Obesity and physical activity.

This paper discusses the epidemiological evidence linking obesity to physical activity. The underlying plausible hypothesis is that the feedback from energy expenditure to appetite may be weak at low levels of physical activity and that sedentary lifestyles therefore favour positive energy balance and weight gain. Obesity is widespread in developed countries and appears to have a marked secular trend. An analysis of time-budget surveys reveals that the time required for earning a living and domestic work has declined appreciably over recent decades. This negative secular trend is associated with a substantial decline in the energy spent on these activities. The contraction of work time has resulted in a converse expansion of free time, but the bulk of this is spent on passive leisure. Thus, at least for western societies, the overall energy expenditure has fallen for some decades and lifestyles have become increasingly more sedentary. The review of a large data set on energy expenditure under free-living conditions indicates that, despite their phenomenally diverse rates of obesity, there is no systematic difference between developed and developing societies. Multivariate regression analysis of body mass index on physical activity level (PAL) reveals a weak but statistically significant inverse relationship in men but not in women, and establishes that the risk of obesity increases sharply at a PAL of less than 1.80. In conclusion, a critical level of PAL has been identified, below which the chances of being overweight become substantial. The use of time is modelled contextually with its energy cost to show the extent to which energy expenditure may be modified. This has relevance from a policy standpoint, allowing a more focused approach for obesity prevention.