Years ago white bread was, well, the greatest thing since sliced bread. The wealthy ate white bread, while everyone else ate the dark, dense, chewy bread.

But today, white bread's refined character is seen as a drawback. Not only does it contain fewer nutrients and less fiber than whole grain bread, it appears to be far more likely to make you obese. So you may want to think twice before reaching for another slice.

White bread starts to break down and taste sweet almost as soon as you begin eating it, leaving you wanting more.

Research presented at the European Congress on Obesity found that people who ate two or more pieces of white bread a day were much more likely to be overweight or obese compared to those who ate less than one slice of white bread a week.

The study followed over 9,000 Spanish graduate students for five years. While total bread intake wasn’t linked to obesity, the students who ate only white bread, and had two or more pieces of it a day, were 40 percent more likely to be overweight or obese after five years.

Bread itself wasn’t the problem because students who ate both whole grain and white bread didn’t have a higher risk of weight gain. Neither did those who ate only whole grain bread.

So what’s the problem with white bread?

Professor Miguel Martinez-Gonzalez, lead author of the study, explains that white bread starts to break down and taste sweet almost as soon as you begin eating it, leaving you wanting more. He compares it to the consumption of soft drinks.

On the other hand, whole grain breads are made up of more complex carbohydrates containing fiber which take longer to break down and be absorbed by the body. He believes whole grain bread can protect against obesity.

You don't have to quit eating white bread entirely. The best take-away message from this study is to eat more whole grains. The Dietary Guidelines for Americans suggest, “Make at least half of your grains whole grains,” and that’s not so hard to do.

Keep in mind that because these findings were delivered at a meeting, they have not yet been peer-reviewed in preparation for publication in a medical journal and are considered preliminary. While the results clearly suggest an association between eating too much white bread and obesity, they do not show a cause and effect relationship.