Adjustment in living systems: The biological model
To cater for the dynamic nature of energetics in living systems, a second phase of thinking in the field incorporated rates of change, i.e:
Rate of change of energy stores= Rate of energy intake (EI) – Rate of energy expenditure (EE)
The use of ‘rates’ in this equation allows for changes with time and hence allows for the effects of changing body mass on EE and EI. This would mean that a small change in energy balance (e.g. an increase in food) would not lead to a large increase in body fat because as body mass increases, there will be an increase in EE (through increased metabolic rate and the increased energy costs of moving a bigger body around). This will then balance the increased energy intake and the individual would thus again be in energy balance, although at a slightly higher body weight. Using this approach, it has been calculated that the extra slice of toast for the man mentioned above would only add around 2.7kg of weight over 40 years, which is clearly a much more realistic figure. This approach highlights the need to look for the cause of a chronic imbalance between energy intake and expenditure, rather than small, absolute differences of either one (which is the ‘calorie counting’ approach). However, this model still offers no clues about what causes a chronic energy imbalance in the first place. For that, a further explanation became available in the 1980s.
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