Monday, August 23, 2010

Walking and cycling to work prevents obesity?


I'll admit up front that I can't access the full text of the article Walking and Cycling to Health: A Comparative Analysis of City, State, and International Data, so I can't comment on methodology.

The study was large in that it looked at 14 countries, all 50 US states and 50 of the largest US cities for a relationship between active transportation (walking and cycling to work) and obesity.

The abstract (and the media) strongly suggest that walking and cycling to work helps to prevent obesity.

Now there's no doubt that a daily human powered commute, unless you happen to live next door to work, would in fact burn a significant number of annual calories and consequently could easily play a role in weight.

There's also no doubt in my mind that at least here in North America, it's a rather unique person who walks or cycles to work.

Anectdotally I can think of 4 people off the top of my head who without fail cycle to work, rain, shine, sleet, snow or hail.

They're all health nuts.

Whether or not all daily human powered commuters are health nuts isn't something I can claim, but I'd be willing to wager that the lifestyle habits of human powered North American commuters are dramatically different than the folks who rely on internal combustion engines (Europeans may be different as their built environments may make walking or cycling a more commonplace choice).

At the very least, controlling for the dietary choices of the self-propelled would be extremely important in the correlation/causation discussion, and while I can't with any certainty claim that the authors didn't control for diet, if they did, it'd have been a herculean feat given how many different geographic locations they studied.

But let's say that's not the case.

Let's say that in fact diet's not a player in these findings.

Now let's look at Great Britain.

According to the Journal of Physical Activity and Health, nearly 40% of folks in Great Britain walk or cycle to work as you can see in that graph up above. According to that same graph, obesity rates in Great Britain are up at roughly the 22% mark.

Australia and Canada, both with rates of obesity nearly identical to Great Britain's, show active transport rates 1/4 and 1/2 of Great Britain's respectively. Shouldn't Canadians and Australians be markedly heavier (or Brits markedly lighter)?

Ok, so a couple of outliers don't disprove everything do they?

Fine. Well how about time?

Wouldn't rate of change over time matter more than absolute rates when it comes to a discussion of obesity rates and the impact of an intervention that presumably had been there as part of the culture, prior to the most recent sets of measurements? What I'm getting at is that if active transportation is indeed an effective vaccine against an increasingly obesogenic environment, then perhaps we might expect to see not only lower rates of obesity, but also slower rises in those rates in countries where there is more active transportation.

Looking at Great Britain again, according to the BBC, obesity rates have nearly quadrupled over the course of the past 25 years whereas in the US, according to the CDC, over the past 25 years obesity rates haven't quite doubled. How is it that despite all this active transportation Great Britain's obesity rates are rising nearly twice as fast?

Correlation doesn't prove causation which is precisely why I'm not getting excited about this study and also why I'm not trying to claim that active transportation actually raises the rate at which countries gain weight.

My take on all of this?

Comparing between countries isn't likely to be helpful as countries have widely divergent food and eating environments which if not controlled for, would make interpretation of results very challenging.

Comparing within countries (cities and states) isn't likely to be helpful as folks who cycle to work likely live far healthier lifestyles than those who don't and would consequently have a multitude of factors that would contribute to their being slimmer than their driving neighbours.

But maybe I'm making a fool of myself with this post in that the authors did explore all of these issues and really, walking and cycling to work are crucial. Don't think I'll lose sleep tonight worrying about that.

Bottom line for me. Walking or cycling to work is an exceedingly healthful thing to do, and would in fact likely help you to manage your weight, but increasing bike lanes and walking paths aren't likely interventions that're going to impact a whole population, they're ones that might impact a small, already health conscious subset.

Pucher, J., Buehler, R., Bassett, D., & Dannenberg, A. (2010). Walking and Cycling to Health: A Comparative Analysis of City, State, and International Data American Journal of Public Health DOI: 10.2105/AJPH.2009.189324

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  1. I just took a quick readthrough of the paper. They did not take into account nutrition, etc. From the conclusions of the article:

    "Another limitation of our analysis was the inability to control for other factors affecting physical activity levels, obesity, and diabetes. In particular, no comparable city, state, and international data on nutrition (e.g., caloric intake) or genetics (e.g., family medical history) were available for inclusion in the analysis. Finally, small sample sizes and unavailability of data for control variables restricted the statistical analysis to graphs, correlations, and bivariate regressions. In an analysis of only 47 US cities, 50 US states, and 14 countries, incorporating many control variables would have been difficult, even if the data were available. Our results should therefore be interpreted with caution."

  2. Thanks Justin.

    Caution indeed.

  3. Scott Stubbe11:23 am

    It is true that having infrastructure that is more bike and walking friendly will not automatically encourage more healthy behaviors. However it is equally true that a lack of safe and convenient infrastructure will definitely deter such activity, and in my travels in North America I do have to say there I have seen much more of the latter than the former.

    You make the conclusion of your article seem to sound like you believe that only die-hard every day of the year human powered commuters would see a positive effect. Would not those who only skip driving in good weather also see health benefits? It certainly could be easily argued that those are the people who are the most deterred by poor infrastructure. The possibility here is that tens or even hundreds of thousands of people could potentially reap many health benefits from the diversion of a percent or two more of local and national transportation budgets. Is that not a worthy goal?

    It is true that there is no guarantee that people will take full advantage of this opportunity. But it is also equally true that not everybody takes advantage of other anti-obesity treatments they receive, including surgeries? If the will to change is not there, change will not happen. But the more obstacles there are, the less likely the will to change will persevere.

    It is true many factors were not considered. But nothing is ever truly black and white. But the physics are indisputable, more activity = more calories burned. I agree there is not enough cynicism for the "results" of such studies. However in this case perhaps you expressed a bit too much.

  4. Bethany11:32 am

    Last summer I cycled to work 3-5 days a week. 1/2 hour one way. I did lose some weight. And I was working in a bakery! More important than losing the weight, though, was the sense of body awareness and the pleasure I took in that part of the day. At the end of the summer I was stronger and felt healthier and more confident in my body and what it can do. This led to me walking and running, and I am still cycling for transportation.

    Cycling in particular gives me great joy, which makes me a happier person, which benefits everyone! It also lets me leave the car behind much of the time, which is great for the environment, and for my wallet.

  5. Anonymous6:35 pm

    I work about 9 miles from home, and have thought many times about riding to work. However, since its 9 miles of blind corners, no shoulder and a 50mph road that everyone goes 60mph, theres no way that I will head down that road on my bike. The alternative route is a 12 mile dirt road with an equal amount of speeders and blind corners.

    I would love to see an alternative route, but as the two routes go through a relatively narrow valley, I see no way that this is going to happen.

    I think this is probably a common issue in North America. Our infrastructure was developed for driving, and driving fast. We tend to work further from where we live (a 45 minute to hour commute seems to be the norm). We're too spread out to make bike commutes really practical for everyone, let alone walking commutes.

  6. Anonymous2:50 am

    one explanation would be that walking or cycling is not the cause of better weight control allone but an indicator of a healthier lifestyle alltogether of just a less toxic environment (surrogate marker)
    people living in an environment that allows safe cycling and walking to work or school will most probably also shop for healthier foods and care more about nutrition

  7. Anonymous10:37 am

    It's chicken-and-egg. I lost 60 lbs; regular cycle commuting was part of that, and has also been significant in maintaining that loss. My body became more athletic, and I wanted to fuel it properly for better performance.

    I tend not to ride when there's snow on the ground (though I see lots of people bking year-round), and I notice it's much more of a struggle in winter to maintain my weight when I have to spend 2 hours each day on the bus rather than my bike.

    There is no doubt in my mind that lifestyle fitness promotes overall health. Multi-use paths and bike lanes are worth the investment.

  8. Anonymous5:03 pm

    When I lived in Vancouver (nice weather) after my knee surgery (5 years of waiting and wait gain), I started walking home from work (3 kms.) It was the best exercise for me: A tread mill I could turn off, but when I was only 1/2 way home, I still had to get the rest of the way! But now that I live in Edmonton, a car-friendly city where it can be 40 below for months, walking is not such an easy prospect. Cities need to be designed to be more pedestrian friendly if they want us to get out and walk more.