The study was small, including only 78 people with normal tear production, says researcher Reiko Arita, MD, PhD, a clinical researcher at the University of Tokyo School of Medicine and an associate professor at Keio University in Japan.
She also found those with certain genetic variations had more tears in their response to caffeine. These variations differ among ethnic groups.
"However," she says, "I suppose a third to a half of the population has the possibility to respond to caffeine.''
The study is published in the journal Ophthalmology.
About 5% of the U.S. population has dry eye syndrome, usually over the age of 50, according to the American Academy of Ophthalmology. Those who have it are not able to maintain a healthy layer of tears to coat the eye. The eyes can sting and burn.
Treatments include wetting drops or artificial tears, lubricating ointments, hot compresses, and prescription medicine to increase tear production.
For most people, moderate amounts of caffeine is not harmful, according to the National Institutes of Health. However, too much caffeine can make you restless, anxious, and irritable. It may also keep you from sleeping well and cause headaches, abnormal heart rhythms, or other problems. If you stop using caffeine, you could get withdrawal symptoms.
Some people are more sensitive to the effects of caffeine than others and should limit their use of caffeine. So should pregnant and nursing women. Certain drugs and supplements may interact with caffeine.
Caffeine for Dry Eye: Study Details
In earlier research, Arita found that caffeine users were less likely to get dry eye. "Actually, this study was started because a patient with dry eye told me that his eyes were more comfortable when he had a cup of coffee after lunch than without having a cup of coffee," Arita says.
Women are more likely than men to have dry eye, as are older people. Certain medicines as well as laser eye surgery can increase the risk of getting dry eye.
Arita also knew caffeine was likely to stimulate tear glands, since it also increases saliva, digestive juices, and other secretions.
It's also known that people differ in their response to caffeine, depending on their DNA. Arita's team analyzed the DNA of everyone in the study. They looked at two genetic variations that affect caffeine metabolism.
None of the men or women had dry eye, eye allergies, or other problems. They participated in two sessions, separated by six days or less. In one, they took either a caffeine capsule or a placebo, which looked the same.
The amount of caffeine given depended on a person's weight; those who weighed 127 to 165 pounds, for instance, got a 400 milligram caffeine capsule. One 8-ounce cup of brewed coffee contains about 100 milligrams of caffeine.
During the second session, they were given the other capsule.
Arita's team measured the total volume of tears. Everyone had more tears after taking the caffeine capsule than the placebo. Some participants reported caffeine side effects such as increased heart rate, sweating, trembling, palpitations, and wakefulness.
Those who had the genetic variations had higher tear production than those who didn't.
Arita can't explain yet exactly why caffeine appears to boost tear volume.
Caffeine for Dry Eye: Perspective
The idea could have merit but needs much more research, says Pedram Hamrah, MD, attending physician and surgeon at Massachusetts Eye and Ear and assistant professor of ophthalmology at Harvard Medical School.
He reviewed the findings but did not take part in the study.
"All they are showing is in a normal person who doesn't have dry eye, if you give them caffeine they have some increase in tear production right after," he says. It's not certain if the effect lasts, Hamrah says.
Most who have dry eye also have a high rate of tear evaporation, he says. The caffeine research doesn't address that.
Hamrah reports serving as a consultant for ReVision Optics, Allergan, Alcon, and Fovea Pharmaceuticals.
"It is logical that caffeine might stimulate tear production in many individuals," says Marguerite McDonald, MD, an ophthalmologist in Lynbrook, N.Y., and a clinical professor of ophthalmology at NYU Langone Medical Center. She also reviewed the findings.
The increased tear production is linked to certain genetic markers, she says. This explains why some people have a reaction to caffeine and others have little response. The research "may lead us to new medical therapies for dry eye," says McDonald, who is also an adjunct clinical professor of ophthalmology at Tulane University Health Sciences Center in New Orleans.
McDonald reports consultant work for Allergan, Alcon, Bausch + Lomb, and Abbott.