People with a parent or sibling diagnosed with glaucoma should consider limiting caffeine to the amount in two cups of coffee a day, says Louis Pasquale, MD, an ophthalmology professor at Mount Sinai Health System in New York City.
“It’s a suggestion,” he says. “It’s not something set in stone, but if you’re interested in reducing your risk of the disease, it’s something I would definitely entertain.”
Pascale and his colleagues reported their findings in Ophthalmology.
Glaucoma causes damage to the optic nerve, often from increased pressure from a buildup of fluid inside the eye. It can cause blindness.
The disease is more common in people with close family members who have it. And researchers have identified variants in multiple genes that are more common in people with glaucoma.
In previous studies, people who take large doses of caffeine have had temporary increases in eye pressure. So researchers have wondered whether getting caffeine over a long period of time could pose a risk.
To help answer that question, Pasquale and his colleagues looked at the records of more than 100,000 people. The records came from the UK Biobank, a large survey conducted in the United Kingdom. The records included information on patients’ genes, how much tea or coffee they drank, their eye pressure, and whether they had glaucoma.
Looking at all the participants together, the researchers did not find any increased risk of glaucoma from caffeine. In fact, they found that people who got the most caffeine actually had slightly lower eye pressure.
But when they looked only at people whose genes were associated with glaucoma, they found that caffeine was linked to higher average eye pressure and also to a higher risk of glaucoma.
That was particularly true when they looked at the 25% of people who got more than 321 milligrams of caffeine -- the equivalent of three cups of coffee -- a day. When these people were also in the top 25% of genetic risk for increased eye pressure, they had a much higher risk of glaucoma. They were almost four times more likely to have the disease than people who did not consume caffeine and were at the lowest 25% genetic risk.
The researchers found that the combination of caffeine and genes increased the risk of glaucoma more than the genes alone.
The researchers could not find a statistical link to higher eye pressure or glaucoma for coffee, only for tea. But that is likely because of a statistical problem, says study co-author Anthony Khawaja, associate professor at the University College London Institute of Ophthalmology. “The answer is quite simple− the UK is a tea drinking nation!” he says in an email. There were probably not enough people in the database who drink large amounts of coffee to analyze their glaucoma risk.
People in both the top 25% for genetic risk and top 25% for tea drinking (3 to 6 cups of tea a day), were almost three times as likely to have glaucoma compared to those in the lowest 25% for tea drinking and lowest 25% for genetic risk.
However, this type of study cannot prove that caffeine increases the risk of glaucoma or high eye pressure, even among people with a genetic risk, Pasquale says. He and his colleagues would like to do a follow-up study in which they give caffeine to groups of people with different genetic risks, then compare the effects on eye pressure.
Patients should not rely on changing their caffeine consumption as a way of treating their glaucoma, says Asaf Achiron, MD, an ophthalmologist at the Sackler Faculty of Medicine in Tel Aviv, Israel, who has studied the relationship of lifestyle and eye pressure. Caffeine did not have a big effect for most people in the study, he points out.
“Glaucoma is a complex disease. In some cases it continues to progress even when [eye pressure] is lowered by the drops so clearly there are more issues here than pressure,” he says in an email.
Still, the finding might be worth mentioning to patients, he says. “I don't discuss tea with my patients just measure their [eye pressure]. Perhaps I should.”