Consuming low-calorie drinks may increase the risk of putting on weight, according to scientists in the United States.

They have suggested that people who choose diet drinks containing artificial sweeteners tend to overcompensate and consume more calories than those who do not.

Although the rise in obesity has corresponded with a growth in low-calorie soft drinks, designed to make keeping weight down easy by replacing sugar with saccharine or other sweeteners, scientists who conducted experiments using rats at Purdue University, in Indiana, have suggested that the opposite may be happening.

They found that rats fed on yoghurt sweetened with saccharine ate more calories, gained more weight and put on more body fat than rats that were given yoghurt sweetened with glucose.

Susan Swithers and Terry Davidson, who conducted the experiments, have suggested that, by breaking the connection between a sweet sensation and high-calorie food, the use of saccharine changes the body’s ability to regulate how many calories it consumes. “The data clearly indicate that consuming a food sweetened with no-calorie saccharine can lead to greater body-weight gain and adiposity than would consuming the same food sweetened with a higher-calorie sugar,” they conclude in their report, which is published in the journal Behavioural Neuroscience.

They admit that their results may seem counterintuitive and might not be welcome to nutritionists and doctors who have long recommended low-calorie or no-calorie sweeteners. But they say that their findings match emerging evidence that people who drink more diet drinks are at higher risk of obesity and metabolic syndrome, a collection of medical problems such as abdominal fat, high blood pressure and insulin resistance that put people at greater risk of heart disease and diabetes.

People and animals learn that eating certain foods has consequences. Sweet tastes signal calories. If that link is broken, the researchers suggest, then the individual loses the ability to judge how many calories are being consumed. One controversial theory is that calorie consumption is signalled by a rise in body temperature after eating. The greater the rise in body temperature, the more aware the individual is that a lot of calories have been consumed.

In the experiments the rats that were used to eating low-calorie yoghurt showed a smaller rise in temperature after eating a different, calorie-loaded meal. It appeared that their ability to detect calories had been blunted, leading to overeating.

Normally, the researchers say, sweet foods provide a stimulus that strongly predicts that someone is about to take in a lot of calories and their ingestive and digestive reflexes gear up for that intake. But when false sweetness is not followed by lots of calories the system gets confused. Thus, they argue, people on low-calorie diets may eat more – or expend less energy – than they otherwise would. If their theory is correct, then artificial sweeteners such as aspartame, sucralose and acesulfame K, which taste sweet but do not provide calories, could have similar effects.

The results did not surprise Richard Cottrell of the Sugar Bureau. He said that there had been a number of studies that suggested a link between use of low-calorie sweeteners and weight gain and that dietary studies in humans “do not support the idea that avoiding sugar is a predictor of low body mass index, quite the contrary”.

Dr Cottrell said that the evidence suggested that people who chose low-calorie drinks tended to consume more calories. “The advocates of low-calorie products tend to rely on evidence from covert substitution experiments, where the subjects don’t know their food has been doctored. But even these experiments suggest that compensation kicks in slowly over a few days. People who know they are ‘banking some goodness’ by taking, say diet drinks, are inclined to overindulge.”

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