Researchers at the Harvard School of Public Health (HSPH) found that vigorous physical exercise during early adulthood is correlated with a decreased risk of Parkinson’s disease in men, according to a study released on Monday.

The study also noted that women who exercised in early adulthood experienced no statistically significant reduced risk of developing the disease.

Parkinson’s disease is a condition that affects more than half a million people, with more than 50,000 new cases reported annually. Most cases are diagnosed after the age of 50 and major symptoms include tremor, stiffness, and a marked decline in the speed of movement.

The study followed 48,000 men and 77,000 women for a period of 14 years, from 1986 to 2000.

Each subject was healthy at the outset and underwent an evaluation every two years that assessed many factors about their physical activity and personal health. By the end of the study, clinicians had diagnosed 387 cases of Parkinson’s disease.

From the data, researchers were able to discern that men who exercised the most experienced a 50 to 60 percent lower risk of developing Parkinson’s disease than those who were not active. The type of exercise was not a factor into this risk reduction.

Alberto Ascherio, a senior author of the study and Associate Professor of Nutrition and Epidemiology at HSPH, was excited about the results of the study and said it sheds some light on what he termed a “mystery disease.”

Ascherio also added that “the publication of [our] results will provide incentive to future large-scale studies” of the disease.

However, Michael A. Schwarzschild, Associate Professor of Neurology at the Harvard Medical School and the Massachusetts General Institute for Neurodegenerative Disease, and also a senior author of the study, cautioned that this study “does not prove that physical activity is protective against Parkinson’s disease.”

While Schwarzschild stressed that this study is epidemiologically based and thus does not establish causality, he added that “laboratory animal research of Parkinson’s disease provides convergent data that raise the possibility that physical activity can prevent development of disease.”

Although the study showed a statistically significant correlation between physical activity and reduced risk of Parkinson’s for men, no such evidence was seen when the researchers examined data for women.

“In our data, the main difference was that women were not as active as men,” Ascherio said.

Parkinson’s also has a lower incidence rate in women than men. Other epidemiological studies have also observed differences in the effect of Parkinson’s on men and women.

In one recent study, researchers reported a “strong link between caffeine use and coffee consumption and a decrease in the risk of Parkinson’s disease in men,” according to Schwarzschild.