Drinking hot tea could be linked to a lower risk of having an eye condition that can lead to blindness, research has suggested – although experts say the study does not show that the brew offers any protective effect.
Glaucoma is an eye condition in which the pressure of fluids inside the eye damages the optic nerve – and can lead to blindness if left undetected. Many are unaware they have the condition, and while the risk of glaucoma increases with age, it can also affect babies and children. About 57.5 million people are thought to have the condition worldwide.
Now researchers delving into the question of whether caffeine could affect pressure inside the eye say they have found that drinking hot tea appears to be linked to a lower risk of glaucoma.
Writing in the British Journal of Ophthalmology, researchers in the US describe how they analysed data from a 2005-2006 nationwide health and nutrition survey, looking at the results of eye examinations from 1,678 participants aged 40 or over.
They also analysed the participants’ responses to a questionnaire about how often they had drunk coffee, hot tea, soft drinks or iced tea in the past year, and whether those drinks were caffeinated or decaffeinated.
In total, 84 participants were found to have glaucoma, with diabetes more common and smoking less common for those with the condition than the participants taken as a whole.
There was no link between drinking coffee, soft drinks or iced tea and having glaucoma, whether the drinks were decaffeinated or not, nor between glaucoma and decaffeinated hot tea.
But, the team noted, there did appear to be a link to hot tea in general, with those consuming more than six cups of the brew a week less likely to have the condition even when factors such as age, body mass index, smoking and diabetes were taken into account.
Overall, the chance of having glaucoma – compared with not having it – was 74% lower among those who drank hot tea in such quantities.
“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.
The team suggests that certain plant chemicals such as flavonoids and other antioxidants found in tea could be offering a protective effect, adding that caffeinated tea might be richer in antioxidants than decaffeinated tea.
But they admit that it is difficult to draw any conclusions since the study only offers a snapshot in time and does not look at whether drinking hot tea actually lowers the risk of glaucoma.
It is also a puzzle why there is no link between iced tea and lower glaucoma risk.
And there are other limitations, including a lack of data on the type of tea drunk, that participants could have drunk multiple types of beverage, possible errors in diagnosis of glaucoma, and the fact that very few participants had glaucoma – and very few of them drank hot tea. What’s more, the survey relied on participants self-reporting what they had drunk, with the authors noting the association could even be a matter of chance.
“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.
Melanie Hingorani from the Royal College of Ophthalmologists urged caution. “Absolutely nothing is proven or even strongly evidenced – there are all sorts of things wrong with it,” she said of the study. All those over the age of 40 should have an eye-check every one to two years, she said, and more regularly for those at risk, such as those with diabetes or a family history of glaucoma. If detected, Hingorani added, glaucoma can be tackled – although not reversed – with drops or other treatments.
Naveed Sattar, professor of metabolic medicine at the University of Glasgow, said that while the study was interesting, it did not mean that anyone should starting increasing their hot tea consumption.
“I would not say in any way that this proves that drinking tea prevents glaucoma,” he said, adding that the study did not take into account other habits or socioeconomic factors that could be at play, and that robust clinical trials would be needed to show if tea had any benefits for glaucoma risk.
Instead, Sattar said, individuals should focus on other lifestyle choices – such as having a good quality diet and staying active – to reduce the likelihood of developing conditions known to be linked to an increased risk of glaucoma, such as diabetes.
“Drinking tea without sugar is certainly beneficial because it is a very low calorie drink; whether it has got other magical benefits like preventing glaucoma I think is open to question,” he said.