Courtney Black says artificial sweeteners left her 10kg heavier – a nutritionist weighs in

Be honest: when you’re choosing a can of Coca-Cola, do you reach for the full-sugar version – or opt for Diet Coke in the name of health?

A 375ml can of Coca-Cola contains almost 40g of sugar, while a 355ml can of Diet Coke contains around 125–190mg of aspartame – a low-calorie sweetener around 200 times sweeter than sugar.

Sweeteners have traditionally been marketed as a way to cut sugar and calories and, in turn, support weight management. But in recent years, they’ve faced increasing scrutiny, perhaps partly due to growing concerns about ultra-processed foodsand questions around the long-term health effects of ingredients such as aspartame.

Now, fitness influencers including Courtney Black are adding fuel to the debate. In a recent reel, Black claimed that during her ‘artificial sweetener phase’ she was ‘bloated, holding fat and 10kg heavier’.

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So, can artificial sweeteners really cause weight gain – or is something else going on? We asked nutritionist Kim Pearson, who specialises in weight loss, to separate fact from fiction.

Sweeteners haven’t been definitively shown to cause weight gain, says Pearson. The evidence is much more nuanced.

‘Some observational studies have found an association between higher consumption of sweeteners and higher BMI or obesity risk, but these studies cannot establish that the sweeteners themselves caused the weight gain,’ she explains. ‘There is a possibility that people who are already struggling with their weight may be more likely to choose artificially sweetened products.’

While some studies have associated artificial sweeteners with increased body weight, there is also evidence from randomised controlled trials that replacing sugar with non-sugar sweeteners can reduce calorie intake and may produce a small reduction in body weight in the short term.

‘I think it’s misleading to tell people that having a small amount of artificial sweetener is likely to cause weight gain,’ Pearson adds.

However, she stresses that sweeteners aren't a magic solution for weight management.

‘They may help someone who is eating a lot of sugar to reduce their intake, but that doesn't necessarily mean they support long-term weight management,’ she says.

‘The evidence is mixed, which is important to acknowledge,’ says Pearson.

The World Health Organization (WHO) currently advises against using non-sugar sweeteners as a means of controlling body weight or reducing the risk of chronic diseases. However, randomised controlled trials generally paint a more favourable picture than observational research, Pearson points out.

‘When non-sugar sweeteners replace sugar, they can reduce energy intake and may lead to modest reductions in body weight, with much of the evidence from studies being relatively short-term,’ she says.

‘Observational studies, on the other hand, have found associations between higher sweetener consumption and obesity, as well as some cardiometabolic outcomes. But these associations do not prove that the sweeteners were the cause, as factors such as reverse causality and other differences between sweetener users and non-users may also play a role.’

Pearson’s view is that we shouldn't demonise sweeteners – but we shouldn't necessarily rely on them, either.

‘I don't think we should rely on them as a weight loss strategy,’ she adds. ‘Where possible, I'd encourage people to work on reducing their taste for sweet foods and drinks.’

Not all sweeteners affect the digestive system in the same way.

‘It's important to distinguish sweeteners such as aspartame, sucralose and stevia from sugar alcohols, or polyols,’ says Pearson.

Polyols are more likely to cause gastrointestinal symptoms because some aren't completely absorbed in the small intestine. The unabsorbed portion can draw water into the gut and, once it reaches the large intestine, be fermented by gut bacteria, producing gas.

Depending on the individual and the amount consumed – higher doses are more likely to cause problems – this can lead to bloating, abdominal discomfort, gas and diarrhoea.

‘People vary in their tolerance, so something that causes significant bloating in one person may have no noticeable effect on someone else,’ Pearson explains.

Examples of polyols include sorbitol, maltitol and xylitol. ‘These are often used in sugar-free sweets, chewing gum, protein bars and some other low-sugar products,’ she adds.

Not through bloating alone, says Pearson.

‘It’s highly unlikely,’ she says. ‘I would be very sceptical of attributing a 10kg increase to bloating alone.’

Bloating can make someone feel considerably larger and cause visible abdominal distension. Body weight can also fluctuate because of changes in fluid retention and the amount of food in your digestive system. But that doesn't explain a 10kg increase.

‘A 10kg increase would be very difficult to explain by intestinal gas or bloating alone,’ Pearson says.

If someone has genuinely gained 10kg on the scales over a relatively short period, she'd want to establish what's contributing to that change rather than assuming it's all body fat – or dismissing it as bloating.

‘Changes in fluid balance, constipation, menstrual cycle, carbohydrate and sodium intake and, depending on the circumstances, certain medications or medical conditions can all influence body weight.’

‘The important point is that feeling bloated and gaining 10kg are not interchangeable things,’ she says.

Persistent or substantial unexplained weight gain should be properly assessed rather than automatically attributed to sweeteners or digestive symptoms.

Pearson doesn't think everyone needs to avoid them completely.

‘For someone who consumes a lot of sugar, switching to a sugar-free option may be a useful way of reducing free sugar and calorie intake,’ she says.

But she doesn't recommend consuming unlimited amounts simply because something is labelled ‘sugar-free’.

‘I would encourage people to focus on reducing the overall sweetness of their diet and choosing minimally processed foods and unsweetened drinks,’ she says.

‘I’d also encourage people to pay attention to their individual response. If someone regularly experiences bloating, abdominal discomfort or changes in bowel habit after consuming products containing sweeteners, it makes sense to remove the sweeteners (and products containing them) and notice whether symptoms improve.’

There isn't good evidence that occasionally consuming artificial sweeteners will make you gain weight. Equally, replacing every sugary food or drink with an artificially sweetened alternative isn't necessarily the answer to long-term weight management.

Courtney Black’s experience may be genuine, but it doesn’t demonstrate that artificial sweeteners caused her to gain 10kg. And if you're regularly experiencing bloating or unexplained changes in your weight, it's worth looking at the bigger picture rather than assuming one ingredient is responsible.