Why do astronauts have bad eyes? Scientists have taken a closer look at astronauts and have found that low-gravity environments may have a negative effect on eyesight.

Scientists previously identified biochemical differences in astronauts experiencing vision issues, specifically that metabolites in the one-carbon pathway were elevated. The study was followed up by examining a handful of differences in enzymes in the one-carbon pathway.

The scientists also collected blood samples from astronauts for DNA, biochemical and metabolic analyses; they also measured vision and ocular parameters. This revealed two genetic differences were significant factors in whether individuals developed vision issues in low-gravity environments.

"We've identified a genetic link in astronauts with vision issues," said Scott Smith, one of the researchers, in a news release. "We still don't know the mechanism, or what is causing the vision issues, but being able to narrow down who to study should help refine research and hopefully will speed finding the cause and a way to treat, or ideally to prevent, these problems from happening."

More bad news for long duration human space travel. To read more, click here.