The energy cost of growth.

Energy requirements for growth can be separated into two components. One is the requirement for substrates from which to form the components of the new tissue, the other is the extra energy needed to meet the metabolic cost of growth. The first requirement is, of courae, absolute in so far as I g of new tissue containing, say, 25% fat and 15% protein cannot be deposited without a dietary supply of about 13.4 kJ of appropriate energy. This is in fact the average energy content of tissue deposited in the rapidly growing malnourished child recovering on an energy-rich milk diet (Spady, Payne, Picou & Waterlow, 1976). The requirement will be variable as the composition of the new tissue varies. Thus, weight gain in older animals tends to involve the deposition of more energy because of the higher fat content. Similarly, in the malnourished child, measurements of muscle mass by Jackson, Picou & Reeds (1976) have clearly shown that new tissue can contain from as little as 5 to as much as 30 kJ/g according to the fat content. However, this paper will be concerned with the second component of the energy cost of growth, the metabolic cost observed as heat production associated with growth.