Animation of GPS-equipped pizza deliverers in Manhattan - from PBS's America Revealed (click for more)

We’re used to seeing lots of claims that suburban sprawl is a key cause of the obesity epidemic afflicting the nation’s children and youth.

Here’s a new study that contradicts the received wisdom. While it would be premature to regard it as definitive, it suggests the relationship between sprawl and physical activity might be more complex than is usually acknowledged.

As part of her doctoral research at Queens University in Ontario, epidemiologist Laura Seliske compared the physical activity of 12-17 year olds against the degree of sprawl in 33 Canadian metropolitan areas.

She got data on the level of physical activity of 7,017 young people who responded to the Canadian Community Health Survey in 2007-08. She developed an urban sprawl score for each metro based on dwelling density, proportion of the population living in the urban core, and the proportion of detached dwellings.

Using multilevel logistic regression analysis, she sought to determine whether urban sprawl was associated with active transportation (≥30 minutes/day); with moderate-to-vigorous physical activity (≥60 minutes/day); and with whether respondents were either overweight or obese.

There are three surprising findings from her research. She found:

• No relationship between the degree of sprawl and the extent to which young people are overweight or obese.
• Children aged 12-15 years who live in more sprawled cities are more likely to engage in moderate to vigorous physical activity than those living in more compact areas.
• Children aged 12-15 years who lived in more sprawled cities are also more likely to use active transport modes (i.e. walking, cycling, skating) as a means of transport.

The relationships are statistically significant but nevertheless modest. What’s unusual though is the absence of the strong positive relationship between density and physical activity that is commonly assumed.

Dr Seliske says her findings aren’t really unusual – they’re consistent with other research:

In contrast to what has been reported in adults from several countries, less compact urban areas may encourage physical activity in young people. Slater et al found that adolescents living in sprawling counties had higher rates of sports participation, with a 20 unit increase in the county-level urban sprawl score (approximately 1 SD) was associated with a 3.1% increase in sports participation. Furthermore, Mecredy et al found that Canadian youth exposed to less densely connected streets were 21% (95% CI: 9-34%) more likely to be active for at least 4 hours/week outside of school compared to those exposed to more densely connected streets.

A plausible explanation is low density suburbs provide a relatively safe environment in which teens can play, walk and cycle. Parents are more relaxed because there are fewer cars than in compact areas and they’re less worried about stranger danger.

Dr Seliske emphasises that these findings differ from those for adults, where many studies find lower levels of physical activity in more sprawled areas (although there’s another whole debate here about whether that’s due to the environment or the result of factors like self-selection). Her key point is the contradictory findings suggest that the mechanisms which explain the association vary by age.

That variation is illustrated by the difference in the level of physical activity of 12-15 year olds compared to 16-19 year olds. The latter age group are old enough to drive. Access to a car is better in more sprawled areas, leading to behaviours in this older group more like those of adults living in the same sorts of locations.

This is only one study and of course it’s got its limitations, not least the fact it’s based on another country. As always caution should be exercised in extrapolating to local circumstances.

Still, I find the argument that younger children and teens will be more physically active in a suburban location than in a denser area intuitively appealing. That’s certainly been my household’s experience in moving from the inner city to the suburbs (and I can testify a short cul-de-sac is brilliant for very young children – has to be short though).

A key downside of sprawl however is there’s a greater likelihood young persons will take up driving once they come of age.