A new study suggests that vigorous physical activity may increase the risk for vision loss, a finding that has surprised and puzzled researchers.

Using questionnaires, Korean researchers evaluated physical activity among 211,960 men and women ages 45 to 79 in 2002 and 2003. Then they tracked diagnoses of age-related macular degeneration, from 2009 to 2013. Macular degeneration, the progressive deterioration of the central area of the retina, is the leading cause of vision loss in the elderly.

They found that exercising vigorously five or more days a week was associated with a 54 percent increased risk of macular degeneration in men. They did not find the association in women.

The study, in JAMA Ophthalmology, controlled for more than 40 variables, including age, medical history, body mass index, prescription drug use and others.