They found when there was a ‘mismatch’ between sweetness and calories - as is often the case with diet drinks or foods because they are not as sugary - the calories fail to trigger the body’s metabolism. Reward circuits in the brain also did not register that calories had been consumed, which could lead to eating more.

Commenting on the paper, Dominic Dwyer, Professor of Psychology at Cardiff University, said: “What the paper does imply, correctly in my view, is that mismatches between calories and sweetness interfere with metabolism of calories in a way that could have negative impact on weight gain, diabetes, heart disease etc. but that determining the link between the unprocessed calories and metabolic health needs future work.

“The most important implication is namely the fate of calories consumed in the mismatch conditions.

“These are not efficiently metabolised at the time of ingestion and thus processed later and/or stored either of which could drive weight gain and interfere with metabolism.”

Tam Fry, of the National Obesity Forum, added: "This research should be enough to convince you that artificial ingredients, whether they be in food or drink, can screw up your system even though they may sound healthy.