We do not eat the same foods every day and we do not perform the same physical activity every day. Because of the mismatch between energy intake and energy expenditure, it might be expected that small and constant fluctuations in body weight occur over time. This is true, but the changes are much smaller than might be expected through calculation of the energy intakes and energy expenditures, and this relates to the complex set of homeostatic mechanisms that most complex animals, including humans, possess. These mechanisms make allowances for changes to energy balance through small and frequent adjustments to appetite and energy expenditure in response to body weight changes. The feedback mechanisms for this process are complex and involved both neuronal and hormonal signals, many of which provide their afferent signal to the hypothalamus of the brain. Here micro adjustments in resting energy expenditure, appetite or physical activity levels can be made to match energy intake to energy expenditure.
Larger than normal fluctuations in short term body weight may be a signal that normal body weight regulatory mechanisms are beginning to degrade. Large fluctuations in short term body weight therefore predict 12 month weight gain.
Therefore over time, the body weight of most individuals remains stable within a few pounds of body mass. However, the obesity disease is characterised by a breakdown of these control mechanisms such that resting metabolic rate, appetite, catecholamine release, fuel utilisation, substrate availability along with many other regulatory mechanisms. This leads to an inability to efficiently control body weight, and this results in larger fluctuations in short term body weight. Researchers have assessed the short term changes in body mass in a group of individuals, and found that following these changes over a 6 month period was a reliable way to predict weight gain after 12 months. In this respect, those individuals with the largest fluctuations in body weight had the greatest risk of weight gain at 12 months. The authors concluded that the heightened weight variability was likely a signal that normal regulatory mechanisms for body weight had started to degrade in those individuals that showed the most variable body weights.
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