Even diet soft drinks may be linked with increased insulin levels, alarming new research has revealed.
Researchers from Brazil and Portugal found adults who drink both regular and diet drinks had higher levels of insulin in their saliva.
"Insulin levels increased 1h after soft-drinks ingestion (regular and diet), while the levels did not change for low sucrose content and water + sweeteners test-drinks," said the study, which appeared in the journal Food Research International.
Prior studies have linked diet fizzy with multiple risk factors, including heart disease and diabetes, but the researchers also found aspartame excretion in sample of participants' saliva - which they said is the first study to report such findings.
Aspartame is one of the most used sweeteners in the world and has been deemed a "possible carcinogen" by World Health Organization (WHO) experts.
The latest study included 15 adults who had no history of unusual blood sugar levels or diabetes.
"Theoretically, insulin should not be released after drinking diet soda since it should not raise blood sugar and not call for insulin to be released," said Karen Z Berg, a New York dietician who wasn't involved in the study.
"It's no secret that both regular soda and diet sodas have negative health implications. This study, while super small, had an interesting approach to studying saliva after consumption of these beverages," Berg told Medical News Today.
She said people often drink diet fizzy drinks to control blood sugar levels and weight but "more and more research is coming out that is proving these beverages have the opposite effect".
While the WHO earlier this year dubbed aspartame a "possible carcinogen", the UN agency said it remained OK to consume at levels previously agreed in 1981 - which is below 40mg/kg per day.
A person weighing between 60 and 70kg would need to drink more than 9-14 cans of diet fizzy drink every day to go over that level, the WHO said.
Nonetheless, if "consumers are faced with the decision of whether to take cola with sweeteners or one with sugar, I think there should be a third option considered - which is to drink water instead", said WHO nutrition head Francesco Branca.