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It's rare in life to have your cake and eat it. But are low-calorie sweeteners the guilt-free way to be naughty?

Nobody is going to claim that regularly drinking full-sugar pop is good for you with a 500ml bottle of cola containing around 200 calories.

But a diet version can come in at just the one calorie.

Simple logic would suggest that swapping a full sugar drink for a diet version cuts calories from your diet.

And yet such drinks have a mixed reputation.

A fresh review by Imperial College London has argued there is "no solid evidence" that low-calorie sweeteners are any better for weight-loss than full-sugar drinks.

And they challenged the idea that such drinks are automatically healthier.

Meanwhile there is public concern about some sweeteners and groups of scientists have argued that low-calorie sweeteners may lead to weight gain and increase the risk of developing type 2 diabetes.

So do they have a place in our shopping baskets?

"A lot of people assume they must be healthy choices because they are not sugared beverages, but the critical thing for people to understand is we don't have the evidence," said Prof Susan Swithers, from the US's Purdue University.

Studies looking at large groups of people have shown obese people tend to drink more fizzy diet drinks than those of a healthy weight.

A study of US adults in the American Journal of Public Health showed 11% who were a healthy weight, 19% of those who were overweight and 22% who were obese drank diet beverages.

And a study in the journal Obesity that followed 3,700 people for eight years showed those consuming the low-calorie sweeteners put on the most weight.

The researchers were left asking the question: "Are artificial sweeteners fuelling, rather than fighting, the very epidemic they were designed to block?"

But it is impossible to determine cause and effect in such studies. Are the drinks causing weight gain or are obese people turning to diet drinks in an effort to control their weight?

Different types of sweetener

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Aspartame: Odourless, white crystalline powder that is derived from two amino acids

Saccharine: The first artificial sweetener ever synthesised in 1879

Stevia: Sweetener derived from the South American Stevia plant

Sweeteners - NHS Choices

Body disruption

Prof Swithers' experiments on rats suggest the drinks alter the way the body deals with normal sugar, which could lead to weight-gain.

When sugar hits the tongue it gives us that delicious hit of sweetness, but also tells the body that food is on the way.

But with zero-calorie sweeteners that same message is sent, but no food arrives. The argument is the link between sweetness and calories has been broken.

Prof Swithers told me: "We think the diet sodas may be bad because they make it hard to deal with the sugar you are consuming.

"When the animals get real sugar they're not as good at processing it, their hormonal responses get blunted, their blood sugar levels go up and it leads to weight gain."

She also points out another problem - compensation. When you know you are taking calories out of one part of your diet you tend to eat more somewhere else.

"I had a diet beverage therefore I can have a cookie," she said - it's the same effect that has been well documented after we hit the gym.

Aspartame

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Aspartame is one of the best known low-calorie sweeteners, but is also the most controversial.

The sugar substitute, also known as E951 in Europe, is 200 times sweeter than sugar and has been linked to a series of scare stories since it was introduced to foods in the 1980s.

Claims have included allergies, premature births and cancer.

Pepsi quoted public distrust in the stuff as the number one reason people were ditching their diet soda in the US.

It is often described as one of the most tested food ingredients in the world.

And a review by the European Food Safety Authority in 2013 concluded that there were "no safety concerns" including for pregnant women and for children.

Meanwhile scientists at the Weizmann Institute of Science in Israel showed that low-calorie sweeteners altered the balance of bacteria inside the guts of rats.

Our body's cells are outnumbered 10-to-one by bacteria, viruses and fungi growing on or in us and this "microbiome" has a huge impact on health.

The study, in the journal Nature, showed that low-calorie sweeteners altered the animals' metabolism and led to higher blood sugar levels - an early sign of developing type 2 diabetes.

Seven human volunteers then spent seven days consuming very high levels of low-calorie sweeteners. In half of people the results mirrored those in the animals.

But Prof Peter Rogers, from the University of Bristol, is far from convinced by such studies.

He says most of the animal research used levels of sweeteners that showed "little relation" to how they are used in real life.

And that it was "equally plausible" that low calorie sweeteners "may actually decrease one's desire for a sweet dessert".

More from our Diet Debate series:

Read: Is breakfast a waste of time?

Watch: How healthy is your breakfast?

Read: Low-fat or high-fat - does it matter?

Read: Is butter back and is saturated fat good for you?

Weight-loss

Prof Rogers was part of a review, which included researchers funded by the food industry, of the evidence on low-energy sweeteners.

The results, published in the International Journal of Obesity, showed people lost weight when asked to replace sugary drinks with low-calorie sweetened ones.

It showed they lost around 1.2kg on average when people were on the diet for between four and 40 months - an effect broadly the same as for switching to water.

Prof Rogers said: "We clearly found that consuming low-calorie sweeteners, in place of sugar, reduced energy intake and body weight."

And that while people consuming sweeteners did end up eating more than those who stuck to sugary drinks, they still consumed fewer calories overall.

He concludes: "They're not going to do all the work for you, but it's a way to enjoy the pleasure of sweet, without the penalty of calories in our obesogenic society."

Water

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Although experts say that in an ideal world we'd all be drinking water, a study in Obesity journal even suggests "pre-loading" with water half an hour before eating actually helps people lose weight.

But even staunch critic of low-calorie sweeteners, Prof Swithers, argues they may have a role as a halfway house.

"A diet beverage would be useful to have in your diet as a transition, so if you're drinking regular soda every day and find it too difficult to stop," she said.

They should help with some weight-loss, at least in the short-term. The big question is whether we'd all be better off by just adjusting to a diet that's less sweet.

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