Data from a large retrospective study confirm the protective role of tea
The results of a large retrospective study published in the British Journal of Ophthalmology analysed the effects on glaucoma risk of caffeine intake through the consumption of coffee, tea and soft drinks.
The glaucomathe 'silent thief' of sight, is one of the leading causes of blindness and low vision in the world, with an estimated prevalence of 57.5 million people that could reach 65.5 million by 2020 if current trends are maintained.
The causes behind the onset of this serious eye disease are many and varied, ranging from elevated intraocular pressure (IOP) to blood pressure disorders, oxidative stress and autoimmune processes that combine to cause damage to the optic nerve and ganglion cells, leading to progressive visual field loss.
It was hypothesised that the intake of caffeine increases the risk of glaucoma by promoting increased IOP and levels of homocysteine, an amino acid produced by methionine metabolism whose increase is believed to pose risks to the cardiovascular, nervous, and bone systems, particularly through increased production of free radicals and oxidative stress.
Researchers at Brown University, led by Dr Connie Wu, looked at data from 2005-2006, collected from the National Health and Nutrition Examination Survey (NHANES). The NHANES is a programme initiated in the early 1960s with the aim of verifying the nutritional and health status of adults and children in the United States through the examination of a nationally representative sample of approximately 10,000 people, subjected to interviews, clinical examinations and blood samples.
For 12 months, 1,678 of the individuals in the sample underwent an eye examination, complete with fundus photos. 5% of the sample (84 adults) developed glaucoma and were monitored for caffeine intake through drinks.
The final data excluded a correlation between glaucoma risk and consumption of regular or decaffeinated coffee, iced tea and soft drinks.
Surprisingly, however, those who drank at least one cup of hot tea per day had a lower risk of glaucoma, an effect that was not observed in those who drank iced tea or hot decaffeinated tea.
In summary, daily consumption of iced tea reduced the risk of glaucoma by 74%.
Why does tea reduce the risk of glaucoma?
Researchers hypothesise that tea has a protective activity due to its high content of flavonoids and substances of plant origin that have antioxidant, anti-inflammatory, anti-cancer and neuroprotective activity. Numerous studies have already demonstrated these effects of tea substances in the prevention of cardiovascular diseases, cancer and diabetes, and more specifically in counteracting the oxidative and neurodegenerative aetiology present in the pathogenesis of glaucoma.
In addition, flavonoids have been shown to be able to inhibit the VEGF (Vascular Endothelial Growth Factor) receptor and thus may have a potentially useful action in preventing neovascular glaucoma, fibrotic tissue formation after glaucoma surgery and neurodegeneration.
Regarding the fact that iced tea does not produce the same protective effects as hot tea, the explanation could be precisely that the amount of flavonoids contained in iced tea is significantly lower than in hot tea and decaffeinated tea, as has already been shown in a previous study.
Before it can be concluded that tea consumption can be regarded as a behaviour to be recommended for the specific purpose of preventing glaucomatous pathology, however, further investigations are needed to overcome the observational and cross-sectional nature of the data collected, in order to have a more direct correlation between beverage consumption and the occurrence of glaucomatous pathology, as well as to collect more precise data on quantities (there were no cup size specifications) and also on the correlation with the different types of glaucoma.
Sources
Wu CM, Wu AM, Tseng VL, et al. Frequency of a diagnosis of glaucoma in individuals who consume coffee, tea and/or soft drinks. Br J Ophthalmol. 2018 Aug;102(8):1127-1133.
Hakim IA, Hartz V, Harris RB, et al. Reproducibility and relative validity of a questionnaire to assess intake of black tea polyphenols in epidemiological studies. Cancer Epidemiol Biomarkers Prev 2001;10:667-78.
Dr. Carmelo Chines
Direttore responsabile
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