Why exactly are so many people overweight?

The great thing about devoting 10 years of your life to studying nutrition and public health is that no matter how much you learn, everyone else seems to think they know at least as much as you do about it. Despite the $100,000 price tag on my degrees, apparently my opinion isn't actually any more valuable than anyone else's. Case in point: when I was doing my thesis, I worked with a high school food service director who loved to tell me his theories about the childhood obesity epidemic. When I said something about the food in school cafeterias, he would go on a tirade about how sedentary kids are these days and how much different it was back in the good ol' days when he was growing up doing manual labor on the farm (I kid you not). I nodded in feigned agreement, all the while thinking "no it's the darn french fries they're buying from cafeterias like yours."

I have had a lot of conversations like this. Among the "causes" that have received blame for the obesity epidemic are food preservatives, artificial sweeteners, hormones in meat, a dieting culture, and of course laziness and lack of self-control. Each of these (with the exceptions of the last two, which I do not think are factors) may in fact play some intermediary role, but the variables that ultimately affect weight can be narrowed down to just two: energy intake and energy expenditure (i.e. calories in and calories out). So which of those is responsible for the dramatic increase in obesity we've seen in recent decades? Well, the obvious answer is "both," certainly. But, it is unlikely that they both contribute exactly equally. So what is more important? Everyone has a different view, but if you want the evidence-based (hence the name of the blog) answer, it's......

...

...

...
...

...

CALORIE INTAKE

After deciding to post on this topic today, I discovered that I already covered it about a year ago (here). But, I figure, if I didn't remember it, probably you didn't either. So, here I am blogging about it again. In the original post, I referenced a really interesting study, but I didn't describe the results in detail. Today I will. You may or may not be interested in the details, depending on how much thinking you feel like doing right now, but here they are.

The study, by Swinburn et al. (2009), concluded that the increase in calorie intake in the U.S. since the 1970's can entirely explain the increase in obesity that occurred during the same time period. How did they figure this out? They used food supply data to determine how much more Americans are eating now than they did 30-40 years ago (about 350 calories/day for kids and 500 calories/day for adults). Then, they used that information to predict how much more people would weigh based on the increase in calories. If it turned out that people weighed more than that, that would suggest that a decrease in physical activity was responsible for some portion of the collective weight gain. However, if people weighed the same as what was predicted, that would mean that the extra calories did it.

The results: Children weighed almost exactly what was predicted, while adults weighed more than 5 lb. LESS. Assuming that the data and calculations are accurate (this assumption is open to criticism), this means that physical activity has stayed about the same for kids and increased for adults (the extra activity is what would account for adult weights being less than predicted). I have to admit, I find it hard to believe that the video-game-addicted, internet-connected kids of today get the same amount of exercise as the 1970's set and that adults today are more physically active. But, the study does make a strong case for diet being the primary, if not the only, culprit in the obesity epidemic.

It makes sense when you consider, as the principal investigator of the study noted:

"To return to the average weights of the 1970s, we would need to reverse the increased food intake of about 350 calories a day for children (about one can of fizzy drink and a small portion of French fries) and 500 calories a day for adults (about one large hamburger). Alternatively, we could achieve similar results by increasing physical activity by about 150 minutes a day of extra walking for children and 110 minutes for adults..."

Other ways to save 500 calories:

Have 2 slices of pizza instead of 4

Eat only half of your restaurant entree

Skip dessert when you go out to eat

Have 2 slices of toast with jam instead of a muffin with butter

Have 3 cans of diet soda instead of regular soda

Skip the rolls and butter with dinner

Have a low-fat ice cream sandwich instead of a large bowl of Ben & Jerry's

So what sounds easier to you? Doing one of the above every day? Or walking for an hour and half every day? I know what I'd rather do.