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There has been a marked increase in body weight across much of the developed world. This has taken place, even though data suggest that there has not been an increase in calories consumed. This leads to a puzzle. If calories are declining, why are people gaining weight?

Changes in the nature of work and leisure, housework and other activities have led to substantial reductions in the strenuousness of daily life. In ongoing work, we are investigating how changes in purchased foods correspond to changes in time use and the strenuousness of activities. It appears that weight gain has resulted from a faster decrease in activity levels than in calories consumed, leading to an excess of calories.

The aim of this research is to help inform policy by increasing our understanding of the factors that have driven the rapid rise in obesity. The results do not say that food is not a problem, but that we need to consider both - calories ingested and calories expended.