In what will come as a shock to those who consume 'diet' labelled fizzy drinks hoping to avoid weight gain, a study has such drinks actually stop you losing weight rather than help you.
Scientists claim the low calorie sugary drinks are packed with chemicals which disrupt the body's metabolism.
It will come as bad news to millions of people who believe diet versions of soft drinks help them lose weight. And the new claims have been slammed as "baseless" by the British Soft Drinks Association.
Massachusetts General Hospital researchers found the sweetener aspartame is the culprit which works against slimmers.
Mice it tested by giving it to them in their drinks gained more weight - and developed more issues with their metabolism - than those who weren't.
Aspartame contains a product called phenylalanine which it's claimed disrupts one of the body's enzymes which is key to preventing metabolic disorders.
MailOnline reported that it was similar to a condition seen in people with Type 2 diabetes or heart disease.
The report's senior author Dr Richard Hodin said, "Sugar substitutes like aspartame are designed to promote weight loss and decrease the incidence of metabolic syndrome, but a number of clinical and epidemiologic studies have suggested that these products don't work very well and may actually make things worse.
"We found that aspartame blocks a gut enzyme called intestinal alkaline phosphatase (IAP) that we previously showed can prevent obesity, diabetes and metabolic syndrome.
"So we think that aspartame might not work because, even as it is substituting for sugar, it blocks the beneficial aspects of IAP."
The researchers followed four groups of mice for 18 weeks - with two groups being fed a normal diet, one being given drinking water with aspartame and the fourth being given just plain water.
The other two groups were given a high-fat diet - together with either plain water or water infused with aspartame.
The mice on the normal diet that also got aspartame drank an amount roughly equivalent to an adult human drinking three and a half cans of diet drinks every day.
Those on a high fat diet which also got aspartame drank the equivalent of almost two cans.
Weight-wise, when the trial ended there was little difference between the weights of the two groups on mice on a normal diet.
But the mice given a high-fat diet - and also received aspartame - put on more weight than those mice on the same diet that were given just plain water.
Moreover, those mice in both diet groups that were given aspartame registered higher blood sugar levels than those who weren't. And both groups given aspartame displayed higher levels of inflammation.
Dr Richard Hodin said, "People do not really understand why these artificial sweeteners don't work.
"There has been some evidence that they actually can make you more hungry and may be associated with increased calorie consumption.
"Our findings regarding aspartame's inhibition of IAP may help explain why the use of aspartame is counterproductive.
''While we can't rule out other contributing mechanisms, our experiments clearly show that aspartame blocks IAP activity, independent of other effects."
Cola makers hit back
But the British Soft Drinks Association hit back at the claims - telling the MailOnline that they were baseless.
BSDA director General Gavin Partington said: "These claims are being made by a study conducted on mice and run contrary to the overwhelming body of scientific evidence.
"Decades of scientific research, including human clinical trials, show that low-calorie sweeteners, such as those in diet drinks, have been found to help consumers manage their calorie intake when part of an overall healthy diet."