Comparative Biochemistry and Physiology Part A 121 (1998) 423 – 429 Heat production and substrate oxidation in rats fed at maintenance level and during fasting Andre ´ Chwalibog a, *, Kirsten Jakobsen b , Anne-Helene Tauson a , Grete Thorbek a a The Royal Veterinary and Agricultural Uniersity, Department of Animal Science and Animal Health, Bulowsej 13, 1870 Frederiksberg, Copenhagen, Denmark b Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, 8830 Tjele, Denmark Received 26 May 1998; received in revised form 17 September 1998; accepted 5 October 1998 Abstract A total of 36 Wistar rats were fed a commercial diet to a stipulated live weight of 75 g (Group A), 100 g (Group B) and 225 g (Group C). All rats were measured in energy balance experiments, in which the animals were fed near maintenance level, followed by a period of fasting with measurements of the gas exchange. The rats in Group A, B and C were fasted for 2, 3 and 4 days, respectively. The minimum heat production on the last day of fasting for all groups was proportional to metabolic body weight (kg 0.75 ) with a regression: heat production, kJ day -1 =321 ×kg 0.75 (R 2 =0.994). In rats fed near maintenance level, heat production was provided by oxidation of carbohydrates in 80 – 85%, oxidation of protein was 10 – 15%, while oxidation of fat contributed less than 10%. It is suggested that in the fasting period, the contribution to the total heat production from oxidized carbohydrate and fat depended on the size of the fat depots, a large fat depot giving rise to fat oxidation. On the last day of fasting, 24, 51 and 90% of the total heat originated from fat oxidation in Group A, B and C, respectively. © 1998 Elsevier Science Inc. All rights reserved. Keywords: Carbohydrate oxidation; Fasting; Fat oxidation; Heat production; Indirect calorimetry; Maintenance; Protein oxidation; Rat 1. Introduction Rats have been used extensively as model animals for studies on energy metabolism during high, normal and low food intakes. Restricted food intake or even peri- ods of fasting are frequent situations in both humans and animals. In farm animals, these situations may even be practised in certain periods of production in order to avoid excessive fatness, metabolic diseases or birth difficulties. During periods of restricted feeding, fasting or starvation, many studies have focused on the metabolic depression, which has been found repeatedly in both animals and man, and on the factors explaining it. In periods where a weight loss occurs, some efforts have been made to estimate the loss of body fat and protein from studies on changes in body composition. In rats, this was done with carcass analyses [9,21] or by calculation of weight loss with assumed chemical com- position of the weight loss [15]. In human beings, the composition of the weight loss and the energetic contri- bution to overall heat production (HE) or metabolic rate measured during some defined situation have been performed in healthy [23] and diseased subjects [22] by various methods. Measurements of gas exchange in respiration units have been used in indirect calorimetry to estimate en- ergy metabolism. Recently, measurements of gas ex- change have been used to calculate the oxidation of protein (OXP), the oxidation of carbohydrate (OX- CHO) and the oxidation of fat (OXF) in the intact body of different species [2,4,6,18,20]. * Corresponding author. Tel.: +45 35 283044; fax: +45 35 283020; e-mail: ac@kvl.dk 1095-6433/98/$ - see front matter © 1998 Elsevier Science Inc. All rights reserved. PII S1095-6433(98)10153-8