Diabetes damages every part of the body, from the brain to the feet. High blood sugar, the hallmark of diabetes, wreaks havoc on blood vessels. It makes sense that keeping blood sugar under control should prevent diabetes-related damage — but how low to push blood sugar is an open question.
A study published in today’s issue of The New England Journal of Medicine (NEJM) provides reassuring evidence that so-called tight blood sugar control is good for the heart and circulatory system.
“Tight blood sugar control represents a new age of diabetes care,” says Dr. David Nathan, professor of medicine at Harvard Medical School and director of both the General Clinical Research Center and the Diabetes Center at Massachusetts General Hospital.
The hazards of high blood sugar
Type 2 diabetes is marked by high levels of blood sugar. Over time, high blood sugar damages small blood vessels throughout the body. This is called microvascular disease. The damage can lead to kidney failure, nerve pain, amputation, and blindness.
But the leading cause of complications and death in people with diabetes is cardiovascular disease, which involves the body’s larger blood vessels. (This is also called macrovascular disease.) About two-thirds of people with diabetes die from heart disease, stroke, or other cardiovascular problems.
A good measure of blood sugar is the hemoglobin A1c (HbA1c) test. It reveals a person’s average blood sugar level over the previous three months. People without diabetes have an HbA1c level under 5.7%; an HbA1c level of 6.5% or greater usually indicates diabetes.
For some people with type 2 diabetes, a healthy diet and regular exercise can keep blood sugar in check, but many need medication as well. People with diabetes are usually urged to aim for tight blood sugar control, which translates into an HbA1c level below 7%.
Research has shown that tight blood sugar control can reduce the risk of microvascular complications. But the effect of tight blood sugar control on cardiovascular disease has been murkier. The new NEJM report suggests that tight blood sugar control also has cardiovascular benefits.
The benefits of tight control
The report is a 10-year follow-up of the Veterans Affairs Diabetes Trial. This trial enrolled 1,791 military veterans with type 2 diabetes who were an average of 60 years old. These veterans were randomly assigned to either “intensive” therapy intended to bring blood sugar down to a lower HbA1c target, or “standard” therapy with a higher HbA1c target. In each group, the target blood sugar level was achieved with a combination of oral diabetes medications and insulin injections, if needed.
During the five-and-a-half-year trial, the intensive-therapy group had an average HbA1c level near 6.9%. The standard-therapy group had an average HbA1c level near 8.4%.
More than 1,600 of the trial participants were followed for another five years. During this time, researchers compared how many participants had a cardiovascular event, such as heart attack or stroke, between the intensive therapy and standard therapy groups.
The results were heartening, both for doctors who have championed tight blood sugar control and for people with diabetes who have worked hard to achieve it. Heart attack and stroke risk in the tight control group was 17% lower than among those whose blood sugar levels floated a bit higher. That translates into nearly 9 fewer heart attacks and strokes per 1,000 people.
The study had another positive finding. It reinforced what Dr. Nathan calls “metabolic memory.” As he explains, “an early period of intervention seems to have durable effects over time.”
The active part of the VA trial lasted for about five-and-a-half years. After that, the veterans’ medical care was no longer supervised by the research study team. Within three years, the average HbA1c level in the intensive-therapy group had crept upward, reducing the difference in levels between the intensive and standard groups from 1.5% to somewhere between 0.2% and 0.3%. And yet, the intensive therapy group continued to reap cardiovascular benefits years later.
The effects of intensive treatment don’t last forever, cautions Dr. Nathan. At some point, he says, “metabolic memory becomes metabolic amnesia.” But the longer you keep blood sugar under tight control, the longer the benefits are likely to last.
Although tight blood sugar control can help prevent diabetes-related damage, it has some drawbacks. People aiming for tight control can experience bouts of low blood sugar (hypoglycemia), which can be very dangerous. Tight control can also be difficult to achieve, sometimes requiring multiple medications that may have harmful side effects of their own.
Earlier research had suggested that people with long-standing diabetes and established heart disease may not benefit from tight blood sugar control as much as those with newly diagnosed diabetes. But the new NEJM report shows that it’s never too late to control blood sugar. The veterans enrolled in this trial had had poorly controlled diabetes for several years before the study began. And 40% had cardiovascular disease when they enrolled in the trial. So not only did intensive treatment reduce cardiovascular disease risk, it did so in older people with long-standing diabetes, many of whom already had heart trouble.
Good blood sugar control is important for everyone with diabetes. Current guidelines from the American Diabetes Association recommend aiming for an HbA1c level of less than 7%. But the guidelines also recognize there’s no one-size-fits-all rule. If you have diabetes, ask your doctor if tight blood sugar control is right for you.