It is a truism that energy cannot be created or destroyed. Therefore it is of interest both clinically and in terms of basic science that increased energy intake (food) in man is not invariably accompanied by commensurate increases in body mass (Jequier and Tappy, 1999). The apparent simplicity of the energy balance equation e.g. , belies the complexity of the expenditure component (McMinn, 1984). Intake being easily measured as calories in food and drinks consumed, whereas the calorie cost of life is portioned between basal requirements, physical activity, post-prandial thermogenesis and thermoregulation (Joosen and Westerterp, 2006). In addition it is apparent that the engine of cellular respiration while efficient does not convert 100 per cent of dietary energy to either adenosine triphosphate (ATP) or body tissue (Heymsfield et al., 1995). Observations of energy balance in man therefore require an understanding of these components and an appreciation of the metabolic fate of dietary macronutrients. Observations of (relative) long-term weight stability in free living humans suggest strongly a set point of some sort, albeit one which frequently appears to be overridden (Levine, 2007). This defence of body weight can largely be explained through systems regulating the hunger-satiety cycle and feedback from energy (adipose) stores through circulating products of fat cells (e.g. leptin) providing proportional feedback to central centres involved in modulating feeding behaviour and energy expenditure (Flier and Maratos-Flier, 1998, El-Haschimi and Lehnert, 2003). It is clear, however, that inter-individual differences also occur and that qualitative aspects of dietary choice, variations in metabolic response to food ingestion and different inherited tendencies towards body composition can all influence energy balance in the whole person (Zheng et al., 2009). Dietary protein for example stimulates greater post prandial thermogenesis than fat or carbohydrate (Johnston et al., 2002), and metabolic rates vary between individuals and also within individuals according to body mass and composition.
Excerpt from my PhD thesis.