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Harvard Heart Letter | May 2008
ACCORD’s discord on blood sugar control
Tight control still essential, but no consensus on how low you should go.
High
and wildly fluctuating levels of sugar (glucose) and insulin in the
bloodstream are the hallmarks of diabetes. They can injure nerves and
small blood vessels, which can lead to vision loss, kidney failure,
erectile dysfunction in men, or the loss of a foot or leg. Damage to
larger blood vessels is also common — nearly two out of three people
with diabetes die of a heart attack, stroke, or other form of
cardiovascular disease.
A big question for managing
diabetes has long been “How low should you go with blood sugar?” Is it
necessary to drive down blood sugar to the level seen in healthy people
without diabetes, or can it linger a bit above that?
A key
test, called hemoglobin A1c, offers a time-lapse look at blood sugar
levels over several weeks. People without diabetes tend to have a
hemoglobin A1c level under 6%; in folks with the disease, it may be 8%
or higher. Keeping hemoglobin A1c under 7% (what’s known as tight
control) helps guard against vision loss, kidney failure, and
amputations. Tight control helps younger adults with type 1 diabetes
ward off angina, heart attacks, or procedures to open or bypass
cholesterol-clogged coronary arteries. Whether tight control prevents
heart attacks and strokes in older folks with type 2 diabetes,
especially those who already have heart disease, is still up in the air.
Uncertain role for tight control
The
Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial was
launched in 2001 to test three prevention strategies in adults with
long-standing type 2 diabetes:
-
tight control of blood sugar (target hemoglobin A1c below 6%)
-
tight control of systolic blood pressure (target below 120 mm Hg)
-
raising protective HDL and lowering harmful LDL cholesterol
In
February 2008, the National Heart, Lung, and Blood Institute abruptly
halted the blood sugar arm of the trial because there had been more
deaths in the intensive glucose control arm than in the standard blood
sugar control group. Half of the deaths were the result of
cardiovascular disease. The blood pressure and cholesterol parts of the
trial are still under way.
The results shocked many
diabetes experts, who had expected tight blood sugar control to have
the same payoff for cardiovascular disease as it does for protecting
small blood vessels and nerves among people with type 2 diabetes. Why
didn’t it work like that?
It is possible that trying to
keep blood sugar in the “normal” range might somehow sap the reserves
of people with type 2 diabetes and make it extra difficult to survive a
heart attack or stroke. It is also possible that rapidly reducing
average blood sugar and insulin levels might destabilize
cholesterol-filled plaque, making these sometimes fragile pools more
likely to burst and cause a heart attack or stroke.
Stick with blood sugar targets
The ACCORD results do not
mean that controlling blood sugar is harmful. It is the right thing to
do for everyone with diabetes. Recommendations to aim for a hemoglobin
A1c level under 7% still stand. This isn’t always easy to do, and only
about half of Americans with diabetes hit this target. But it remains
an excellent goal for most people. Based on the ACCORD results, letting
blood sugar drift a bit above 7% might make sense for people who have
had type 2 diabetes for a long time — more than 10 years — especially
those who have already developed heart disease.
Given the
importance of tight control for preventing vision loss, amputations,
and other complications of diabetes, though, a goal of 7% makes sense
for most people with diabetes.
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