Back in May we told you about a NASA study that found that long space flights can impact astronauts' eyes and vision. Now NASA thinks they may know why this is the case.

View of Earth from space

A new NASA study found a link between abnormally low levels of a B vitamin called folate and the vision problems some astronauts experience during space flight. But when the researchers checked the diets of these astronauts, there was no evidence that they had a vitamin deficiency. The researchers began to suspect another cause, one that can look a lot like a vitamin deficiency but is harder to pronounce: enzyme polymorphisms. (Patience, dear reader — soon this will all make sense.)

Polymorphisms are small biological variations that are hereditary. An example of a common polymorphism in humans is the condition called lactose intolerance — trouble digesting cow's milk products.

Enzymes play key roles in the genetic instructions that are crucial to the normal function of bodily processes. When an enzyme doesn't work well, it can slow down an entire "assembly line" that governs a bodily process. Some enzyme polymorphisms may not work well in challenging conditions, like microgravity during space flight.

Earth-based studies gave NASA scientists the tip-off on enzyme polymorphisms. Here's how they connected the dots. Certain enzyme polymorphisms have been found in people who suffer from migraine, stroke or other diseases linked to intracranial pressure, which means the build-up of excess pressure within the head. Intracranial pressure is thought to be a cause of astronauts' eye and vision problems, as well. In microgravity conditions, bodily fluids shift from the lower body toward the head. When an astronaut's body doesn't efficiently process this excess fluid, intracranial pressure can result. So, it's likely that astronauts who have eye and vision problems have enzyme polymorphisms that make them susceptible to developing intracranial pressure.

The next research steps will be to test for the presence of enzyme polymorphisms and verify whether they are related to astronauts' eye and vision changes. The researchers say they're excited about this new piece of the puzzle, but that it's too soon to reach any conclusions.

NASA's quest to understand astronauts' vision problems began when a survey showed that about 23 percent of short-flight and 49 percent of long-flight astronauts had problems with near and distance vision during their missions. In some, vision problems continued long after they returned to Earth. NASA then studied seven astronauts who had lived in space for six months or longer and found that all had vision and eye anatomy abnormalities. The most common change was flattening of the back of the eyeball. Changes in the retina, the light-sensitive area at the back of the eye, and the optic nerve were also found.

NASA just approved eight eye/vision studies for the next stage of astronaut health research. This work may eventually improve eye care on Earth as well as in space, especially for diseases of the retina and of the optic nerve.    

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