Is your sugar-free diet drink actually making you fat... or could the artificial sweeteners actually give you cancer?

• People keeping an eye on their weight often switch to sugar free soft drinks
• Studies show two-thirds of the fizzy drinks purchased are low or zero-calorie
• However, there are claims that zero-calorie drinks could promote weight gain
• Others claim that the artificial sweeteners can even cause cancer 

It is caffeine, not the sweetener that gets people hooked...

Sirin Kale spent 27 years 'addicted' to Diet Coke. At one stage, the 32-year-old journalist from London was glugging up to seven cans a day, spending roughly £500 a year on her habit. Each morning she'd 'pad to the kitchen' for a swig before starting her day.

'I'd get anxious if I didn't have any in the fridge as bedtime approached, and run to the shop in the middle of the night to ensure there was a cold can waiting for me in the morning,' she told The Guardian last year.

'I recently spent a year on prescription medication for a stomach condition that was almost certainly triggered by my over-consumption of Diet Coke.'

When she attempted to quit last January, she was surprised at how tough it was to kick the habit. The first week brought agonising headaches, eased only by a sip of Diet Coke. Eventually, hypnotherapy worked.

Sirin, pictured above, says: 'Though I still think about Diet Coke, it doesn't consume my thoughts like it used to. I am not constantly monitoring how many cans I have in the fridge.'

Sirin's story appears to offer proof that diet drinks — and the super-sweet artificial sweeteners that they're loaded with — can be addictive. But most experts say that physical addiction to sugar or sweet food, similar to those to a drug or alcohol, isn't possible.

'In order for a substance to be physically addictive, you have to experience withdrawal symptoms or a "come down" when you stop having it,' says psychologist Kimberley Wilson.

The official definition of addiction also requires the person to have cravings for the substance that impact their ability to function normally — which is yet to be proven for sweet foods.

In Sirin's case, the unpleasant physical symptoms can be explained by withdrawal from caffeine. A can of Diet Coke contains the same amount as two-thirds of an espresso.

Research has shown that sugar can light up the brain's pleasure centres. 'We do have an in-built preference for sweet foods because they are high in calories,' says Wilson, who specialises in eating behaviours.

'When we eat sugar, levels of a stress hormone called cortisol get suppressed. So it is entirely feasible that people feel real stress relief when they eat or drink something sweet. Over time, it can become a habit.'

In the 1970s, lab studies showed that mice fed aspartame developed tumours — but it was later revealed the animals had been fed the equivalent to 20 cans of Coke a day.

'No one would ever consume this amount, and no similar effect has ever been found in humans,' says Chris Corpe, nutrition expert at King's College London.

Despite this, and no doubt aware of all the bad press, in 2015 drinks giant Pepsi removed aspartame from its diet drinks in the US, replacing it with sucralose — but it didn't do this in the UK or anywhere else around the world.

Numerous reviews by public-health bodies over the past four decades have all concluded that sweeteners are safe. As for other studies that show people with diets high in sweeteners have an increased risk of cancer, Dr Mellor says: 'These people tend to be the people who are also eating little fresh, home-cooked food and more processed, possibly high-calorie meals.

'A lot of people turn to diet drinks and low-calorie products because they need to lose weight, which means they are at a higher risk of cancer anyway. Studies looking at people who eat lots of processed foods find the same relationship.'

So what of the claims that sweeteners, far from helping us stay slim, actually have the opposite effect?

Animal studies certainly seem to suggest this.

In 2016, Australian scientists found that rats fed a diet containing artificial sweeteners went on to consume more food when they were allowed to eat freely than those not given the sweetener diet.

Scans on the animals showed consuming sweeteners increased activity in areas of the brain associated with pleasure — more so than when they were fed normal sugar.

Professor Greg Neely from the University of Sydney, who led the research, explained that, long-term, consumption of an overly sweet artificial flavour increased the animals' preference for high-calorie food — making them binge. Human studies have come to similar conclusions. Experiments involve participants fasting overnight, then being given drinks containing either artificial sweetener or sugar. After each drink, volunteers are allowed to choose snacks from a buffet — and the amount they eat is monitored.

In some studies, people consume more food after the sweetened drink than the sugary drink. Blood tests also show raised levels of hormones that trigger feelings of hunger, such as ghrelin, in those who had consumed drinks with sweeteners compared to those who had swallowed drinks with sugar.

But Dr Mellor says research such as this proves little. 'Lab studies like these are not a fair representation of the way people eat,' he says. 'If you make people fast, they will eat more than they usually would when they're allowed to — because they'll be very hungry.

'A sweetened drink like Diet Coke won't provide any calories, so you'd expect people to be less satisfied by that than by a full-sugar version. In reality, most people won't be drinking these drinks with nothing else in their stomach, having starved themselves for hours on end.'

So what about the blood tests showing raised hunger hormones after the artificially sweetened drink?

'Some studies show this, but others show the total opposite, so it's not clear what's going on,' answers Dr Mellor.

In the 1970s, lab studies showed that mice fed aspartame developed tumours — but it was later revealed the animals had been fed the equivalent to 20 cans of Coke a day

The latest theory is that sweeteners have a damaging effect on the trillions of bacteria that live in our gut. Pioneering this research is a team of Israeli scientists who used faecal samples to analyse gut bacteria before and after participants drank artificially sweetened drinks. They discovered that giving saccharin, sucralose or aspartame to people over the course of a few weeks disrupted the healthy bacteria in the gut. These bacteria feed off the partly digested food in the bowel, producing compounds that send signals to the brain which tell us if we're hungry or full.

The sweeteners also seemed to increase levels of a type of bacteria linked to damaged bowel tissue.

But other scientists say these results need to be viewed with caution. Firstly, there is not yet consensus about the exact combination of gut bugs — out of the 100 trillion in an average bowel — that is needed for healthy appetite regulation.

'The one thing we know about gut bacteria is that they are changing all the time and are never static,' says Dr Mellor. 'The bugs that are most likely to impact our health are the ones that remain there for a long period of time — not those that appear, or disappear, over a weekly period. The only way you can test this — and its impact on appetite — is by putting tubes in people's bowels for weeks at a time as they eat, which is clearly impossible.'

Dr Mellor adds: 'Some sweeteners, like aspartame, are absorbed into the bloodstream very quickly and do not even make it into the gut.'

So why do studies show that those who consume diet-drinks are more likely to be overweight?

For instance, one, in 2015, found that the average waist circumference of people who drank diet soda daily over a ten-year period was nearly four inches greater than those who didn't drink them.

The most likely explanation is a scientific phenomenon known as reverse causality — or what some diet experts call 'the Big Mac and Diet Coke theory'.

'Essentially, this means the people who consume a lot of diet drinks are more likely to eat high-fat, high-salt and high-sugar foods, and are more likely to be overweight in the first place,' says Dr Mellor.

So does that mean diet drinks do little to keep off the pounds?

Earlier this year, the World Health Organisation published one of the most conclusive reports on the health impact of artificial sweeteners to date.

The paper, which scrutinised 283 international studies, including controlled experiments and observational research on all common sweeteners, concluded that artificial sweeteners may lead to a 'small reduction' in body fat in the short term, with no significant impact on long-term heart health risk.

The authors noted some studies showed negative health effects but that evidence was 'inconclusive'.

Banning certain commonly used sweeteners entirely might be equally as detrimental.

For instance, one, in 2015, found that the average waist circumference of people who drank diet soda daily over a ten-year period was nearly four inches greater than those who didn't drink them.

Dr Sarah Berry, senior lecturer in nutritional sciences at King's College London, says: 'If you stop people having diet drinks, some of them may well switch to high-sugar, high-calorie drinks instead.'

Meanwhile, consultant dietician Rebecca McManamon is keen to highlight one important benefit of diet drinks that is rarely considered. 'Currently we have a crisis in dental health across the UK — with record levels of tooth decay in children and huge waiting lists for dentist appointments,' she says.

'Although diet drinks increase the risk of some enamel erosion, because they are acidic, the absence of sugar makes them far less harmful. It is all very well telling people to drink water instead but, for many, these drinks are a staple part of their daily routine and they find it hard to give them up.

'But if you can convince patients to switch to a non-fizzy version, you're sparing them tooth damage, at the very least.'