Diet drinks do not help people lose weight, a study has found, because they eat more to compensate.
Calorie-free drinks like Diet Coke and Sprite Zero increase someone's hunger and desire to eat, according to scientists.
Volunteers given artificial sweeteners found in these drinks should have lost weight by avoiding the sugar packed into normal fizzy drinks.
But they simply made up the difference in calories at lunch, ensuring they stayed the same size.
The findings, published in the International Journal of Obesity, were described as "surprising" by the lead author, who gave people three different kinds of sweeteners, or sugar, before recording their food intake for the rest of the day.
Dr Siew Ling Tey, of the Agency for Science, Technology and Research in Singapore, said: "The energy 'saved' from replacing sugar with non-nutritive sweetener was fully compensated for at subsequent meals in the current study, hence no difference in total daily energy intake was found between the four treatments."
The research follows a recent study showing just two glasses of diet drinks a day could double someone's risk of developing diabetes, with a team at Karolinska Institute in Sweden suggesting they may stimulate appetite, causing people to put on weight.
The latest experiment looked at aspartame, an artificial sweetener found in Diet Coke and Coke Zero Sugar, and stevia, found in celebrity-backed health drinks like glacéau vitaminwater.
It compared them with monk fruit extract and with conventional sugar, found in full-calorie fizzy drinks.
In only the second study to investigate the effect of artificial sweeteners and sugar on food intake, 30 men picked at random were given 500ml drinks containing each ingredient, then allowed to eat as much fried rice for lunch as they wanted.
Those given the diet drinks ate up to 80 calories more at lunchtime, the study found, with those given aspartame consuming the most during the day.
They did not binge eat, but described being more hungry after the diet drinks, meaning they lost no weight from the switch away from sugar.
The study states: "One of the concerns with NNS (non-nutritive sweetener) consumption is the increased appetite, which may lead to overcompensation for the energy saved.
"The present study found that although desire to eat, hunger and prospective consumption ratings were higher after consuming NNS preloads compared to sucrose preload, there was no evidence of overconsumption on NNS preload days."
Conflicting research has found diet drinks can help people lose weight, while protecting them from the spike in their blood sugar from calorific drinks.
However, after having lunch, when people given the low-calorie beverages ate more, their blood sugar two hours later was higher than those given sugary drinks.
Animal studies suggest that artificial sweeteners could cause people to absorb more glucose.
However Gavin Partington, director general of the British Soft Drinks Association, said: 'Decades of scientific research has shown that low calorie sweeteners, such as those found in diet drinks, help consumers manage their weight as part of a calorie-controlled diet.'