And some insurance companies pay doctors extra if their diabetic patients get their levels very low.

The low-blood-sugar hypothesis was so entrenched that when the National Heart, Lung and Blood Institute and the National Institute of Diabetes and Digestive and Kidney Diseases proposed the study in the 1990s, they explained that it would be ethical. Even though most people assumed that lower blood sugar was better, no one had rigorously tested the idea. So the study would ask if very low blood sugar levels in people with Type 2 diabetes  the form that affects 95 percent of people with the disease  would protect against heart disease and save lives.

Some said that the study, even if ethical, would be impossible. They doubted that participants  whose average age was 62, who had had diabetes for about 10 years, who had higher than average blood sugar levels, and who also had heart disease or had other conditions, like high blood pressure and high cholesterol, that placed them at additional risk of heart disease  would ever achieve such low blood sugar levels.

Study patients were randomly assigned to one of three types of treatments: one comparing intensity of blood sugar control; another comparing intensity of cholesterol control; and the third comparing intensity of blood pressure control. The cholesterol and blood pressure parts of the study are continuing.

Dr. John Buse, the vice-chairman of the study’s steering committee and the president of medicine and science at the American Diabetes Association, described what was required to get blood sugar levels low, as measured by a protein, hemoglobin A1C, which was supposed to be at 6 percent or less.

“Many were taking four or five shots of insulin a day,” he said. “Some were using insulin pumps. Some were monitoring their blood sugar seven or eight times a day.”

They also took pills to lower their blood sugar, in addition to the pills they took for other medical conditions and to lower their blood pressure and cholesterol. They also came to a medical clinic every two months and had frequent telephone conversations with clinic staff.