Age-related vision loss is common and devastating. But new research suggests that physical activity might protect our eyes as we age.
There have been suggestions that exercise might reduce the risk of macular degeneration, which occurs when neurons in the central part of the retina deteriorate. The disease robs millions of older people of clear vision. A 2009 study of more than 40,000 middle-aged distance runners, for instance, found that those covering the most miles had the least likelihood of developing the disease. But the study did not compare runners to non-runners, limiting its usefulness. It also did not try to explain how exercise might affect the incidence of an eye disease.
So, more recently, researchers at Emory University in Atlanta and the Atlanta Veterans Administration Medical Center in Decatur, Georgia, took up that question for a study published last month in The Journal of Neuroscience. Their interest was motivated in part by animal research at the VA medical center. That work had determined that exercise increases the levels of substances known as growth factors in the animalsí bloodstream and brains. These growth factors, especially one called brain-derived neurotrophic factor, or BDNF, are known to contribute to the health and well-being of neurons and consequently, it is thought, to improvements in brain health and cognition after regular exercise.
But the brain is not the only body part to contain neurons. The retina does as well, and the researchers wondered whether exercise might raise levels of BDNF there, too.
To test that possibility, the researchers gathered adult, healthy lab mice. Half of these were allowed to remain sedentary throughout the day, while the other animals began running on little treadmills at a gentle rodent pace for about an hour a day. After two weeks, half of the mice in each group were exposed to a searingly bright light for four hours. The other animals stayed in dimly lit cages. This light exposure is a widely used and accepted means of inducing retinal degeneration in animals. It doesnít precisely mimic the slowly progressing disease in humans, but causes a comparable if time-compressed loss of retinal neurons.
The mice then returned to their former routine -- running or not exercising -- for another two weeks, after which the scientists measured the number of neurons in each animalís eyes. The unexercised mice exposed to the bright light were experiencing, by then, severe retinal degeneration. Almost 75 per cent of