I. Measurements were made of energy balance by direct calorimetry, and of nitrogen balance in groups of lean and congenitally obese ('fatty') Zucker rats at body-weights of 200 and 350 g given a highly digestible semisynthetic diet at 14.0 or 18.4 g/rat per 24 h. 2.Losses of food energy and N in faeces were very small. The fatty rats lost much more N in urine than did lean rats. Despite this the proportion of gross energy that was metabolized was 0.92 for both fatty and lean rats.3. In all trials, fatty rats lost a smaller proportion of metabolizable energy (ME) as heat and deposited less as protein than thin rats but deposited much more as fat. 4.The amounts of ME required to deposit I kJ of protein and I kJ of fat respectively were shown by regression analysis to be 2 . z~ (fo.16) and 1.36 (k0.06) kJ respectively. These values agree extremely closely with recent, more tentative, estimates based on assumptions as to maintenance requirement which the present experiments were able to circumvent. It may be concluded with confidence that the energy costs of depositing I g of protein or fat are almost identical at 53 kJ ME/g.The energy cost of fat and protein deposition is simply the increment of food energy (usually expressed as metabolizable energy (ME)) required to promote a defined increment in body protein or fat.The energy cost of fat deposition can be measured with precision in adult animals since, in these circumstances, energy retention as protein is small and the amount of ME required to maintain energy balance (so-called ' maintenance requirement ') does not differ much between successive measurements made of metabolic heat production at different levels of ME intake. There is general agreement that in simple-stomached species such as the rat and the pig the energy cost of fat deposition ranges from about 1.4 kJ ME/kJ fat deposited for foods consisting predominantly of carbohydrate to 1-15 for foods rich in triglycerides (ARC/MRC Committee, 1974).The energy cost of protein deposition has been more difficult to assess. First, even during rapid growth the amount of energy deposited as protein is small relative to that deposited as fat or dissipated as heat. Second, the division of ME between maintenance requirement and that for protein and fat deposition changes continuously during growth and these changes are linked in such a way that changes in maintenance, protein and fat deposition show marked autocorrelation. Kielanowski (1965) recognized this when first he used multiple regression analysis in an attempt to partition ME intake between maintenance, protein and fat deposition and since then a series of reports has used his approach to describe the efficiencies of protein and fat deposition in pigs (Kielanowski

The efficiency with which an animal can utilize the metabolizable energy (ME) contained in the food it eats is determined by the amount of heat (H) it produces in metabolism. This paper is devoted to an analysis of the factors which affect H in animals. The topic is of perennial interest to students of both animal and human nutrition concerned respectively with the efficient use of animal feedstuffs by livestock and with problems experienced by man in maintaining energy balance through adult life.There are three ways by which one can approach the analysis of metabolic heat production. (I) Analysis by external inputs. Analysis of H according to measurable variables in the whole animal and its environment, namely size of animal, quantity and quality of food intake, behaviour and activity and the thermal environment. These forms of analysis require a little further explanation. The analysis according to external input is the classic approach of the calorimetrist measuring energy flow in the whole animal in response to changes, e.g. in food intake or ambient temperature. Measurements of this sort are very numerous and while they may not be very profound in a metabolic sense, they are usually very precise and the limited conclusions that can be drawn from them can usually be drawn with confidence. Analysis according to internal input refers to the 'lower level' of the modeller who uses his knowledge of stoichiometry and the power of his computer to derive a more elegant and comprehensive picture of the flow of energy through an animal and the likely consequence to the animal of any perturbation of the inputs to the system. The disadvantage of this approach is that, at present, the uncertainty attached to some of the estimates can be large.The philosophy behind the approach to analysis by internal output is that the metabolism of the animal is not so much driven by the amount of energy flowing into the system but pulled along by the requirement of different organs and tissues for energy substrates to regenerate (principally) ATP from ADP produced as a direct consequence of the work done by those tissues in support of maintenance, and, for example, growth or lactation. This form of analysis can be further subdivided into analysis by form or function. Analysis by form involves the measurement or estimation of H in different organs (e.g. the gut and liver). Analysis by function involves measurement of the contribution to total metabolic rate of energetically expensive processes such as protein synthesis.

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Bristol BS18 7 0 U Metabolizable energy (ME) and protein contained within the food consumed by an animal during growth are directed (almost) entirely towards heat production and the deposition of body protein and fat. The partition of nutrients between these three compartments or between the various organs and tissues in the growing animal is obviously determined by the amount of ME available for growth, the availability of other nutrients (protein, minerals, etc.) relative to ME and the animal's intrinsic 'growth plan' which is a function of its age, genotype and its physiological state.I wish to propose that for most practical purposes the partition of ME and protein during growth into body protein, fat and heat can be described by a model of extreme simplicity (Fig.

I. Heat losses associated with the utilization of metabolizable energy for synthesis of protein and of fat during growth mere studied in Zuekcr rats selected for normal leanncss or congenital obesity.z. Measurements of energy and nitrogen balance were made on groups of four lean and four fat rats offered food ad lib. and kept at zzo. Balance trials were also conducted on groups of fat or lean rats offered restricted amounts of food at two levels and kept at 22' or 2 8 ' . The medium rations offered to fat and lean rats were the same. The energy and N contents of frlt and lean rats were determined by carcass analysis at 35 d and at about 90 d of age.3. At ad lib. intake, fat rats ate 38 74 more than lean rats. Heat losses and N balance were similar. When fat and lean rats were pair-fed, heat loss and N retention werc lower in fat rats.Absolute values and changes during growth of heat loss (kJ/rat per 24 h) were closely correlated with values obtained for S balance.4. Carcass analysis showed that energy retention in protein was, on average, 75 "/b in lean rats but only 14 % in fat rats. Estimates of energy retention from slaughter experiments and balance trials agreed well, but marked discrepancies existed between the different estimates of N retention.5 . The net efliciencies of utilization of metabolizable energy for growth in lean and fat rats were 0.485 and 0.614 respectively. The energetic efficiencies of net protein synthesis and net fat synthesis were estimated to be 43 and 65 6. The interactions between appetite, growth and activity in the development of obesity in the Zucker rat are discussed.respectively.The efficiency with which a growing animal can synthesize protein and fat dcpends primarily on the amount that it eats. The greatcr the intake of metabolizable energy (ME) in excess of maintenance requirement, the greater the gross efficiency of energy retention. The net efficiency with which fixed increments of a particular food can promote energy retention in an animal may be obtained by measuring the increment of energy retained (kJ) per IOO kJ consumed in excess of maintenance (Blaxter, 1962).Evaluation of different foodstuffs as sources of net energy, e.g. for ruminants, is usually done by measuring their capacity to promote energy retention, principally as fat, in the mature animal (Agricultural Research Council, 1965 ; Nehring, 1969). This is necessary to ensure that the maintenance energy requiremcnt of the animal docs not differ significantly between successive trials and so can be determined with precision (Blaxter, Clapperton & Wainman, 1966). Such experiments cannot however predict the extent to which the efficiency of utilization of energy in the growing animal is affected by the partition of retained energy between protein and fat. The scheme proposed by the Agricultural Research Council (1965) for determining the energy requirements of ruminants for growth avoids the issue by assuming that the efficiency of utilization of ME for production is constant throughout growth, although of course ...