Overnutrition in rodents during early postnatal life induced by breeding in small nest sizes leads to distinct metabolic changes, which remain throughout life even under conditions of free access to normal food. An accelerated body fat accumulation and a probably higher efficiency of energy utilization render this animal model suitable for the study of relations between the development of an obese state and its metabolic causes [ 1, 21. There is increasing evidence from studies in human beings [3,4] as well as in rodents [S, 61 that a higher body fat content is associated with a lower thermogenic response to food ingestion, assumed to indicate an increased efficiency of energy utilization. First results concerning postprandial thermogenesis in postnatally overfed rats are presented here.
The experiment was carried out with male rats of the Wistar strain (Wist Schoe, VEB Versuchstierproduktion, Schonwalde near Berlin, GDR), which were grouped immediately after birth in nests of 2 or 12 animals. After weaning (4 weeks) the animals were kept in single cages and fed ad libitum with standard pellets (type R, VEB Versuchstierproduktion). At an age of 7-9 weeks the energy expenditure ofthe rats was determined by indirect ca!orimetry in an open system. For this purpose the animals were transferred into metabolic cages (Simax, CSSR), supplied with a constant stream of fresh air (80 I/h, (25 f 1) "C, 100% relative humidity). Oxygen consumption and carbon dioxide production were estimated paramzgnetically and by infrared absorption using Permolyt 2 and Infralyt 4 (VEB Junkalor, Dessau, GDR), respectively. To compare energy intake the rats got at 8 a.m. a semisynthetic diet (casein 15%, wheat starch 53 %, sucrose lo%, sunflower oil 15 %, cellulose 2 %, salt mixture 4 %, vitamine mixture 1 %) limited to 500 kJ metabolizable energy per kg0.75.