Energy in vs energy out
Although the rise in obesity is due to many factors, the root cause is the imbalance between energy in (through the food choices we make) and energy out (mainly through physical activity). A lot of attention has been paid to the types of food we are eating, but data shows that average energy intake may actually be declining. This suggests that physical activity must have been declining even faster. And walking has been declining the most. Because of cars.
Walk, don't drive
As car ownership increases, many journeys that would once have been made on foot are being replaced by car travel. This has been going on ever since mass car ownership got under way in the 1960s. Each year, a greater proportion of the time spent travelling is by car, and the distance travelled on foot declines.
A study published in August 2007 showed that main car drivers walk only half the distance and for half the time of adults in non-car owning households. This equates to a deficit of 56 minutes of walking every week for these drivers, relative to adults in non-car households. As the authors of the study say,
"Over a decade we calculate that this could lead to a weight gain of more than 2 stones...for most obese adults their weight gain has accumulated over many years. Small energy imbalances on a daily basis can lead to major weight gain over a decade and more, and our calculations indicate that this decline in walking is in itself enough to account for much if not all of the recently observed upsurge in obesity."
Source: Adrian Davis, Michael Fergusson & Carolina Valsecchi, Unfit for Purpose: How car use fuels climate change and obesity (London, Institute for European Environmental Policy, August 2007)