How much does it cost? Teaching physiology of energy metabolism in mice using an indirect calorimetry system in a practical course for veterinary students

In endothermic mammals total energy expenditure (EE) is composed of basal metabolic rate (BMR), energy spent for muscle activity, thermoregulation, any kind of production (such as milk, meat or egg production) and the thermic effect of feeding. The BMR is predominantly determined by body mass and the surface to volume ratio of the body. The EE can be quantified either by direct or indirect calorimetry. Direct calorimetry measures the rate of heat loss from the body, whereas indirect calorimetry measures oxygen consumption and carbon dioxide production and calculates heat production from oxidative nutrient combustion. A deep and sustainable understanding of EE in animals is crucial for veterinarians in order to properly calculate and evaluate feed rations, during special circumstances such as anaesthesia or in situations with increased energy demands as commonly seen in high yielding livestock. The practical class described in this manuscript provides an experimental approach to understand how EE can be measured and calculated by indirect calorimetry. Two important factors that affect the EE of animals (the thermic effect of feeding and the effect of ambient temperature) are measured. A profound knowledge about the energy requirements of animal life and its measurement is also relevant for education in general biology, animal and human physiology and nutrition. Therefore, this teaching unit can equally well be implemented in other areas of life sciences.

Circadian rhythms in resting metabolic rate account for apparent daily rhythms in thermic effect of food

Context Daily variation in thermic effect of food (TEF) is commonly reported and proposed as a contributing factor to weight gain with late eating. However underlying circadian variability in resting metabolic rate (RMR) is an overlooked factor when calculating TEF associated with eating at different times of the day. Objective To determine whether methodological approaches to calculating TEF contribute to the reported phenomena of daily variation in TEF. Design, Setting and Participants: Fourteen overweight to obese but otherwise healthy subjects, had their resting and postprandial energy expenditure measured over 15.5 hours at a clinical research unit. TEF was calculated for breakfast, lunch and dinner using standard methods (above a baseline and premeal RMR measure) and compared to a method incorporating a circadian RMR where RMR was derived from a sinusoid curve model and TEF was calculated over and above the continuously changing RMR. Main Outcome measures TEF at breakfast, lunch and dinner calculated with different methods. Results Standard methods of calculating TEF above a premeal measured RMR showed that morning TEF [60.8kcal ± 5.6] (mean ± SEM) was 1.6 times greater than TEF at lunch [36.3kcal ± 8.4], and 2.4 times greater than dinner TEF [25.2kcal ± 9.6] (p=0.022). However, adjusting for modelled circadian RMR nullified any differences between breakfast [54.1kcal ± 30.8], lunch [49.5kcal ± 29.4], and dinner [49.1kcal ± 25.7] (p=0.680). Conclusions Differences in TEF between morning and evening can be explained by underlying circadian resting energy expenditure, which is independent of an acute effect of eating.