Etiology of obesity: excess intake or decreased physical activity
Stated simply, obesity is the end-result of positive energy balance, or an increased energy intake relative to expenditure. It is often stated, or assumed, that obesity is simply the result of overeating or lack of physical activity. However, the etiology of obesity is not as simple as this, and many complex and interre-lated factors are likely to contribute to the develop-ment of obesity; it is extremely unlikely that any single factor causes obesity. Many cultural, behav-ioral, and biological factors drive energy intake and energy expenditure, and contribute to the homeo-static regulation of body energy stores, as discussed earlier. In addition, many of these factors are influenced by individual susceptibility, which may be driven by genetic, cultural, and hor-monal factors. Obesity may develop very gradually over time, such that the actual energy imbalance is negligible and undetectable.
Although there are genetic influences on the various components of body-weight regulation, and a major portion of individual differences in body weight can be explained by genetic differences, it seems unlikely that the increased global prevalence of obesity has been driven by a dramatic change in the gene pool. It is more likely and more reasonable that acute changes in behavior and environment have contributed to the rapid increase in obesity, and genetic factors may be important in the differing individual susceptibilities to these changes. The most striking behavioral changes that have occurred have been an increased reliance on high-fat and energy-dense fast foods, with larger portion sizes, coupled with an ever-increasing seden-tary lifestyle. The more sedentary lifestyle is due to an increased reliance on technology and labor-saving devices, which has reduced the need for physical activ-ity for everyday activities. Examples of energy-saving devices are:
● increased use of automated transport rather than walking or cycling
● central heating and the use of automated equip-ment in the household, e.g., washing machines
● reduction in physical activity in the workplace due to computers, automated equipment, and elec-tronic mail, which all reduce the requirement for physical activity at work
●increased use of television and computers for enter-tainment and leisure activities
●use of elevators and escalators rather than using stairs
●increased fear of crime, which has reduced the like-lihood of playing outdoors
●poor urban planning, which does not provide adequate cycle lanes or even pavements in some communities.
Thus, the increasing prevalence, numerous health risks, and astounding economic costs of obesity clearly justify widespread efforts towards prevention.
The relationship between obesity and lifestyle factors reflects the principle of energy balance. Weight maintenance is the result of equivalent levels of energy intake and energy expenditure. Thus, a discrepancy between energy expenditure and energy intake de-pends on either food intake or energy expenditure, and it is becoming clear that physical activity provides the main source of plasticity in energy expenditure. In addition, lifestyle factors such as dietary and activ-ity patterns are clearly susceptible to behavioral mod-ification and are likely targets for obesity prevention programs. A second, yet related, reason that control of the obesity epidemic will depend on preventive action is that both the causes and health consequences of obesity begin early in life and track into adulthood. For example, both dietary and activity patterns responsible for the increasing prevalence of obesity are evident in childhood.