of the normal growth process. Although there may be aspects that can be perceived of in this way, it is not justified to consider that this is necessarily true for the entire process.
During growth, the changes in form are most readily identifiable as an increase in stature and mass, but also include more subtle variations in the composition of the body and changes in the relative size of the different organs and tissues (WIDDOWSON, 1970). The coordinated development and refinement of function may be more difficult to quantify than changes in size, and are not as clearly characterised.
Growth, as an increase in mass, represents an increase in the net energy content of the body. The energy density of the increased mass will vary with the nature of the tissue being deposited, from 35 kJ/g for adipose tissue to 5.6 kJ/g for lean tissue (JACKSON, PICOU and REEDS, 1977). There is an energy cost associated with the actual deposition of this tissue which is determined by the nature of the initial substrate and the pattern of tissue to be deposited (PULLAR and WEBSTER, 1977). To the extent that growth represents an elaboration and refinement of form, there is an energy cost associated with the process of remodelling. The energy cost of remodelling gives the appearance of inefficiency to the overall process of net deposition. It is not altogether clear whether remodelling can be used to account for the relative inefficiency of protein deposition during growth. In childhood, for example, 1.4 g of protein have to be synthesised for the net accretion of 1 g of protein (WATERLOW and JACKSON, 1981). During the growth of normal children, the absolute requirement for energy to satisfy all these components is small relative to the energy required for maintenance: around 15 to 30% at birth and falling to only 5% at 1 year of age (FAO/WHO/UNU, 1985). Although it is possible to adopt a factorial, biochemical approach to determining the relative demands in energy for each of the processes, it is much more difficult to determine the real physiological cost of all the processes associated with growth at the level of the whole body. It is for this reason that we have only a limited appreciation in detail of the real costs. One way to get around this problem is to study situations in which the entire process has been intensified. The advantage in studying catch-up growth is that there is an acceleration of the whole process with an intensification of the changes in time. As a result, the proportion of the total energy intake devoted to tissue deposition and remodelling may reach more than 50% of the total (ASHWORTH, 1974).