Several studies have found that people who regularly down diet soda are more likely than people who don't to have certain risk factors for those chronic diseases -- like high blood pressure and high blood sugar.
And one recent study became the first to link the beverages to the risk of actual heart attacks and strokes.
Still, researchers have not been able to say whether it's the sugar-free drinks, themselves, that deserve the blame.
Many factors separate diet- and regular-beverage drinkers -- and, for that matter, people who stick with water. Overall diet is one.
So this latest study tried to account for people's general diet patterns, said lead researcher Kiyah J. Duffey, of the University of North Carolina at Chapel Hill.
She and her colleagues used data on more than 4,000 Americans taking part in a long-term study of heart health. They were all between the ages of 18 and 30 when the study began in the mid-1980s.
Over the next 20 years, 827 study participants developed metabolic syndrome -- a cluster of risk factors for heart problems and diabetes including extra weight around the waist, unhealthy cholesterol levels, high blood pressure and elevated blood sugar.
The researchers found that young adults who drank diet beverages were more likely than those who didn't to develop metabolic syndrome over the next 20 years.
DIET MATTERS TOO
The picture became more complex when Duffey's team considered the role of diet as well.
The lowest risk of metabolic syndrome was seen among people who drank no diet beverages and stuck to a "prudent" diet -- one rich in foods like fruits, vegetables, whole grains and fish.
Meanwhile, people who also ate a prudent diet but did drink diet beverages had a somewhat higher rate of metabolic syndrome -- but not by much.
Over 20 years, 20 per cent of those men and women developed metabolic syndrome. That compared with 18 per cent of prudent eaters who didn't regularly have diet drinks, Duffey's team writes in the American Journal of Clinical Nutrition.
Participants with the highest rate of metabolic syndrome -- at 32 per cent -- were those who drank diet soda and downed the typical "Western" diet. That means lots of meat, processed foods and sugar.
Duffey's team weighed factors other than diet, too, like people's weight and exercise habits at the start of the study.
With all of that considered, healthy eaters who steered clear of diet drinks still had the lowest risk of developing metabolic syndrome -- more than one-third lower than Western-style eaters who did drink diet beverages.
"I really think it's overall diet that's important," Duffey said in an interview.
If you want to cut calories, replacing sugary drinks with sugar-free versions will do that, she noted. "But if the goal is a broader impact on your health," she said, "you need to consider the whole diet."
NO CLEAR ANSWERS YET
Duffey stressed that this study was observational -- meaning it followed people over time, looking for links between eating and drinking habits and the risk of metabolic syndrome.
That type of study can't prove that diet drinks have a negative effect on cardiovascular health, as some researchers have theorized, or whether some other factor is responsible.
That is still "definitely an open question," said Hannah Gardener, a researcher at the University of Miami Miller School of Medicine who led the recent study linking diet beverages to an increased risk of heart disease and stroke.
"We're still a long way from making any public health guidance" on diet beverages and health, Gardener said.
It's estimated about one-third of U.S. adults have metabolic syndrome -- with older age and obesity being prime risk factors.
There's some evidence from animal research that artificial sweeteners can end up boosting appetite and food intake. But no one knows yet if that translates to humans.
Whether diet drinks have specific health effects or not, Gardener agreed on the importance of overall diet.
"It's very important to have a healthy, balanced diet," she said.
And if you enjoy your sugar-sweetened drinks, Gardener said, "you shouldn't think that simply switching to diet (drinks) is going to be enough without taking into consideration your overall diet."
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