A new study suggests aerobic exercise may slow the progression of diseases that destroy the retina and eventually cause blindness. In mice exposed to harmful bright lights, the ones that had regularly run on treadmills had much better function in their retinas than mice that had not been exercising.
The researchers suggest the findings, which they report in The Journal of Neuroscience, point to exercise as a possible treatment for slowing down human eye diseases like age-related macular degeneration (AMD), a leading cause of blindness in the elderly.
AMD results when photoreceptors - nerve cells that sense light - in the retina at the back of the eye start to die.
According to the American Academy of Ophthalmology, more than 2 million Americans age 50 and over have advanced AMD, the stage that can lead to severe vision impairment.
Although both animal and human studies have suggested exercise may slow down the progress of neurodegenerative diseases or injury, there is little information about how it might affect vision.
It has also been suggested that aerobic exercise works by stimulating a protein called brain-derived neurotrophic factor (BDNF), which helps brain cells grow and stay healthy.
In this new study, Dr. Machelle Pardue and colleagues, from the Atlanta VA Center for Visual and Neurocognitive Rehabilitation and Emory University, test the effect of aerobic exercise on retinal cells undergoing degeneration.
Exercise reduced loss of light-sensing cells in mice
They worked with two groups of mice, an exercise group and a non-exercise group. The exercise group ran on treadmills for an hour a day on 5 days per week for 2 weeks, while the non-exercise group was placed on stationary treadmills during the same periods.
After 2 weeks of exercise, some mice from both groups were exposed to very bright lights (bright enough to damage their retinas) for 4 hours, and the others were exposed to dim lights.
The bright lights "caused 75% loss of both retinal function and photoreceptor numbers," note the researchers.
"However," they write, "exercised mice exposed to bright light had 2 times greater retinal function and photoreceptor nuclei than inactive mice exposed to bright light."
Plus, they found the exercised mice had 20% higher levels of BDNF protein than the non-exercised mice.
Exercise protects the retina, probably by raising levels of BDNF
To test whether it was BDNF that was mediating the effect of exercise, the researchers injected the mice with a drug that blocked the protein. They found it reduced retinal function and photoreceptor counts in the exercised mice to "inactive levels."
The researchers conclude their findings "suggest that aerobic exercise is neuroprotective for retinal degeneration and that this effect is mediated by BDNF signaling."
Dr. Pardue says:
"This is the first report of simple exercise having a direct effect on retinal health and vision. This research may one day lead to tailored exercise regimens or combination therapies in treatments of blinding diseases."
Dr. Michelle Ploughman, of Memorial University of Newfoundland, where she studies the effect of exercise on the brain, says the findings give us new clues about how aerobic exercise affects BDNF and protects the brain, and adds:
"People who are at risk of macular degeneration or have early signs of the disease may be able to slow down the progression of visual impairment."
Funds from the National Eye Institute, Department of Veterans Administration Affairs, and the Katz Foundation, helped finance the study.
Meanwhile, Medical News Today recently reported how UK scientists used gene therapy to restore some sight in people with an incurable form of inherited blindness called choroideremia. They replaced a defective gene in their retinas with a working version of the same gene.