Could one cup of hot tea per day have a protective effect against the onset of glaucoma?
Glaucoma is an eye condition characterized by damage to the optic nerve, which may result in partial or total loss of eyesight. Risk factors for developing glaucoma include age, a medical history of diabetes, obesity, and hypertension.
According to recent data from the National Eye Institute, in 2010 alone, 1.9 percent of the North American population aged 40 and over was diagnosed with a form of glaucoma.
Furthermore, some studies hypothesized that the consumption of other caffeinated and non-caffeinated drinks could also influence the risk of developing glaucoma.
So far, this notion has not been verified, since most of the research addressing the link between drinks and the risk of heightened intraocular pressure referred to small, and thus inconclusive, population samples.
Recently, scientists from Brown University in Providence, RI, and the University of California in Los Angeles have decided to compare how the consumption of various drinks — including hot tea, decaffeinated tea, iced tea, coffee, and soft drinks — influence the risk of glaucoma.
"No study to date has compared the effects of caffeinated and decaffeinated coffee, tea, and soft drinks on glaucoma," write the researchers.
"The objective of this study," they add, "is to examine the association between consumption of various caffeinated and decaffeinated beverages and glaucoma."
The results of the study were published yesterday in the British Journal of Ophthalmology.
Lower risk for tea drinkers
Lead study author Connie Wu and her colleagues analyzed data sourced from the 2005–2006 National Health and Nutrition Examination Survey, which gathered the medical data of around 10,000 people.
The survey used a range of tools, including interviews, physical examinations, and blood samples, aiming to give a detailed pictured of health in the United States population.
The team chose the 2005–2006 survey because it also gathered data on glaucoma diagnoses. That year, 1,678 participants agreed to share full eye test results, and of these, 84 adults were found to have a form of glaucoma.
As a part of their assessment, the participants were quizzed on their drinking habits, including how much coffee, hot tea, decaffeinated tea, soft drinks, and iced tea they had drunk over the past year, and how often.
The researchers found that the participants who drank hot tea every day had a 74 percent lower risk of developing glaucoma than those who didn't.
To ensure the consistency of these results, the team also checked for potential confounding factors, such as a history of diabetes and smoking habits.
No links were found between glaucoma risk and any other type of beverage taken into account in the study, including coffee — both caffeinated and decaffeinated — as well as decaffeinated tea, iced tea, and soft drinks.
Is the relationship causal?
The scientists warn that this is only an association noted in an observational study, so no cause-effect relationship should be inferred without further analysis.
The study also had other limitations, such as the small number of participants with glaucoma and a lack of detailed information about the timeline of diagnosis.
Other missing information refers to how much of the beverage the hot tea drinkers actually had each day, what kind of tea they consumed, and how it was brewed, which may have swayed the findings.
Still, the study authors note in their paper that "[t]ea contains phytochemicals and flavonoids [types of active chemical compounds found in plants], which have been observed to have anti-inflammatory, anticarcinogenic, antioxidant, and neuroprotective properties associated with the prevention of cardiovascular disease, cancer, and diabetes."
Thus, the researchers suggest, it wouldn't be so far-fetched to consider that the consumption of tea could have a protective metabolic effect.
Wu and colleagues also refer to existing studies that have proposed that glaucoma may, in part, be an effect of oxidative stress and neurodegeneration, which are two processes linked to aging and breakdown at cellular and molecular levels.
Taking into account the potential protective effect of hot tea consumption when it comes to cell aging and damage, the researchers suggest that further efforts should be dedicated to investigating the role of this common, and much-loved, beverage.
"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 team concludes.