An intriguing study has found a correlation between drinking tea and a reduced risk of glaucoma. However, don’t start brewing an extra cup just yet, because no causation has been established.

Glaucoma is an eye condition in which fluid pressure inside the eye causes systematic damage to the optic nerve. Most types of glaucoma have no symptoms and can creep in and lead to blindness if they are not detected and treated early. Writing in the British Journal of Ophthalmology, researchers in the US describe the association between consumption of coffee, tea or soft drinks and this condition, which affects over 2.7 million people in America alone, and almost 60 million worldwide.

They surveyed 1,678 participants of the 2005–2006 National Health and Nutrition Examination Survey (NHANES). Across them, the prevalence of glaucoma was 5.1% (84 people). There was no significant correlation between glaucoma and consumption of caffeinated and decaffeinated coffee, iced tea and soft drinks, and glaucoma. But when it came to hot tea, things were quite different. As it turns out, tea drinkers are at a significantly lower risk of glaucoma.

Participants who consumed at least six cups a week were 74% less likely to have glaucoma when compared with those who did not consume hot tea. To make things even more interesting, there was no change for patients who drank decaffeinated hot tea.

“In summary, individuals who consumed hot tea were less likely to have a diagnosis of glaucoma compared with those who did not consume hot tea,” the authors write.

Of course, this is just a correlation and no causation has been properly explored, but the fact that the same results didn’t carry over to decaf tea seems to suggest that certain plant chemicals — such as flavonoids and other antioxidants found in tea — provide a protective effect to the eyes. But this still doesn’t prove anything, and it doesn’t really explain why the same effect wasn’t reported in iced tea.

There are also other limitations to the study, including a lack of information on the tea that was drunk, a limited sample size for both people with glaucoma and tea drinkers, and possible errors in diagnosis. Still, this is good news if you’re a tea drinker.

“Tea drinkers should feel comfortable about drinking tea but should realise that the results are preliminary and drinking tea may not prevent glaucoma,” said Anne Coleman, co-author of the research from the University of California, Los Angeles.

But if you want to reduce your risk of glaucoma, there are other things you should focus on. The biggest risk factors for glaucoma are still an unhealthy diet and a lack of physical activity. So while tea is still probably good for you (especially without sugar), staying fit and eating healthy food is still the best thing you can do.

Journal Reference: Connie W. Mu et al. Frequency of a diagnosis of glaucoma in individuals who consume coffee, tea and/or soft drinks. http://dx.doi.org/10.1136/bjophthalmol-2017-310924

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