The news comes from two extraordinary studies appearing in the Sept. 21 issue of Annals of Internal Medicine. Both focus on a test called hemoglobin A1c -- HbA1c -- which measures average blood sugar levels over the past three months. Normal HbA1c levels range from 4% to 6%.
High HbA1c is known to be a marker for blindness, kidney disease, and nerve damage in people with diabetes. Now it also predicts heart disease in people with diabetes, report Johns Hopkins researcher Elizabeth Selvin, MPH, and colleagues.
That's a major finding, if not a major surprise. The shock comes in a British study that looked at HbA1c levels in people with and without diabetes. Regardless of whether a person had diabetes, the study shows that every 1% increase in HbA1c ups the risk of death -- from all causes -- by 24% for men and 28% for women. Nearly three-fourths of the deaths in the study came in people with "moderately elevated" HbA1c levels: between 5% and 6.9%.
"These are important studies because they show we should be concerned about blood glucose elevations even in people who do not have diabetes," Laurence S. Sperling, MD, director of the Emory Heart Center risk reduction program, tells WebMD.
3 Most Important Things in Diabetes: Control, Control, Control
In people with type 2 diabetes, they found, every 1% rise in HbA1c added 18% to a person's risk of heart disease. And there was a 28% jump in risk of "peripheral vascular disease" -- the problems in tiny blood vessels that lead to blindness, kidney failure, and amputation. The findings were similar, but not statistically significant, for people with type 1 diabetes.
"In normal people, HbA1c ranges from 4% to 6%. But in people with diabetes it can range from 6% to 20%," Selvin tells WebMD. "For people who have diabetes, good blood sugar control is an HbA1c of less than 7%. Over 8% is poor control. Over 9% is a very important sign of very poor control. Every 1% change is clinically significant."
People with diabetes already know they're supposed to keep their blood sugar under control. Now that advice is even more urgent. After all, Selvin notes, heart disease accounts for 70% to 80% of deaths in people with diabetes.
"The important question is whether HbA1c levels predict heart disease in diabetes. Our study says this is the case," Selvin says. "So if you keep good blood sugar control -- with good diet, proper exercise, and effective medication -- this can reduce your risk of heart disease."
High HbA1c Risky for Everybody
If you don't have diabetes, your blood sugar is still an issue. A huge issue, find Kay-Tee Khaw, MD, and colleagues at the University of Cambridge School of Clinical Medicine, England.
Khaw's team checked HbA1c levels in more than 10,000 45- to 79-year-old men and women for six years. Their findings:
- 72% of deaths were in people with HbA1c levels between 5% and 6.9%.
- The lowest rates of heart disease and death were among those with HbA1c levels under 5%.
- For men, every 1% increase in HbA1c beyond 5% meant a 24% increase in risk of death from all causes.
- For women, every 1% increase in HbA1c beyond 5% meant a 28% increase in risk of death from all causes.
- Even after taking into account traditional risk factors for heart disease, like high blood pressure, cholesterol, and previous heart attacks, there was a 21% increased risk of cardiovascular disease for every 1% increase in HbA1c beyond 5%.
"About 4% of the population have diabetes and [only] 27% of the population have HbA1c levels less than 5% -- so the majority of the population have HbA1c levels that are less than optimal," Khaw tells WebMD in an email interview.
Some people without diabetes are at higher risk than some people with diabetes, notes Hertzel C. Gerstein, MD, of McMaster University in Hamilton, Ontario.
"An HbA1c level of 6.59% in a non-diabetic individual predicts a higher cardiovascular risk than an HbA1c level of 5.5% in a well-controlled diabetic individual," Gerstein writes in an editorial accompanying the Khaw and Selvin studies.
Epidemic Heart Disease Looms
There's evidence that Americans' and Canadians' HbA1c levels are going up. Even a small increase in our average blood sugar levels -- of 0.1% to 0.2% -- means the nation faces a massive epidemic of heart disease.
"But this epidemic is not inevitable -- it may be thwarted if we take action now," Gerstein writes. "We know that lifestyle changes can dramatically reduce the incidence of diabetes and slow the HbA1c increase in both non-diabetic and diabetic individuals."
Meanwhile, Sperling urges primary care doctors to pay close attention to patients' blood sugar levels and other heart risk factors. Just because a person doesn't have full-blown diabetes, full-blown high blood pressure, or very high cholesterol doesn't mean they don't need help.
"Average Joe America goes to his doctor and hears he has borderline high blood pressure, his bad cholesterol is a little high, his good cholesterol is a little low, his blood sugar is getting up there -- no red flags, just borderline," Sperling says. "But now we know that being borderline carries a significant risk. Doctors have to recommend realistic lifestyle changes and consider medications to lower these risks."
Khaw says that measuring HbA1c helps doctors identify patients who would benefit from cholesterol- or blood-pressure-lowering medication. But controlling blood sugar ultimately depends on a person's diet and activity level.
"Control of obesity and increased physical activity can reduce risk of diabetes and lower HbA1c levels," Khaw says in the email. "Other lifestyle behaviours that appear to be beneficial ... include high fruit and vegetable intake."