December 14, 2017

expert reaction to study looking at drinking hot tea every day and risk of glaucoma

Scientists examine the association between various drinks and glaucoma risk, in a new study, published in the British Journal of Ophthalmology.

Prof. Chris Hammond, Frost Chair of Ophthalmology, King’s College London, said:

“These results are interesting and add to the increasing consensus that tea contains antioxidants and other compounds that are good for our health.  Glaucoma becomes more common with age, and is a significant cause of blindness in the UK.  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, UCL, said:

“This survey does not show that drinking hot tea is linked to lowering your risk of developing glaucoma.  Participants were interviewed once and were not followed up, so the authors could as easily have reported that people with glaucoma are less likely to drink hot tea.  The authors did not explore how, biologically, this association might work, which means the results from this study say nothing about one thing causing another.

“Participants self-reported how many times a day or week they consumed each drink, as well as their smoking and diabetes status, which may introduce inaccuracies into the study.  The actual amount consumed of each drink was not recorded.  Participants were not asked about their water consumption or alcohol consumption, which may be of interest in a dietary study like this.  No adjustment was made for whether participants had other eye disorders or not.

“By looking at lots of different drinks, the chances increased of the investigators finding at least one drink that would, purely by chance, appear to be associated with either an increase or decrease in the prevalence of glaucoma.  Very few participants with glaucoma reported drinking hot tea at least once a week; it wouldn’t take much variation in the observed data to completely change the conclusions of this study.  A much larger study, with repeated interviews over time, is needed to better understand what the associations might be 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.  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.”

Catherine Collins, Registered Dietitian, said:

“Population studies often throw up spurious claims between foods and health – and this is one of those times.  This research, based on dietary survey data collected 12 years ago, appears to show daily drinking of hot tea reduced the risk of the eye condition glaucoma by 74%, an effect not seen in those taking other hot or cold drinks.

“Tea is a healthy drink, rich in antioxidant polyphenols such as tea catechins and other flavonoids.  Their levels in tea depends on brewing time, size of cup or mug, and type of tea used.  However, in this research it wasn’t the type of tea or its strength that appeared protective, rather the temperature at which it was drunk.  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.

“But don’t let that stop you having a cup of tea – it’s a healthy, sugar free drink which may also help protect against tooth decay¹.”

1 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841993/ Goenka P, Sarawgi A, Karun V, Nigam AG, Dutta S, Marwah N.  ‘Camellia sinensis (Tea): Implications and role in preventing dental decay’.  Pharmacognosy Reviews.  2013;7(14):152-156. doi:10.4103/0973-7847.120515.

* ‘Frequency of a diagnosis of glaucoma in individuals who consume coffee, tea and/or soft drinks’ by Connie M Wu et al. published in the British Journal of Ophthalmology on Thursday 14 December 2017.

Declared interests

Dr Graham Wheeler: “I am employed by UCL, have a visiting researcher position at the MRC Biostatistics Unit at the University of Cambridge, am a Fellow of the Royal Statistical Society, and a voluntary research committee member for Chiltern Music Therapy, a not-for-profit organisation providing music therapy services.  I have no financial conflicts of interest.”

Catherine Collins: “No conflict of interest declared.”