Air date: August 7, 2011

Host: Jeremy Shere, PhD

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Among the greatest inventions in history, fat-free snack foods has to rank somewhere in the top 10. I mean, what’s better than being able to dig into a bag of potato chips without the guilt of consuming hundreds of grams of fattening fat?

Except that the stuff that replaces natural fat may end up making you fatter.

That’s according to Susie Swithers, a psychologist at Purdue University. In a recent experiment she put one group of rats on a diet that included fat free chips, and another that included regular potato chips.

"The basic finding of the study was that if rats were eating a high fat diet and we gave them access to potato chips that were made with a fat substitute, they ended up gaining more weight than rats that only had access to high fat, high calorie potato chips."

Which seems totally counter-intuitive, right? If fat free chips have less calories than regular chips, why would the fat free rats gain more weight?

"So we know that animals are terminating meals, they stop eating using signals that tell them something about what’s going to happen later. They include things like the release of hormones from the gut, from the intestines."

In other words, when animals are lunching on a high fat, high calorie snack, those hormones tell their bodies to expect lots of calories and to stop eating at a certain point. But fat substitutes can confuse things. Because the substitute tastes like fat, the body hunkers down for a big calorie load. But when it doesn’t come, the body’s natural response is to eat more.

That’s Swithers' theory, anyhow. Of course, it’s not clear that what happens in rats will necessarily happen in people. But the takeaway for us chronically overweight humans, Swithers says, is that eating low fat or fat free foods is no magic bullet. It doesn’t guarantee weight loss or eating less. And it might result in exactly the opposite.

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I’m Jeremy Shere.

Additional Resources:

• Read more on Swither's research in the Purdue University newsroom.