Can Eliminating a Can of Soda a Day Keep You From Getting Fat?

Kevin Drum ponders an ad claiming that a single sugar-laden soda a day will make you gain ten pounds in a year:

Now, there are a bunch of things you might say about this right from the start. Maybe governments shouldn't be in the business of running nanny state ads about personal nutrition. Maybe this particular ad was disgusting and shouldn't have been released. Maybe obesity isn't really that big a deal in the first place. But those weren't the issues at stake. Rather, it was this single sentence in the ad:
Drinking 1 can of soda a day can make you 10 pounds fatter a year.
What, I thought, could be wrong with that? A can of sugared soda contains about 150 calories, and adding 150 calories a day to your diet would almost certainly produce a ten-pound weight gain over the course of a year or so. There are some caveats, of course:

    • If you cut out 150 calories elsewhere, you won't gain any weight.
    • Your exact weight gain will depend on your age, current weight, etc.
    • If you have a miracle metabolism, you might not gain any weight at all.

This all seems pretty obvious, and while you'd probably mention it in a longer piece, it hardly seems necessary in a 30-second spot. But it turns out the scientists, especially Michael Rosenbaum of Columbia, seemed to think it should all be included. The ad, he said, was "misleading in that there is no reference to energy output changes."

I think there's a subtler problem with the ad.  It treats the soda consumption as completely unrelated to your metabolism, your level of exercise, and so forth. But this is not really the state of the art thinking on calorie consumption.  The "input/output" model of obesity is correct at some level, but it's far too simple.  If you are consistently over your calorie needs by as little as 100 calories a day--the number of calories in a largish slice of bread--you'll gain ten pounds in a year.  If you're under your calorie intake by as little as 100 calories, you'll lose ten pounds in a year.  If adding or subtracting calories like that could actually mechanically produce weight gain or loss, we'd all be obese, or starving to death.

So why doesn't this happen to more people?  We understand it intuitively on the starvation side--if you accidentally go under your calorie count one day, your appetite compensates by making you hungrier.  If you do it too many days in a row, your metabolism slows down.

What most people don't appreciate is that the same thing happens on the other side.  That's probably because in our culture, most of us would like to be eating a little bit less than we are.  But experiments with students and prisoners who deliberately set out to gain weight show that appetite decreases, and metabolism increases, if people consistently overeat.  That's why most of us stay in a relatively stable band, rather than seeing our weight fluctuate wildly.  Gina Kolata argues that most people have a pretty narrow band of weight that their bodies are comfortable in--about 10-30 pounds.  Stay at the low end, and you'll feel a bit hungry; get to the high end, and you may lose some of your interest in food.  But it's actually very hard to bust out of that band on either side.

So while adding a can of soda to your diet and changing absolutely nothing else would indeed make you gain ten pounds, the act of consuming those calories will change other things.  It may supress your appetite for other foods, or your body may boost your metabolism a bit to compensate.

Now, there are some mitigating counterarguments--some people think that soda calories are different from other sorts of calories, because the body doesn't respond to liquid calories the same way as it does to normal foods that come packed with some fiber and fat.  But that's not really the argument the ad is making.  In fact, it suggests drinking low-fat milk instead, which has about the same number of calories as soda.  To be sure, those calories come bundled with things like calcium, which are good for you--but if the 140 calories from a can of Coke are going to make you fat, so will the 120 calories in contained in the same amount of lowfat milk.