Conventional thinking among doctors who treat people with type 2 diabetes has been the lower the blood sugar levels, the better.

Many doctors are now taking a second look.

A massive study of diabetics with a high risk of heart disease known as ACCORD has found that lowering blood sugar levels to what is considered normal for healthy people proved deadly for some, researchers said on Wednesday.

Older patients who underwent intensive therapy to reach that level had higher rates of death than a group of patients in the same study who were treated more conservatively.

“The study was designed asking the question if you really control blood sugar, do you actually prevent death?” said Dr. Faramarz Ismail-Beigi of the University Hospitals Case Medical Center in Cleveland, Ohio, one of the study’s researchers.

“I think in the intensive group, the answer is no. It doesn’t prevent heart attacks and stroke,” he said.

Patients in the intensive treatment group of the ACCORD study were aiming for a hemoglobin A1c level - a standard measure of blood sugar - of 6 percent or below. They achieved an average of 6.4 percent, whereas the more conservative treatment group had A1c levels of about 7.5 percent.

“We can move millions of people into this zone with a certain amount of resources,” Ismail-Beigi said. To move them all down to 6, you would need five to 10 times more resources. In terms of public health, it makes a huge difference.”

Dr. James Dove, president of the American College of Cardiology, said the study was a bit unsettling.

“I think it offers some concern and caution,” Dove said in a telephone interview. “The standard theory has been the lower the blood sugar the better off it was for the patient in decreasing the side effects of diabetes.”

Researchers at the National Heart, Lung and Blood Institute, which funded and organized the ACCORD trial, said they will study why patients in the aggressive arm of the study fared worse.

By Julie Steenhuysen