There has recently been a surge of research into the effect that exercise could have on dry eyes.- perfect timing for anyone about to take part in the London marathon.
The studies agree that regular exercise, specifically aerobic exercise like running, can help improve dry eyes. It’s really interesting to see that the studies show improvements in both objective measures of dry eyes and patient-reported symptoms. These don't always align, since like any discomfort condition (back pain, toothache) there is quite a lot of room for individual pain threshold and subjectivity.
The most recent study published in Experimental Eye Research found that even one session of exercise per week could improve 'Tear Breakup Time TBUT'. This shows eyecare professionals how quickly tears evaporate off the eye: the quicker they evaporate, the more likely the eye is drying out between blinks. An earlier study found that participants reported significantly higher 'Dry Eye-related Quality of Life Score' after working out three times a week for 10 weeks.
The reasons why exercise, such as running, might help improve dry eyes is not fully understood, and more research is needed. But it could very well be due to increased blood flow around the body and reduction in systemic inflammation. Dry eyes is an inflammatory condition, so anything that might bring down chronic inflammation in the body would certainly help!
That being said, many runners do also report that their symptoms are worse when running and that their eyes constantly stream when they're outside. This has less to do with running itself and more to do with windy conditions which can exacerbate already dry eyes, since the wind wicks away tears from the eye, leaving them more dehydrated. This can lead to a gritty, irritated feeling or even a red eye after running. Sometimes the eyes go into overdrive and produce overly watery eyes to try to overcompensate.