Nutritional obesity and body composition

It will be abundantly evident from other papers in this symposium that obesity is not a common feature in modem farm animals nor should it be inferred from the inclusion of this paper that nutritional obesity is an inevitable consequence of the consumption of animal products. The justification for including this subject rests entirely on the obsession with human obesity in our overnourished western world and the considerable research effort, particularly during the last 15 years, on the development and sequelae of overnutrition, using animal models. However, detailed examination of the voluminous literature reveals that most studies have used the laboratory rat and were undertaken to study food intake or energy metabolism with only a very limited number being primarily designed to examine body composition. The author has therefore been forced to be selective in terms of literature cited and acknowledges di4iculty in reaching unifying themes. It will become clear that the general principles relating to body composition of growing and adult animals which have b&n elucidated and developed during the last 40 years apply equally well during the development of obesity as during undemutrition. It is therefore fitting to begin this paper with a tribute to those who so painstakingly laid the basis of our understanding of growth and body composition and, in particular, to two research groups from Cambridge, namely the late Sir John Hammond and his colleagues (McMeekan, 1940a,b,c; Palsson & Verges, 1952; Palsson, 1955) and McCance & Widdowson and their colleagues (Spray & Widdowson, 1950; Dickerson & McCance, 1961; Cabak et al. 1962; Lister & McCance, 1967; Widdowson, 1968). Hyperphagia and consequential obesity have been induced in the laboratory rat by offering high-fat diets (Mickelsen et al. 1955), by gastric intubation (Cohn et al. I ~ J ) , by offering concentrated sugar solutions (Muto & Miyahara, 1972) or by offering a varied ‘cafeteria’ diet (Sclafani & Springer, 1976). Spontaneous obesity was observed in the squirrel monkey (Saimiri sciureus) given semi-purified diets (Ausman et al. 1981) and obesity was induced in sheep by long-term feeding of pelleted diets (Blaxter, 1976; Blaxter et al. 1982). The present paper will primarily examine the extent to which the means of inducing nutritional obesity in the laboratory rat affects chemical and anatomical aspects of body composition but will also take account of species differences where possible. In addition, consideration will be given to ( I ) the effect of dietary fat composition on the development of obesity and on the composition of body fat, and (2) variations in the energy content of weight gain during hyperalimentation or of weight loss during fasting or restricted energy intake.