A cup of hot tea a day can keep glaucoma away, according to a new study published in the British Journal of Ophthalmology.

Glaucoma causes fluid pressure to build up inside the eye (intraocular pressure), damaging the optic nerve. It is one of the leading causes of blindness worldwide, and currently affects 57.5 million people, and is expected to increase to 65.5 million by 2020.

Previous research suggests that caffeine can alter intraocular pressure, but no study so far has compared the potential impact of decaffeinated and caffeinated drinks on glaucoma risk.

So Dr. Anne Coleman from the University of California, Los Angeles, and co-authors looked at data from the 2005-2006 National Health and Nutrition Examination Survey (NHANES).

Among 1,678 participants who had full eye test results, including photos, 84 (5%) adults had developed the condition.

They were asked how often and how much they had drunk of caffeinated and decaffeinated drinks, including soft drinks and iced tea, over the preceding 12 months, using a validated questionnaire.

Compared with those who didn’t drink hot tea every day, those who did, had a lower glaucoma risk, the data showed.

After taking account of potentially influential factors, such as diabetes and smoking, hot tea-drinkers were 74% less likely to have glaucoma.

But no such associations were found for coffee (caffeinated or decaffeinated), decaffeinated tea, iced tea or soft drinks.

“This is an observational study so no firm conclusions can be drawn about cause and effect, and the absolute numbers of those with glaucoma were small,” Dr. Coleman and colleagues noted.

“Nor did the survey ask about factors like cup size, tea type, or the length of brewing time, all of which might have been influential.”

“But tea contains antioxidants and anti-inflammatory and neuroprotective chemicals, which have been associated with a lowered risk of serious conditions, including heart disease, cancer, and diabetes,” they said.

“And previous research has suggested that oxidation and neurodegeneration may be involved in the development of glaucoma.”

“Further research is needed to establish the importance of these findings and whether hot tea consumption may play a role in the prevention of glaucoma,” the researchers concluded.

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Connie M. Wu et al. Frequency of a diagnosis of glaucoma in individuals who consume coffee, tea and/or soft drinks. British Journal of Ophthalmology, published online December 14, 2017; doi: 10.1136/bjophthalmol-2017-310924