Do diet fizzy drinks cause stroke and dementia?

Four glasses of colourful fizzy drinks
21st April 2017

‘Diet’ or ‘low-calorie’ soft drinks that contain artificial sweeteners could increase your risk of stroke, according to the latest news coverage. We look behind the headlines.

Drinking one can or more of diet fizzy drink could mean you are up to three times more likely to have a stroke, research suggests. How reliable is this finding?

The researchers, from Boston University, found that a higher intake of artificially sweetened soft drinks were associated with an increased risk of stroke and dementia.

Those consuming ‘diet’ drinks every day were 2.96 times more likely to suffer an ischaemic stroke (the most common type of stroke) and 2.89 times more likely to develop Alzheimer’s disease than those who drank them less than once a week, the study found. The research also found an increased risk of other types of dementia, not just Alzheimer’s, although the increase in risk was smaller.

Only 3 per cent of the people in the study had a stroke and 5 per cent developed dementia

Non-diet drinks, sweetened with sugar, were not associated with an increased risk of stroke or dementia, the study, published in Stroke, the journal of the American Heart Association, found.

The researchers looked at 2,888 adults aged over 45 (average age 62) for stroke, and 1,484 people over 45 (average age 60) for dementia. The participants were asked to complete a food-frequency questionnaire to see how many sugary drinks they were having.

These people were then followed up for 10 years, during which there were 97 cases of stroke and 81 cases of dementia. To put this into perspective, only 3 per cent of the people in the study had a stroke and 5 per cent developed dementia, so it is still a small number of people developing either condition. This also means that the findings were based on relatively small numbers, increasing the possibility that the results were coincidence rather than a cause and effect.

The people in the study who were drinking artificially-sweetened drinks could have made this choice because of health conditions, like diabetes, which put them at a greater risk of stroke and dementia, regardless of their drink preference.

The study authors say that more research is needed, and suggest this should be clinical trials investigating whether artificially sweetened drinks are associated with important stroke risk factors, such as high blood pressure.

The research

It is important to stress that the researchers found an association, rather than cause and effect, and don’t know why artificial sweeteners are linked to stroke. The study authors point out that previous research has linked diet drinks with increased risk of diabetes, although it’s not clear whether this is related to these findings.

The participants weren’t necessarily representative of the UK population

One issue with the research is that it relied on people honestly remembering and stating how many soft drinks they have consumed over the previous year. The researchers admit in the report that this could ‘introduce error’.

People in the study were asked about their intake of:

  • total sugary beverages (combining sugar-sweetened soft drinks, fruit juice, and fruit drinks)
  • sugar-sweetened soft drinks (high-sugar carbonated beverages, such as cola)
  • artificially sweetened soft drinks (sugar-free carbonated beverages, such as diet cola).

The participants were all from the town of Framingham in Massachusetts, and were part of The Framingham Heart Study. Another limitation is the absence of ethnic minorities taking part in the study. Together this means that it is difficult to apply these results to the UK as the participants weren’t necessarily representative of the UK population.

The questionnaires were carried out between 1991–1995, 1995–1998, and 1998–2001, so are more than 15 years old.

A strength of the research was that its judgement of whether people have had a stroke is thorough.

The researchers took into account people’s lifestyle habits and medical history, to make sure that this rather than sugary drink intake wasn’t causing the results they found.

Bottles of brightly coloured fizzy drinks

The BHF view

Tracy Parker, Heart Health Dietitian at the British Heart Foundation said, “Sugar sweetened beverages are not only bad for our teeth but the excess calories can make us put on weight, which is a risk factor for heart disease.

Artificially sweetened beverages should be drunk as an occasional treat rather than a daily beverage

“Although artificially sweetened beverages may be a better choice for those looking to lose weight or control their blood glucose levels, they should be drunk as an occasional treat rather than a daily beverage.

“The study is interesting and highlights the need for more research in this area, however it doesn’t show a causal link between fizzy drinks and developing stroke and dementia, so the end result is limited. Further research is needed before we draw any definite conclusions.

“Why not try a much healthier daily beverage like water or unsweetened coffee and tea with low fat milk,” she recommends.

Media coverage

The research was widely covered in the media, including The Guardian, The Telegraph, the Express, and the Daily Mail.

The Daily Mail article said: “The team of scientists from Boston University believe the artificial sweeteners including aspartame and saccharine may be affecting the blood vessels, eventually triggering strokes and dementia.” The study did not look at the blood vessels, and the published research does not mention this as a possible cause. In the online version of the story, the article opens with “Boston University researchers found aspartame, a low-calorie sweetener, wreaks havoc on the arteries - as opposed to sugar-sweetened drinks”. The study looked at artificially sweetened soft drinks but did not look specifically at aspartame. And it did not find that they “wreak havoc on the arteries”.

The Guardian coverage highlighted that further research was needed and that these conclusions may not be reliable.

The Express coverage also included a related study by the same researchers, published in the journal Alzheimer's & Dementia, which found a link between sugary soft drinks and memory problems and smaller brain volume.  However it didn’t clearly distinguish that this related to sugary drinks, rather than ‘diet’ drinks which featured in the research on stroke and dementia. 

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