WASHINGTON (AP) - Neil Gilbert doesn't have diabetes, but every day the 21-year-old injects himself with a small dose of insulin. He's hoping it will protect him from ever getting the disease.

When Gilbert was about 14, doctors predicted the Acton, Mass., teen would likely develop Type 1 diabetes within three years. His sister had the disease and medical tests showed his own body was losing its ability to regulate blood sugar.

So Gilbert became one of the first high-risk children enrolled in early studies to see if taking insulin might prevent, or at least delay, diabetes. Seven years later, Gilbert is a healthy college student. He doesn't know yet if the insulin is working or if his doctors' dire disease prediction was just off by a few years.

But "I don't want to be a diabetic," Gilbert says, so he plans to keep taking the shots. "Would I really want to take the chance?"

Now doctors are renewing their hunt for thousands more Americans like Gilbert, a final push to complete another major study they hope will answer whether insulin - either injected or in an experimental pill form - is protective.

The 800-person study actually began in 1995, but doctors still need about 300 more people at risk for developing diabetes to complete enrollment - and finding them is hard.

Doctors must test about 30,000 relatives of diabetics to find 300 who fit the study's criteria, says Dr. Jay S. Skyler of the University of Miami, lead researcher of the study financed by the National Institutes of Health.

"A lot of people have forgotten it's under way," he said in renewing scientists' call for volunteers.

Up to 1 million Americans have Type 1 diabetes, also called juvenile diabetes even though people of any age can get it. Patients' immune systems destroy pancreas cells that secrete insulin, a hormone necessary to convert sugar into energy.

Type 1 diabetics need regular insulin shots to survive. Still, over time, high blood sugar levels put them at risk for blindness, kidney disease and other life-threatening complications.

Most at risk are close relatives of diabetics, who in general are 10 to 20 times more likely to develop the disease than anyone in the general population.

But the only way for family members to know if they are most at risk is to get tested, which is free for study volunteers.

The first step is a fairly easy blood test that detects certain antibodies, or immune cells, that attack insulin-producing pancreas cells.

Some 96 percent of people who take that test get good news: They don't have the dangerous antibodies, and thus are at low risk of developing diabetes despite their family connections. That's the end of their participation - and the reason researchers must test so many thousands of people for the study.

People who have the dangerous antibodies can then choose more sophisticated testing that measures pancreatic function and whether they have a gene that seems to protect against diabetes. Putting those tests together determines people's risk of developing diabetes within the next five years.

Anyone whose risk is more than 50 percent may enroll in the insulin injection part of the study. Anyone whose risk is 25 to 50 percent may enroll in a second part of the study, where participants swallow either an experimental insulin pill or a dummy pill every day.

Some people decide they can't handle knowing just how high their risk is, while others want to try anything that might protect them, Skyler said. Children often are enrolled in his study because "their parents have another child with diabetes and want to do anything they can to protect another little one."

If doctors can fill the study in the next few months, they hope to know in 2002 if insulin shots help prevent diabetes, and in 2004 if the experimental insulin pills have any effect.

For information on finding a clinic participating in the study, check the American Diabetes Association at http://www.diabetes.org/ada/dpt-1.asp on the Internet.

Copyright 1999 The Associated Press. All rights reserved.