Drinking more than one soft drink daily -- even if it's diet pop -- seems to be associated with an increase in the risk factors for heart disease, finds a new study in Circulation: Journal of the American Heart Association.

U.S. researchers studied nearly 9,000 middle-aged men and women over four years and found that, compared to participants who drank less than one soft drink daily, those who said they drank one pop or more per day had:

  • A 31 per cent greater chance of developing obesity,
  • A 30 per cent increased risk for gaining inches around the waist,
  • A 25 per cent chance of developing high blood sugar levels
  • A 32 per cent greater chance of developing lower "good" cholesterol (HDL) levels.

Dr. Ramachandran Vasan, a professor of medicine at Boston University School of Medicine who participated in the study, notes that the research team adjusted their analyses for total caloric intake, saturated fat and trans fat intake, dietary fibre consumption, smoking and physical activity. It is not clear how they adjusted for these important factors.

They still observed a strong association between soft drink consumption and the risk of developing those factors that put one at risk for heart disease.

The fact that artificially sweetened diet drinks were also associated with these risks puzzles the doctors.

"We were struck by the fact that it didn't matter whether it was a diet or regular soda that participants consumed, the association with increased risk was present," said Vasan.

The researchers offer a few possible explanations for the results. One is that diet soda drinkers may eat more at other times. However, the study controlled for dietary fat and still found an association with soda drinking and heart disease risk factors.

Another possible explanation is that if you drink a large amount of liquids at a meal, you are more likely to eat a larger amount of food at the next meal, compared to what you would eat had you consumed more solids at the prior meal.

Other theories are that the high sweetness of diet and regular soft drinks make a person more prone to eat sweet items.

"These are all theories, and experts debate their importance," said Dr. Ravi Dhingra, lead author of the study and an instructor in medicine at Harvard Medical School.

Dhingra points out that they can't say definitely that soda drinking is causing the increase in risk; they simply noted an association.

"Our study was observational, and so right now all we demonstrate is an association. We have not proven causality," he said.

They say more study is needed.

The American Beverage Association is dismissing the study, noting that it is "scientifically implausible" to suggest that diet soft drinks -- a beverage that they note is 99 per cent water -- cause weight gain or elevated blood pressure.

"This study doesn't prove any link between soft drinks and increased risk of heart disease. Its assertions defy the existing body of scientific evidence, as well as common sense. Even the researchers acknowledge that their study can't support a link," said ABA president Susan K. Neely in a statement.

They say all the study does is underscore the need for moderation in our diets.

"All of our industry's beverages -- including regular or diet soft drinks -- can be part of a healthy way of life when consumed in moderation and as part of a balanced lifestyle," the industry group says in a statement.