New research is raising fresh worries about diet soft drinks, noting that people who drink them every day have a higher risk for strokes and heart attacks compared to those who drink no pop at all.

But the researchers are quick to point out that their study does not prove that diet soft drinks cause heart attacks or strokes. They note there could be other aspects about diet pop drinkers that accounts for the increased risk that they observed.

The research was presented this week at the American Stroke Association's International Stroke Conference 2011. It found that people who drank diet pop every day had a 48 per cent higher risk of stroke or heart attack than people who said they never drank the stuff.

The study's lead researcher, Hannah Gardener, an epidemiologist at the University of Miami, says she has no idea why diet soft drinks could be risky.

It may be that people who drink lots of diet pop also tend to have a poor lifestyle that raises their stroke risk. That poor lifestyle could include smoking, drinking too much alcohol, not exercising enough or having high blood pressure and smoking.

However, the researchers tried to take into account these known stroke risk factors. And yet, they still didn't see a change in the link between drinking diet pop and increased stroke risk.

"If our results are confirmed with future studies, then it would suggest that diet soda may not be the optimal substitute for sugar-sweetened beverages for protection against vascular outcomes," Gardener said in a news release.

The study involved more than 2,500 people over the age of 40 who took part in the large, multi-ethnic Northern Manhattan Study (NOMAS).

Researchers asked the study subjects to report how much and what kind of pop they drank and about other aspects of their lifestyle.

They then grouped the subjects into seven categories:

  • no soda (meaning less than one soda of any kind per month)
  • moderate regular soda only (between one per month and six per week)
  • daily regular soda (at least one per day)
  • moderate diet soda only
  • daily diet soda only
  • moderate diet and any regular
  • daily diet with any regular

In all, there were 116 daily diet pop drinkers and 901 people who drank no diet soft drinks at all.

Over the course of nearly 10 years, there were 559 "vascular events" among the group -- including hemorrhagic strokes, the more common ischemic stroke, and heart attacks. Among them, 338 were fatal.

The researchers calculated that people who drank diet pop every day had a 61 per cent higher risk of a vascular event than those who reported drinking no pop. After they adjusted the findings to account for patients' diabetes, waistline size and other lifestyle differences, the increased risk persisted, leaving diet pop drinkers at a 48 per cent increased risk.

The study did not control for body mass index, a family history of heart disease, or cholesterol levels, all of which can increase heart attack or stroke risk.

No significant differences in risk were seen among people who drank a mix of diet and regular soda.

It's not clear why diet pop drinkers would have a higher risk of stroke and other events, but previous research has suggested diet soft drinks can promote a "sweet tooth."

A study published in 2007 by researchers from the University of Alberta found that young animals that became used to diet drinks and articifically sweetened foods tended to overeat during meals of regular-calorie food.

The scientists in that study suspected that diet foods disrupted the animals' ability to learn how various flavours correlate with calories and would then overeat when faced with "regular" food.

Still, Gardener stressed that the study only observed a link and was not strong enough to prove that diet soft drinks are risky. It should also be noted that the study relied on participants filling out self-reported questionnaires – and self reports are often not reliable.

As well, participants filled out the survey at the beginning of the study but not throughout the study, so the findings did not account for possible changes to diet pop drinking habits.

Dr. Maureen Storey, senior vice-president of science policy for the American Beverage Association, said in a statement that there is no evidence "that diet soda uniquely causes increased risk of vascular events or stroke."

"The body of scientific evidence does show that diet soft drinks can be a useful weight management tool, a position supported by the American Dietetic Association. Thus, to suggest that they are harmful with no credible evidence does a disservice to those trying to lose weight or maintain a healthy weight," she said.