Obesity is becoming a worldwide problem, even in developing countries. But diet drinks and food prepared with sugar substitutes do not seem to help curb people's appetites. Now a new study may provide some clues about the ineffectiveness of artificial sweeteners. VOA's Melinda Smith has details.

In the last four decades -- as cities grew larger and worldwide income rose -- the average diet around the world increased by at least 74 calories. A University of North Carolina study credited this caloric boost to the growing consumption of sugar and sugar substitutes. In that same period, U.S. consumption of sugar-free foods nearly doubled.

Whether you call them calorie free sweeteners, artificial sweeteners or sugar substitutes, many of us thought this tasty replacement was as good as the real thing.

Apparently we were wrong, says Professor Susan Swithers of Purdue University. She says, "It's temping to think that by simply consuming a food that has fewer calories, that body weight gain and food intake are automatically going to go down. Our data suggest that, in fact, the opposite might happen."

A Purdue University study was done on two groups of rats. One group was given artificially sweetened yogurt and the other group was given yogurt with glucose, a natural sweetener high in calories. But rats given the artificial sweetener gained 20 percent more weight.

The researchers say the artificial sweetener somehow interrupts the body's ability to regulate or register the amount of calories it has consumed. As a result, metabolism slows down and does not burn as many calories.

Professor Swithers explains. "When they got a sweet-tasting food that didn't deliver those calories, they went and then overindulged in their regular food as a consequence."

Artificially sweetened soft drinks and fruit drinks make up a big share of the American diet. A medical consultant for the U.S. beverage industry, John Foreyt, says it is wrong to compare behavior of rats and people. "The bottom line is that we are not rats. You cannot extrapolate a study with rats to humans."

But a survey of people in 2005 found results similar to the Purdue study on rats. It showed a 41 percent weight gain among people who drank diet soft drinks. The Purdue researchers caution that switching to food and drink prepared with real sugar is still no substitute for cutting back on portions and filling up on exercise.