Sugar shock: eight million Americans don't know they have diabetes.

Exercise is absolutely essential to preventing and controlling diabetes. Because exercise increases the body's sensitivity to insulin, it can reduce the amount of the drug Type I diabetecs need and can prevent the development of Type II.

Despite a ravenous appetite and an unquenchable thirst, 34-year-old Jerry had been losing weight for a few months. He also found himself urinating much more often, and for the first time in his life he woke up several times each night to use the bathroom. Sitting on the doctor's examination table, he was stunned to hear the diagnosis: diabetes.

About 16 million Americans have diabetes. It's the nation's fourth-largest killer, causing more than 160,000 deaths last year alone. Nearly half of all diabetics don't even know they have the disease, an oversight that can lead to blindness, serious infections, amputations, kidney problems, neurological disorders, heart disease, coma, and death. Six out of 10 diabetic men become impotent--for some, the most dreaded complication. Grim-sounding as the litany is, the good news is that the more common type of diabetes can often be prevented entirely.

If One Doesn't Get You...

The disease is actually grouped into two categories. The more serious diabetes mellitus Type I (also known as insulin-dependent or juvenile-onset diabetes) usually strikes before age 30. It is caused by a fluke in the immune system that sends the body's blood-borne armada gunning after a group of cells in the pancreas known as the islets of Langerhans. These cells manufacture insulin, a hormone essential for the distribution of glucose (blood sugar) to cells throughout the body so it can be converted into energy. With the islets impaired or destroyed, blood sugar rises to dangerously high levels, resulting in tissue damage throughout the body. People with Type I diabetes (about 15 to 20 percent of all diabetics) require daily insulin injections to control their blood sugar.

The second, more common form of the disease is diabetes mellitus Type II, which is non-insulin-dependent. It most often strikes obese adults because, for reasons still unclear, the cells in their bodies become less responsive to insulin. People with Type II diabetes generally don't require insulin injections; in fact, they can often cure themselves through a program of exercise, weight control, and diet modification, which can also benefit Type 1 patients, This means a diet high in grains and produce; very low in sugar, fat, and cholesterol; and moderate in protein.

"Exercise is absolutely essential to preventing and controlling diabetes," says Elliot Rayfield, M.D., clinical professor of medicine at Mount Sinai Hospital in New York City and author of Diabetes: Beating the Odds (Addison-Wesley, 1992). "Because exercise increases the body's sensitivity to insulin, it can reduce the amount of the drug Type I diabetics need and can prevent the development of Type II," Rayfield says. An added benefit: Aerobic exercise is a good way to combat cardiovascular disease, which affects diabetics more than the general population.

What You Don't Know...

Jerry was fortunate that his classic symptoms aroused enough concern to send him to his doctor. But for many people the greatest risk of either type of diabetes is a delayed diagnosis (see Know Your Risks, page 23).

Presently there's no way to prevent Type I, as the genetic and environmental factors that lead to the faulty autoimmune response are still poorly understood. Ideally, doctors will someday be able to transplant islets of Langerhans cells into the pancreas of insulin-dependent diabetics.

"Still," says Kathleen Wishner, president of the American Diabetes Association and associate clinical professor of nutrition at the University of Southern California School of Medicine, "while gene therapy may be feasible in the future, we believe prevention--identifying people at risk before they can develop complications--is key."

The important thing to focus on, experts say, is management. "Keep in mind that you control the disease," Rayfield emphasizes. "And in the case of Type II diabetes, through exercise and diet you have the power to prevent the illness and its complications entirely."

People who are at high risk of developing diabetes but aren't tested, as well as diabetics who don't follow their doctors' instructions, often pay a high price. Says Wishner, "The ultimate consequence of poorly controlled diabetes is a significantly reduced life span."

RELATED ARTICLE: Know Your Risks

* Family history. Genetic factors are involved in the development of both Type I and Type II diabetes. If one or more of your parents or siblings has the disease, get your blood-glucose level checked regularly.

* Age. The older you are, the greater your risk of developing diabetes.

* Obesity. Being more than 20 percent over your ideal weight increases your risk of developing adult-onset diabetes.

* Lifestyle. The less you exercise, the higher your risk.

Know the Symptoms

* High blood sugar (hyperglycemia), determined by measuring the glucose level in your blood.

* Frequent urination (polyuria), often the first warning that something is wrong.

* Frequent thirst (polydipsia), the body's way to replacing fluid lost through frequent urination.

* Increased appetite (polyphagia), especially carbohydrate cravings,

* Tingling sensations (paresthesias), often felt in the hands or feet because of decreased circulation to the extremities.

* Blurred vision, the result of high blood sugar allowing more water to enter the lens of the eye, causing, it to expand.

* Weight loss, often in spite of normal or increased caloric intake, may signal undiagnosed or poorly controlled diabetes.

* Frequent infections, especially involving the feet, result from poor circulation. Extreme cases may require amputation.

For more information, call the American Diabetes Association at 1-800-232-6733 and request. The Fitness Book for People With Diabetes.

Jonathan M. Berkowitz, M.D., is a surgical pathologist and medical writer in New York City. This article originally appeared in Men's Fitness magazine.