Many people who feel sluggish and tire easily don't realize they may be diabetic

From Food & Health Correspondent Holly Firfer

ATLANTA (CNN) -- Feeling sluggish, tired? Having trouble concentrating? You might be one of the 8 million Americans who suffers from diabetes and doesn't know it.

The condition is not only a health concern, it can also be expensive for employers.

"Poor control of diabetes can really drive up costs in two ways, one is that it causes an increase in direct expenditures, such as doctor's visits, emergency room visits," said Dr. Donald Simonson of Brigham and Women's Hospital. "It also has the indirect costs which are the losses to the employer."

Losses including work days missed and lack of productivity.

But in a new study in the Journal of the American Medical Association, researchers found people who do not perform well at work because of side effects of adult-onset diabetes can do a dramatic turn-around.

"Patients who were optimally controlled in terms of their blood sugar levels had better physical functioning, better vitality, better sense of well-being and improved cognitive performance," said Marcia Testa of the Harvard School of Public Health.

But to get the symptoms under control, you must first ask a doctor to test for diabetes.

Marcello Pagano, 53, says he didn't realize he was diabetic -- he just thought he was getting old.

"I was drinking a lot of water; I was going to the bathroom a lot in the middle of the night," he said. "My eyesight was going and I was sluggish.

Diabetes is a condition in which the body produces little or no insulin, the hormone that allows sugar to be stored or used for energy.

Without that energy reserve, diabetics can feel sluggish.

However, adult-onset diabetes, or "type 2" diabetes can be controlled with a strict diet, exercise and drugs.

"Controlling the blood sugar now, my eyesight has come back -- I see perfectly well now," Pagano said. "I don't have to get up in the middle of the night. I can sleep better and am more rested and have more energy.

"Life is so much better."