NEW YORK -- Obesity may increase the risk that the eye disease age-related macular degeneration will worsen, a new study shows.

But physical activity may reduce the risk, according to the study published in the Archives of Ophthalmology.

Age-related macular degeneration is a condition in which the macula, a spot at the center of the retina, degrades with time. As the macula deteriorates, it becomes increasingly difficult for people with the condition to focus on objects directly in front of them. Peripheral vision is generally unaffected.

Approximately 30 percent of people who are 75 years or older have some degree of macular degeneration. Most have a mild form, however.

No one knows exactly why the macula begins to degenerate, but some risk factors have been identified, according to the study's lead author, Dr. Johanna M. Seddon, an associate professor at the Harvard Medical School and director of the epidemiology unit at the Massachusetts Eye and Ear Infirmary, both in Boston.

These factors include cigarette smoking, high levels of dietary fat and low levels of antioxidants, Seddon said in an interview with Reuters Health.

Studies have shown that antioxidant vitamin and mineral supplements can reduce the risk of disease progression and vision loss, Seddon said.

Scientists suspect that a possible cause for the disease could be compromised blood flow to the retina, according to Seddon. For this reason, Seddon and her colleagues wanted to look at some of the factors that are also linked with the artery disease atherosclerosis.

For the new study, researchers followed 261 patients who were at least 60 and had signs of mild age-related macular degeneration. Often patients with mild disease have no obvious symptoms, Seddon said. But a doctor can see signs of deterioration in the retina.

The patients were followed for an average of 4.6 years. The patients' height, weight and blood pressure were measured annually. The patients performed waist and hip measurements themselves.

Ultimately, the researchers determined that overweight and obese participants were more than twice as likely to have their macular degeneration worsen to the advanced form of the disease during the course of the study than those who were thinner.

A larger waist-to-hip ratio was also associated with an increased risk of disease progression.

But patients who exercised vigorously at least three times a week had a 25 percent reduction in the risk that their disease would worsen, Reuters reported.

Seddon suggests that anyone in their mid 50s with a family history of macular degeneration have their eyes tested for this condition.