More than 18 million men in the United States, about 18 percent of them, are affected by erectile dysfunction, according to a study by researchers from the Johns Hopkins Bloomberg School of Public Health.

The prevalence of problems was strongly linked with age, cardiovascular disease, diabetes and a lack of physical activity, the researchers said in a news release.

They also said that increased physical activity and measures to prevent cardiovascular disease may also prevent decreased erectile function.

One study author said the information could help doctors get patients to care about their fitness.

"The associations of erectile dysfunction with diabetes and cardiovascular risk factors may serve as powerful motivators for men who need to make changes in their diet and lifestyle," said Dr. Elizabeth Selvin.

For the study, the research team analyzed data from 2,126 men who participated in the National Health and Nutrition Examination Survey. Men who reported being "sometimes able" or "never able" to get and keep an erection were categorized as having erectile dysfunction

Men aged 70 and older were much more likely to report having erectile dysfunction compared to only 5 percent in men between the ages of 20 and 40. Nearly half of all men in the study with diabetes also had erectile dysfunction.

Almost 90 percent of all men with erectile dysfunction had at least one risk factor for cardiovascular disease, including diabetes, hypertension, having poor cholesterol levels or being a current smoker.

The study is published in the American Journal of Medicine.