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Obesity: Diet vs. Physical Activity

Posted by tinako on November 8, 2010

Boyd Swinburn

This is the second podcast from the Yale Rudd Center interviewing Dr. Boyd Swinburn.  I’ll get right to it.

Dr. Kelly Brownell says that often people in their field (Public Health) claim that diet and exercise are equally important.  Even those who might disagree are disinclined to step on any toes, nutrition researchers pitted against physical activity researchers.  Brownell asks, is this an important debate?

Swinburn agrees that the debate has taken a long time to get going.  He states he’s a firm believer in physical activity’s importance both in obesity and other health impacts, but that doesn’t mean it’s contributing to the problem (or solution) equally with diet.  Of course this has very important implications for what’s driving the obesity epidemic and what the solutions might be.

Swinburn in his research has asked the question directly: How much of the rise in obesity over the last 30 years can be attributed to intake vs. physical activity?

Obesity took off in the developed countries all around the world, and even in developing countries, at about the same time, the ’70s and ’80s.  This means something at a global level changed quite quickly.  That argues quite strongly against changes in our genes, parenting styles, even in our physical activity environment, which is of course tightly linked to the built environment.  Most of us live in cities, and while cities do evolve, they don’t evolve that quickly or at the same time.  We also haven’t had a global collapse of willpower in the last 30 years [I criticized the book Fat Land which seemed to suggest this].  All these personal attributes don’t seem as though they could drive a sudden global epidemic.

So it has to be global, it has to be able to change quickly, and the obvious answer is the food supply.  Looking back over the last 30 years and thinking of the four Ps of marketing:

  • Product: They have changed drastically quantitatively and qualitatively: more variety, more energy-dense.
  • Promotion: It’s far more sophisticated, with marketing to children being of particular concern.
  • Placement: It’s everywhere.  Thirty years ago, food was in food shops; now it’s in almost any shop.
  • Price: The relative price of junk food has come down over time relative to other foods and to income.

These four Ps make a big push factor to influence our energy intake [as compared to “pull,” where people are demanding something, and which food manufacturers insist is the driving force behind the food environment].  It seems clear that energy intake is driving the obesity epidemic.

Brownell repeats Swinburn’s point: during the period of rapid increase in obesity, there have been profound changes in the food supply, but less in physical activity patterns.  Swinburn agreed; cars and TV have been around for a while, though use has increased.  Video games are new, walking to school  has declined, but they seem relatively small contributors to the overall energy balance; the food world has changed much more.

Brownell asks how we can know how much people are eating, and Swinburn’s answer is that he is quite skeptical of surveys that ask people what they eat (they clearly underestimate and that effect seems to have increased over time).  Therefore Swinburn puts more credence in USDA statistics which look at the food supply and come up with a per-capita usage across all countries.  This data shows that over the last 30 years, the food energy supply has increased dramatically.  People have come up with different equations for trying to estimate what percentage of the obesity epidemic is attributable to this food energy supply increase.  Using different equations, the answer is very similar – more than 100%.  That is, the food energy supply increase is more than enough to explain the obesity epidemic.  I’ll say it again, all of the increase in weight can be accounted for by the increase in food calories.

Swinburn talks about “push factor” again.  He says we like to think that we have free choice, but we are actually highly responsive to the environment.  For example, the commuter environment in Amsterdam is highly conducive to cycling, and most people do.  The environment in Atlanta is highly conducive to driving, and that’s what people do.  The same is true for the food environment.  If there’s a lot of food that’s tasty, that’s advertised, that’s low price, we’re going to eat it.  So the calories are being pushed, and we’re responding by eating them.

This all has enormous implications for prevention.  Physical activity interventions are likely to have very modest impacts – good for other areas of your health, but unlikely to have a big impact on weight.  Swinburn’s a big advocate of physical activity, but we need to reduce our expectations for what we can achieve.

The policies need to focus on the food side, but this is a highly charged issue.  Politicians don’t like to fight with industry, which is why government policies typically fall on the physical activity side – no powerful industry is opposed to it.  In order to successfully address obesity, we will have to address the food.

One Response to “Obesity: Diet vs. Physical Activity”

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