Findings from a US study found hot tea-drinkers were 74% less likely to have glaucoma, which also found no difference in risk in those drinking decaffeinated and caffeinated coffee, decaffeinated tea, iced tea and soft drinks.
“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 study authors wrote.
The observational study’s limitations such as cup size, tea type, or the length of brewing time, which the research team acknowledged, drew criticism from Catherine Collins, a UK-registered dietitian who dismissed their conclusions as “spurious claims”.
No plausible reason for link
“Tea is a healthy drink, rich in antioxidant polyphenols such as tea catechins and other flavonoids,” said Catherine Collins, a registered dietitian.
“Given the anti-oxidant content of brewed tea remains stable whether you drink it scalding hot or lukewarm, this finding doesn’t offer a biologically plausible reason for reduced glaucoma incidence.
“Together with the small sample size, this makes any association between drinking hot tea and glaucoma highly unlikely.”
Professor Chris Hammond, Frost chair of ophthalmology at King’s College London, agreed that the results added to the increasing consensus that tea’s antioxidants were good for our health.
“However, as this study looked at many dietary factors and is only a snapshot taken at a single time point, further longer term studies in the UK and other populations are needed see if tea drinking really does protect us from glaucoma.”
Dr Graham Wheeler, Bayesian medical statistician at University College London (UCL), suggested a much larger study, with repeated interviews over time, was needed to better understand the associations between different drinks and developing glaucoma.
“Participants were not asked what type of tea they drank, or whether milk, sugar, or other additives were used,” he said.
“The authors analysed each drink separately, so couldn’t explore whether any associations were present when, for example, participants often drank hot tea and coffee.”
Tea testing criteria
The team, from the University of California, Los Angeles, began looking at health data taken from 1678 participants. Among those who had had full eye tests, including photos, 84 (5%) adults had developed the condition.
A questionnaire was used to asses how often and how much they had drunk caffeinated and decaffeinated drinks, including soft drinks and iced tea, over the previous year.
Compared with those who didn’t drink hot tea every day, those who did, had a lower glaucoma risk,
After taking account of potentially influential factors, such as diabetes and smoking, hot tea-drinkers were 74% less likely to have glaucoma.
However, no such associations were found for coffee--caffeinated or decaffeinated--decaffeinated tea, iced tea or soft drinks.
“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, study co-author and professor in-residence at the University of California, Los Angeles (UCLA).
Glaucoma refers to the condition that sees an increase in intraocular pressure (IOP). This can contribute to blood flow abnormalities, which damage the optic nerve resulting in visual field loss.
By 2020 those affected is expected to increase to 58.5 million. Almost half (47%) of these people will reside in Asia while 24% will be European.
The protective qualities of tea were referenced by the team, citing it’s phytochemical and flavonoid content as well as its proven anti-inflammatory, antioxidant and neuroprotective properties.
Additionally, flavonoids have been shown preventative qualities for neovascular glaucoma.
Caffeinated teas may have greater antioxidant capacity compared with decaffeinated teas, possibly explaining why decaffeinated tea did not correlate with decreased glaucoma risk.
Source: British Medical Journal Opthamology
Published online ahead of print, doi: 10.1136/bjophthalmol-2017-310924
“Frequency of a diagnosis of glaucoma in individuals who consume coffee, tea and/or soft drinks.”
Authors: Connie Wu, Annie Wu, Victoria Tseng, Fei Yu, Anne Coleman