A huge U.S. study of middle-aged adults has found that drinking more than one soft drink a day — even a sugar-free diet brand — may be associated with an elevated risk for metabolic syndrome, a cluster of factors that significantly boosts the chance of having a heart attack or stroke and developing diabetes.

"We found that one or more sodas per day increases your risk of new-onset metabolic syndrome by about 45 per cent, and it did not seem to matter if it was regular or diet," Dr. Ramachandran Vasan, senior investigator for the Framingham Heart Study, said Monday from Boston.

'Before we were saying take the diet [drink] and you're OK. Now we're saying: "Watch it." ' —Dr. David Jenkins, St. Michael's Hospital

Because the corn syrup that sweetens most regular soft drinks can cause weight gain and lead to insulin resistance and diabetes, "you would expect to see an association with regular soft drinks — but not diet soft drinks," he said. "Our findings suggest that this is not the case."

Metabolic syndrome is associated with five specific health indicators: excess abdominal fat; high blood sugar; high triglycerides; low levels of the good cholesterol HDL; and elevated blood pressure.

The study included nearly 9,000 observations of middle-aged men and women over four years at three different times. The study looked at how many 355-millilitre cans of cola or other soft drinks a participant consumed each day.

The researchers found that compared to those who drank less than one can per day, subjects who downed one or more soft drinks daily had a:

  • 31 per cent greater risk of becoming obese (with a body mass index of 30 or more).
  • 30 per cent increased risk of adding on belly fat.
  • 25 per cent higher risk of developing high blood triglycerides or high blood sugar.
  • 32 per cent higher risk of having low HDL levels.

But Vasan and his colleagues, whose study was published Monday in Circulation: Journal of the American Heart Association, are unsure what it is about soft drinks that ratchets up the risk of metabolic syndrome.

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Researchers have found that drinking one or more soft drinks a day — even a sugar-free diet brand — may boost a person's risk of becoming obese. ((Mark Lennihan/Associated Press))

"We really don't know," he said. "This soda consumption may be a marker for a particular dietary pattern or lifestyle. Individuals who drink one or more sodas per day tend to be people who have greater caloric intake. They tend to have more of saturated fats and trans fats in their diet, they tend to be more sedentary, they seem to have lower consumption of fibre."

Soft drink consumption declining in Canada

While soft drink consumption is declining in Canada, statistics from 2006 showed that Canadians overall still gulp down an average of 85 litres each per year.

Dr. David Jenkins, director of the Risk Factor Modification Centre at St. Michael's Hospital in Toronto, said previous studies have suggested that diet pops did not have the same effects on weight and health as do naturally sweetened soft drinks.

"The unusual thing that needs comment is they [the study's authors] say that the diet colas are the same as the calorically sweetened colas," said Jenkins. "So I think that is the piece that they've put into this puzzle.… I think we need a lot more scrutiny of that."

Jenkins said he believes that high consumption of soft drinks likely goes along with eating a high-calorie diet.

"I think the disappointing thing is if you thought you were doing [yourself] a major service … by taking diet drinks, this is not helping you," he said. "Before we were saying take the diet [drink] and you're OK. Now we're saying: 'Watch it.' "

Caramel, used to colour colas, is an ingredient that goes through a chemical reaction that has been shown in studies to "be quite toxic," said Jenkins. "It's possible that [such products] increase insulin resistance and cause oxidative stress and damage and all the other things we don't want."