People who drink two litres or more of diet soda or other artificially sweetened beverages per week may have a higher chance of developing a dangerous heart condition called atrial fibrillation, according to a new study.

Atrial fibrillation, or AFib, is a disorder that causes irregular and often rapid heartbeat, which can lead to stroke, heart failure, and other complications. It affects about 33 million people worldwide, according to the World Health Organization.

The study, published in the journal Circulation: Arrhythmia and Electrophysiology, analyzed data from more than 202,000 participants in the UK Biobank, a large-scale biomedical database.

The researchers found that those who consumed two litres or more of artificially sweetened drinks per week had a 20% higher risk of AFib, compared to those who drank none. They also found that those who drank two litres or more of sugar-sweetened drinks per week had a 10% higher risk of AFib, while those who drank one litre or less of pure fruit juice per week had an 8% lower risk of AFib.

The researchers adjusted for factors such as age, sex, body mass index, smoking, alcohol intake, physical activity, diabetes, hypertension, and heart disease.

The study is the first to report a link between artificially sweetened drinks and AFib risk, according to the authors. They speculated that artificial sweeteners may affect the gut microbiome, which is the community of bacteria and other microorganisms that live in the digestive system. The gut microbiome plays a role in metabolism, inflammation, and immunity, and may influence the development of cardiovascular diseases.

The authors cautioned that their study does not prove causation and that more research is needed to confirm the findings and explore the underlying mechanisms. They also noted that the UK participants were mostly white and of European ancestry, so the results may not be generalizable to other populations.

However, they advised that people should limit or avoid artificially sweetened and sugar-sweetened drinks whenever possible, and opt for water or unsweetened tea or coffee instead.

“Do not take it for granted that drinking low-sugar and low-calorie artificially sweetened beverages is healthy, it may pose potential health risks,” said Dr. Ningjian Wang, the lead study author and a researcher at the Shanghai Ninth People’s Hospital and Shanghai Jiao Tong University School of Medicine in China, in a press release.

Other experts also warned of the possible dangers of diet drinks for heart health. Penny Kris-Etherton, PhD, an American Heart Association Nutrition Committee member who wasn’t involved in the research, said this was the first study to report a link between these kinds of beverages and a higher risk of atrial fibrillation. She called for more research on the topic and noted that “in the meantime, water is the best choice, and, based on this study, sweetened beverages should be limited or avoided.”