There is an association between physical activity and body weight. In this regard those that perform more physical activity have lower body weights. It is generally accepted that this association has the physical activity as the causative factor which implies that those who avoid physical activity get fat because they are lazy. However, I have argued that the cause and effect of this relationship have been misunderstood and the causal factor in the association is actually the presence of abdominal obesity which then leads to reductions in physical activity. The mechanisms for this is well described in the literature and involves the development of insulin resistance through consumption of a Western diet causing liver overload syndrome, the development of non-alcoholic fatty liver and the accumulation of abdominal adipose tissue and accompanying metabolic dysfunction, followed by leptin resistance. Once hypothalamus insensitivity to the leptin signal has developed, the weak signal is interpreted as a de facto state of starvation by the hypothalamus.
The counterregulatory energy conservation that results from this perceived starvation state causes a decrease in resting metabolic rate, the thermic effect of exercise and the thermic effect of food. The same mechanism also increases the appetite signal in order to create a positive energy balance. The lack of energy available to physical activity as a result of these mechanisms (perceived starvation) limits the force output from skeletal muscles that rely on the glycolytic or phosphocreatine system and this in turn limits intense physical activity and delays recovery from any exercise. It is not surprising that perceived starvation results in these energy conserving changes, because this is also the fate of those in the de facto starvation state. Observational studies have reported the association between physical activity and obesity, and the authors have often assumed because they are conditioned to do so, that the cause of the association is the physical activity. However, in such studies, there are often clues that can be used to argue against this contention.
For example, in one cross sectional study accelerometers were given to pregnant mothers who attached the devices to their children following birth1. The children wore the devices up to age of four years, and at this point the almost 400 children had their body composition analysed. The authors then looked for associations between body composition and the degree of movement recorded in the accelerometers. The results showed that vigorous physical activity was associated inversely with the degree of total adiposity, percentage body fat, fat mass index and abdominal obesity. However, low and moderate activity and sedentary behaviour were not associated with body composition. Therefore, only vigorous physical activity is associated with improved body composition in young children. This is interesting because if physical activity was the cause of the association between exercise and body weight, then you would expect sedentary behaviour to be associated with a worse body composition. However this was not the case.
The fact that sedentary behaviour was not found to be associated with body composition in this study is problematic for those who believe that body fat accumulates as a result of a simple positive energy balance. Clearly, it did not matter if the child was sedentary or mildly active as the weight, fat mass and lean mass of the children were the same. Only vigorous physical activity was associated with the child’s weight. This suggests that the cause of the association is not the amount of physical activity undertaken, and that the cause and effect of the correlation has been misunderstood. This was evident from the authors own conclusion that suggested that vigorous physical activity should be encouraged to prevent obesity. Of course the logical explanation that results from this data is that only those children without abdominal obesity, insulin resistance and leptin resistance were able to perform activity to more intense levels. That the abdominal obesity was the cause of the association also explains why sedentary behaviour and moderate activity was not associated with body composition.
RdB
