The horrific side effects of the diet fizzy drink we ALL love – as doctors warn why you should think twice before opening your next can

They were developed to reduce our sugar intake and tooth decay – but are diet drinks really better than ‘full-fat’ versions?

One thing is clear: diet varieties have overtaken their sugary counterparts in popularity, accounting for 70 per cent of the UK carbonated drinks market and 60 per cent globally.

Newly inaugurated President Donald Trump is such a fan he’s apparently reinstalled the red ‘Diet Coke button’ in the White House, having fitted it during his first term, which alerts staff to bring him a can immediately (he drinks 12 to 15 a day, it’s been reported).

Last week he was presented with a Presidential Commemorative Inaugural Diet Coke by the CEO of Coca-Cola (the company has a tradition of creating commemorative Coca-Cola bottles to celebrate US presidential inaugurations – but Mr Trump’s was Diet for the first time).

And Elon Musk is another devotee – jokily posting on X last week: ‘I have a drinking problem,’ alongside a photo of six cans of Diet Coke on his bedside table.

But could his habit actually be more of a problem than he thinks?

With nearly ten teaspoons of sugar (35g) in a 330ml can of regular cola, diet fizzy drinks – where the sugar is replaced with artificial sweeteners – would seem to be the healthier choice.

But a raft of studies suggest we may not completely escape the negative effects of fizzy drinks by going sugar-free – and the sweeteners in them could be risking additional problems. 

For instance, the World Health Organisation (WHO) in 2023 designated aspartame – a sweetener widely used in diet sodas – as ‘possibly carcinogenic to humans’.

So should you think twice before opening your next can? Here experts explain the latest thinking about diet fizzy drinks…

Damage to tooth enamel

You might think that by choosing diet drinks you’re protecting your pearly whites but several studies, including one in 2019 by the University of Athens, have found that low-calorie fizzy drinks can also damage the surface of teeth and weaken tooth enamel.

‘While it’s true that non-diet fizzy drinks contain high levels of sugar which can cause tooth decay, the diet versions are not necessarily a solution,’ explains Jasmine Piran, a dentist and clinical director of Smiles By Jasmine clinic in London.

‘This is because all fizzy drinks – even diet versions – have an acidic pH, as they contain ingredients such as citric acid [for a fruity flavour], phosphoric acid [for tang] and carbonic acid [which creates the bubbles].

‘All this acid can cause erosion to teeth, leading to irreversible loss of enamel: once a tooth loses its enamel, it’s gone for ever.’

If you can’t resist a sugar-free drink, sipping it through a straw and rinsing out your mouth immediately afterwards with water can help. Or try chewing a piece of sugar-free gum to increase saliva flow, which helps neutralise acids in your mouth.

Type 2 diabetes

‘When it comes to the most serious potential health risks of diet fizzy drinks, type 2 diabetes seems to have the strongest link’, says Susan Swithers, a professor of neuroscience at Purdue University in the US, who has researched the effects of diet drinks on metabolic health.

A study she conducted in 2015 found that people who drank a lot of carbonated diet drinks had a higher risk of obesity, elevated blood sugar and type 2 diabetes.

And there’s growing evidence suggesting that artificial sweeteners are the culprit.

Most recently a 2023 study in the journal Diabetes Care, involving more than 100,000 people, found that those who consumed more artificial sweeteners had a higher risk of type 2 diabetes.

Women who had more than 18mg of artificial sweeteners per day (by way of comparison, a 330ml can of diet cola provides 200mg) were almost 70 per cent more likely to develop the condition than those who consumed less. The French researchers behind this study said the findings ‘strengthen the evidence that these additives may not be safe sugar alternatives’.

Scientists suspect that despite being calorie-free, some sweeteners may impair the body’s response to glucose and raise blood sugar in a similar way to normal sugar.

Indeed, a 2020 study by Jawaharlal Nehru Medical College in India found that people who consumed artificial sweeteners had higher insulin resistance – meaning their body didn’t respond properly to insulin, which makes it harder to maintain normal bloody sugar levels – compared to those who had none.

Heart disease and stroke

Several studies have linked having one or more cans of artificially sweetened drinks a day to a higher risk of serious heart problems.

For instance, research published last March found people who drank more than seven cans a week of diet drinks had a 20 per cent higher risk of atrial fibrillation (an irregular heartbeat that increases stroke risk) than people who didn’t drink any.

‘It’s important not to assume that low-calorie drinks are inherently healthy,’ warned the lead researcher Ningjian Wang, a professor of endocrinology and metabolism at Shanghai Ninth People’s Hospital in China.

‘Based on these findings, we recommended that people reduce or even avoid artificially sweetened beverages whenever possible,’ he said.

It’s thought that the link might be related to the effect of diet drinks on blood sugar levels.

And a 2019 US study, published in the journal Stroke, involving 80,000 women (aged 50 to 79) found that those who drank two or more diet fizzy drinks a day were 23 per cent more likely to have a stroke and nearly 30 per cent more likely to have heart disease over the 12-year study.

The researchers from Albert Einstein College of Medicine in New York said more research is needed to confirm these findings.

Bloating and diarrhoea 

All carbonated drinks can leave you feeling bloated as the bubbles of gas pass through your digestive system.

But artificial sweeteners in diet drinks – and in some ‘regular’ drinks (added for flavour along with the normal sugar) – can upset those with sensitive guts.

‘Some sweeteners – such as sorbitol, mannitol and xylitol – especially in large quantities, can cause bloating, gas or diarrhoea,’ explains Emma Shafqat, a dietitian based in Surrey.

‘This is because they are poorly absorbed in the gut and can sit there and ferment, which can irritate the gut-lining.’

Insomnia

Could your Diet Coke habit explain why you have problems dropping off at night? Potentially, yes – and it’s not just about the caffeine, which at 46mg per can (around half a cup of coffee) can soon add up, especially if you’re drinking it later in the day.

But there is also some early evidence that artificial sweeteners might have a negative effect on sleep patterns, with a 2016 study on mice by the University of Tokyo finding that moderate amounts of sweeteners could disrupt the sleep-wake cycle. It is not clear exactly why.

Obesity

Donald Trump once quipped ‘I have never seen a thin person drinking Diet Coke’ – was he on to something?

Because artificial sweeteners are often so much sweeter than sugar – saccharin, for example, is approximately 300-500 times sweeter – experts fear they may disrupt how the body perceives sweetness and therefore contribute to weight gain.

The suggestion is that their intense sweetness primes our metabolism to prepare for a large influx of calories – and when the calories don’t arrive, the body ramps up its appetite, so people subsequently overeat fattening foods.

‘Frequent consumption of high-intensity sweeteners may have the opposite effect by confusing the body’s natural ability to manage calories based on tasting something sweet,’ adds Professor Swithers.

In 2021 researchers at Keck School of Medicine in California undertook one of the largest studies to date looking at the effects of artificial sweeteners and appetite.

Using brain scans, they found increased activity in the area responsible for food cravings and appetite in both women and obese people who’d drunk artificially sweetened drinks, and compared them with those having sugary drinks or plain water.

Lead researcher Dr Kathleen Page said: ‘We were able to show that women and people with obesity may be more sensitive to artificial sweeteners. For these groups, drinking artificially sweetened drinks may trick the brain into feeling hungry, which may in turn result in more calories being consumed.’

And in 2023, after examining data from 283 studies, the WHO urged people not to have artificially sweetened fizzy drinks for weight control, stating: ‘Replacing sugars with non-sugar sweeteners does not help with [this] in the long term.’

The WHO review also suggested long-term use was linked to an increased risk of type 2 diabetes and heart disease.

Gut microbiome imbalance 

The importance of having a healthy balance of good bacteria in the gut when it comes to our overall health has become evident in recent years – and now it appears that artificial sweeteners may disrupt this delicate internal ecosystem.

Some animal studies on the sweeteners saccharin and sucralose indicate that they alter the composition of our gut bacteria, potentially leading to dysbiosis – an imbalance in ratio of good to bad bacteria, which can have a negative effect on digestion and health, says dietitian Emma Shafqat. ‘However, there needs to be much more human research into this area.’

But Dr Kristina Rother, a senior researcher who has conducted studies into the effects of artificial sweeteners at the National Institute of Diabetes and Digestive and Kidney Diseases in the US, takes a more robust view, pointing out that studies in animals have consistently shown ‘artificial sweeteners worsen the profile of the gut microbiome – I haven’t seen a study that says sweeteners make it better’.

Lower bone density

Many fizzy drinks contain caffeine and phosphoric acid (used to give fizzy drinks a ‘sharp’ flavour) – and high intakes of both have been linked to lower bone density (resulting in weaker bones), as they are thought to interfere with the body’s ability to absorb calcium.

A 2020 study by Chongqing Medical University in China, looking at nearly 20,000 adults over seven years, found that a high consumption (defined as one or more per day) of soft drinks was associated with an increased risk of bone fracture – even after adjusting for lifestyle factors such has starting body weight and diet habits.

Researchers recommended that reducing soft drinks should be considered as an important strategy for maintaining bone health.

And this potential effect on our bones seems to hold true whether they contain sugar or not, with a 2006 study finding that women who drank sodas – diet or full fat – had significantly lower bone mineral density than those who didn’t, reported the American Journal of Clinical Nutrition.

And a 2019 study at the University of California found that consuming more than two soft drinks per day increased the risk of hip fracture among postmenopausal women by a third.

‘Based on our results, low levels of soda consumption (one can a day or less) won’t increase the risk of fractures in postmenopausal women,’ according to the lead author of the study, Dr Pedro Kremer.

‘However, after a certain level – the equivalent of two cans per day – the risk becomes significantly higher.’

Cancer

In 2023 the WHO declared the artificial sweetener aspartame a class 2B carcinogen, which means it’s a ‘possible’ cause of cancer.

It should be noted, though, that the research also calculated the average person could safely drink 14 cans of aspartame-sweetened diet drinks per day before incurring any risk.

However, another WHO report in the same year looked at 48 human studies investigating possible links between sweeteners and cancer and found no significant association, with the possible exception of bladder cancer, but even then the evidence wasn’t strong.

‘There is no convincing evidence that artificial sweeteners cause cancer and people shouldn’t be worried about getting cancer from diet soft drinks,’ the charity Cancer Research UK told the Mail.

‘Your overall diet is more important than individual foods or ingredients for reducing cancer risk. So aim to eat a healthy, balanced diet, with more fruit, veg and wholegrains – and cut back on red or processed meats and foods high in fat, sugar and salt.’

So, those risks considered, what should you drink instead? ‘If you crave something fizzy you’re going to be better off drinking sparkling water with a squeeze of fruit juice,’ says dietitian Emma Shafqat.