Nearly 21 million Americans have diabetes, most of them the type-2 variety associated with poor diet, too little exercise and being overweight, the Centers for Disease Control and Prevention recently reported.

This represents about 7 percent of the population -- and more than 6 million of these people do not know they have the condition, the CDC said.

"Another 41 million people are estimated to have pre-diabetes, a condition that increases the risk of developing type 2 diabetes -- the most common form of the disease -- as well as heart disease and stroke," the CDC said in a statement.

Diabetes is a lack of control of glucose, or blood sugar. Type-1 diabetes, or juvenile diabetes, is an autoimmune condition in which the body mistakenly destroys the pancreatic cells that make insulin. It affects an estimated 2 million Americans.

Type-2 diabetes once was called adult-onset diabetes but it now affects many children as well. It is the sixth leading cause of death in the United States and can lead to many other diseases.

"Diabetes is a leading cause of adult blindness, lower-limb amputation, kidney disease and nerve damage. Two-thirds of people with diabetes die from a heart attack or stroke," said Dr. Frank Vinicor, director of CDC's diabetes program.

Several genes are known to contribute to diabetes, but it has become more common as the population has become fatter.

The risk of diabetes increases with age. About 21 percent of Americans aged 60 years or older have diabetes, the CDC said.

"Recent studies have shown that people with pre-diabetes can successfully prevent or delay the onset of diabetes by losing 5 percent to 7 percent of their body weight," Vinicor said in a statement.

"This can be accomplished through 30 minutes or more of physical activity most days of the week and by following a low calorie, low fat eating plan, including a diet rich in whole grains and fruits and vegetables," he added.