For decades, researchers believed that if people with diabetes lowered their blood sugar to normal levels, they would no longer be at high risk of dying from heart disease. But a major federal study of more than 10,000 middle-aged and older people with Type 2 diabetes has found that lowering blood sugar actually increased their risk of death, researchers reported Wednesday.

The researchers announced that they were abruptly halting that part of the study, whose surprising results call into question how the disease, which affects 21 million Americans, should be managed.

The study’s investigators emphasised that patients should still consult with their doctors before considering changing their medications.

Among the study participants who were randomly assigned to get their blood sugar levels to nearly normal, there were 54 more deaths than in the group whose levels were less rigidly controlled. The patients were in the study for an average of four years when investigators called a halt to the intensive blood sugar lowering and put all of them on the less intense regimen.

The results do not mean blood sugar is meaningless. Lowered blood sugar can protect against kidney disease, blindness and amputations, but the findings inject an element of uncertainty into what has been dogma — that the lower the blood sugar the better. Medical experts were stunned.

“It’s confusing and disturbing that this happened,” said Dr James Dove, president of the American College of Cardiology. “For 50 years, we’ve talked about getting blood sugar very low. Everything in the literature would suggest this is the right thing to do,” he added.

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