Energy Expenditure and Quantitative Oxidation of Nutrients in Rats (Rattus norvegicus) Kept in Different Thermal Environments and Given Two Levels of Dietary Fiber

A study was performed to investigate the effect of environmental temperature (16°C, 24°C or 32°C) and dietary fiber (DF) on energy expenditure and quantitative oxidation of nutrients in rats. Forty-eight male rats, initial body weight 90–105 g, were allocated to eight groups in two series. The rats kept at 24°C was repeated in both series. Low and high fiber diets (56 vs. 257 g DF/kg dry matter) were studied in 6-week balance experiments. The rats in all groups were offered the same amount of air-dried food. Indirect calorimetry was used to measure the energy metabolism. The difference in heat production (HP) calculated by RQ and CN methods was < 2% and was not affected by environmental temperature and DF. The relation between fat and protein oxidation changed from 1.54 to 1.00 when the ambient temperature changed from 16°C to 32°C. The contribution of carbohydrate oxidation to total HP was lowest at 16°C, whereas the fat and protein oxidation was highest at 16°C. The oxidation of nutrients was not influenced by DF. The additional energy retained at the higher temperatures had a constant ratio between fat and protein, i.e., 70:30. At an energy retention of 65.8 kJ/kg0.75d or less, body fat is mobilized and only protein deposited. Because of higher HP, rats living in the cold environment used more fat as substrate for HP than rats kept in warmer environments. The cold environment results in an increase in the amount of interscapular brown adipose tissue, but no significant difference was found between DF levels.