High efficiency of energy utilization in 'cafeteria'-and force-fed rats kept at 29 degrees

Male, Sprague-Dawley (Charles-River) rats, of initial weight 272 g, were given a powdered stock diet (T1) ad lib., force-fed a synthetic diet (T2) or offered a range of palatable foods in conjunction with the powdered stock diet (T3) or a similar diet supplemented with certain minerals and vitamins (T4). Metabolizable energy (ME) intake (kJ/d) averaged 303, 453, 402 and 383 for T1, T2, T3 and T4 respectively and corresponding weight gains were 5.5, 6.9, 8.2 and 7.9 g/d and were significantly different (P less than 0.001). The intakes of T3 and T4 rats ranged from 10 to 60% above the mean value for T1. Crude protein (CP; nitrogen X 6.25) retentions were similar for T1, T3 and T4 rats and significantly lower (P less than 0.01) for T2 rats. Fat retentions were 1.1, 4.1, 2.9 and 2.4 g/d for T1 to T4 respectively (P less than 0.001). The energy contents of the gain (MJ/kg) were 12.7, 26.0, 16.7 and 14.9 for T1 to T4 respectively (P less than 0.001) and energy retentions (kJ/d) were 70, 179, 139 and 117 respectively (P less than 0.001). A linear regression of energy retention (ER) on ME yielded a slope of 0.78 and a mean energy requirement for zero balance of 510 kJ/kg body- weight0 .75. These results are in conflict with reports of 'diet-induced thermogenesis' in 'cafeteria'-fed rats.