Not So (O)Lean: Eating Faux Fat Makes Rats Real Fat The fat substitute Olean may not be so diet-friendly, study finds.

June 21, 2011 -- Another diet myth bites the dust: Products containing the calorie-less fake fat Olean, of fat-free potato chips fame, may make you gain weight, not lose it.

In a new study released today by Purdue University, researchers found that rats who were fed Olean-containing potato chips as part of a high-fat diet ate more overall and gained more weight than those who were fed a high fat diet and regular, full-fat potato chips.

This counter-intuitive finding shakes the conventional wisdom that substituting lower calorie, lower fat foods for the full-fat versions will help reduce overall caloric intake and encourage weight loss.

"Fat substitutes can interfere with the body's ability to regulate what it eats, and that can result in overeating," said Susan Swithers, lead author and psychology professor at Purdue University.

But overeating may not be the only reason why fat substitutes make you pack on the pounds. Researchers suspect that fake fats actually mess with our body's ability to digest and metabolize food, making us more likely to retain weight from what we eat.

"Our bodies make predictions on what to prepare to digest based on taste and how food feels in our mouth," Swithers said. When something tastes sweet or fatty, our body gears up to digest a high density of calories, stimulating the metabolism and triggering a chain of hormonal secretions to process the fat, calories, and nutrients.

"When we get cues that something is fatty, but no calories arrive -- like with fat substitutes -- our body gets confused," Swithers said. "This confusion can make the body stop preparing to digest fatty food when it does come."

Olean is the brand name for Olestra, a calorieless, fat-free fat-substitute discovered accidentally by Procter & Gamble in 1968. Olestra was approved for use as a food additive in snack foods in 1996 and soon after became famous for its negative gastrointestinal side effects, including intense diarrhea and anal leakage.

High Fat Versus Low Fat Diets and Olestra

In the Purdue study, researchers put rats on either a high fat or low fat diet and allowed them to eat as much as they wanted. The rats were also fed a few grams of potato chips a day: Half the rats received only regular chips, while the other half got a mixture -- some days they got full fat, some days they got fat-free Olestra chips.

Researchers found that rats that ate Olestra-containing chips consumed more of their regular food and gained more weight than those rats that ate the higher-calorie, full-fat potato chips.

At first, this effect didn't seem to hold true for rats on a low-fat diet, until researchers gave these rats access to a high-fat chow. Even though all the rats had stopped being fed potato chips, fat-free or otherwise, those rats that had been consuming Olestra tended to eat more and put on more weight when they were later exposed to a high-fat diet.

This suggests that eating Olestra was changing the way the rats' bodies learned to respond to food, interfering with their natural ability to regulate how much fat was "enough."

"It goes against what you might think -- you remove calories from food and you'll lose weight, but at the end of the day the chemical manipulation of food leads to increased weight. We don't understand exactly why yet, but research continues to show this is true," ABC News Medical contributor Dr. Marie Savard said.

Swithers warned that while rat findings can't be directly applied to humans, rats have very similar biological responses to food as humans. The study was published today in the APA journal Behavioral Neuroscience.

Calls made to Procter & Gamble for comment were not immediately returned.