Physiological adjustment. It has been recognised since the 1980s that overfeeding does not always result in the weight gain predicted by the physics energy balance equation. This is because in humans, the body defends itself against an imbalance between EI and EE by adjusting, within limits, the one to the other. Adjustment refers to the metabolic, physiological and behavioural changes that occur when the equilibrium, or ’settling point’ is altered by a positive or negative energy imbalance. The adjustments tend to oppose the energy imbalance to minimise the effect on body weight changes. This helps to explain why, with such huge variations in energy intake and energy expenditure over a year, there are normally only minor fluctuations in body weight in most people. It also helps to explain the plateaus and rebounds during a fat loss program.

The popular belief (based on the physics model presented above) is that fat or weight loss can be calculated in a linear fashion from the calorie deficit. However, reducing the amount of food eaten over a set period by a total of 3500kcals, will not result in an automatic loss of 0.45kg of fat, which would be predicted as the storage equivalent of 3500kcal. Adjustment in energy expenditure through a decrease in metabolic rate in particular, would mean that the reduction in body stores is much less than expected. The opposite also holds true for a positive energy balance. When more is eaten than is used up as energy, metabolic rate increases as body mass increases and again, the fat gain will be less than predicted.

It is this physiological adjustment which has been typically under-estimated in discussions of weight control, although a range of research has now identified it as a key factor in the long term failure of many programs. Adjustment is a biological protective mechanism which prevents excessive long term gains and losses in body energy stores. It is more effective in some people than others (perhaps partly genetically determined). It may also be more strenuously invoked if the energy deficit and weight loss are large.

The short term adjustments are mainly through appetite control, although changes in the thermic effects of food (TEF) are directly related to meal size. Lethargy can be a marked feature of low energy intakes. In the longer term as fat mass and FFM decrease so do RMR and fat burning (higher RQ). Often the changes are greater than expected and this is called adaptation. For example, the fall in RMR may be much greater than expected for the loss of FFM. The adaptive changes are a sign of active mechanisms in the body opposing major shifts in body weight.

Myth-information. Arguments about the benefits of butter or margarine are largely irrelevant when considering their use in fat loss. Both have equivalent fat energy value (i.e. 9 kcal/g).

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