A New Zealand health expert says an American study linking diet drinks with stroke is a warning that artificial sweeteners are not risk-free.

Published in the journal Stroke, the longterm study of post-menopausal women found that those who consumed higher amounts of diet drink had a 23 per cent greater risk of having a stroke than those who drank small amounts or none.

The higher users also had a 29 per cent higher risk of coronary heart disease and a 16 per cent higher risk of death.

Higher use was defined as consuming the equivalent of two or more cans of "diet drinks such as Diet Coke or diet fruit drinks" a week. Low use was less than one a month.

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The study involved more than 81,000 women in the Women's Health Initiative Observational Study.

Professor Boyd Swinburn, of the University of Auckland, said that although there were some weaknesses in the study, it was a timely warning for New Zealanders.

He said artificially-sweetened beverages were not as harmful as those sweetened with sugar, "however the evidence is stacking up that they are not risk-free".

"The promotion has to be: drink water and milk rather than switching to artificial sweeteners."

Food and Grocery Council chief executive Katherine Rich said, "I haven't examined the report in depth, but the American Council on Science and Health has, and it described it as 'fake' news."

She noted the scientific and health group's comment that the study was hampered by its being based on participants' recollections - a "self-reported median of 11 years previously" - among other flaws.

Swinburn said one of the problems with the study was that women with a higher body-mass index (BMI) were more inclined than others to consume more diet drinks "because they are trying to do something about their weight - that's the reverse causality".

Reverse causality means some people were drinking more diet drink because they had some element of the disease risk being investigated by the study.

The study's authors said previous studies' conclusions on consumption of artificially-sweetened drinks and cardiovascular disease were inconsistent, some finding a correlation, others not.

They acknowledged the weaknesses of their study, but also said, "... it is known that non-caloric artificial sweeteners display unfavourable metabolic effects, which may put consumers at additional risk.

"Intake of saccharin, acesulfame-potassium, or stevia and saccharin have been linked to weight gain and increased adiposity [fat]."

They said experiments had shown artificial sweeteners could cause glucose intolerance in humans.

University of Auckland sugar researcher Dr Simon Thornley, of the lobby group FIZZ - Fighting Sugar in Softdrinks - said no biological mechanism had been proposed by which diet drinks caused cardiovascular disease. This added to doubts that the authors' findings were a true causal link rather than only a statistical association.

"... FIZZ continues to advocate for restricting the sale of sugary drinks and taxing these drinks. There is clear evidence that switching from sugary drinks to diet versions leads to beneficial changes to health markers and weight in the longterm, from randomised trials."

But Thornley said unsweetened drinks, such as water or milk, were ideal, as they did not maintain the habitual preference for sweet and sugary tastes.