The study, published in the journal Investigative Ophthalmology and Visual Science, looked at the coffee habits of 79,120 women and 42,050 men (121,170 total participants). The researchers found them through the Nurses' Health Study, which examined the years 1980 to 2004, and the Health Professionals Follow-Up Study, which tracked the years 1986 to 2004. All participants were at least 40 years old, and none had glaucoma at the beginning of the survey. The researchers, led by Jae Hee Kang from Brigham and Women's Hospital and Harvard Medical School, assessed the participants using questionnaires asking about their caffeine consumption, as well as by examining medical records.
Researchers were looking for incidences of exfoliation glaucoma, the most common cause of secondary glaucoma in the world, especially in people of European descent. It affects about 10% of adults over age 50. Chances of developing glaucoma are six times higher in people with exfoliation syndrome. Glaucoma (high pressure inside the eye that can damage the optic nerve) can, in turn, lead to vision loss or blindness, if left untreated.
Researchers were intrigued by a possible link between glaucoma and coffee because, according to Kang, Scandinavian populations have the highest consumption of caffeinated coffee in the world, and they also have a high incidence of exfoliation glaucoma.
Compared to abstainers, coffee drinkers were significantly more likely to develop exfoliation glaucoma. The study found that adults who drank three or more cups of coffee a day were 34% more likely to develop exfoliation glaucoma. Women with a family history of glaucoma had the highest risk, with their threat of exfoliation glaucoma soaring to 66% if they drank three cups of coffee daily.
Strangely, the correlation did not exist for other caffeinated drinks, like soda or tea, or for decaffeinated coffee.
However, coffee has been linked to a number of health perks, including reducing the risk of superbug infections, diabetes, strokes, and breast and prostate cancer. So Kang emphasizes the need for further study to find direct evidence that heavy consumption of coffee is indeed a risk factor for exfoliation glaucoma. And, apparently, there isn't a risk if you just stop at that second cup, and if you avoid being Swedish.