Association of hemoglobin A1c with cardiovascular disease and mortality in adults: the European prospective investigation into cancer in Norfolk.

BACKGROUND: Increasing evidence suggests a continuous relationship between blood glucose concentrations and cardiovascular risk, even below diagnostic threshold levels for diabetes.
OBJECTIVE: To examine the relationship between hemoglobin A1c, cardiovascular disease, and total mortality.
DESIGN: Prospective population study.
SETTING: Norfolk, United Kingdom. PARTICIPANTS: 4662 men and 5570 women who were 45 to 79 years of age and were residents of Norfolk. MEASUREMENTS: Hemoglobin A1c and cardiovascular disease risk factors were assessed from 1995 to 1997, and cardiovascular disease events and mortality were assessed during the follow-up period to 2003.
RESULTS: In men and women, the relationship between hemoglobin A1c and cardiovascular disease (806 events) and between hemoglobin A1c and all-cause mortality (521 deaths) was continuous and significant throughout the whole distribution. The relationship was apparent in persons without known diabetes. Persons with hemoglobin A1c concentrations less than 5% had the lowest rates of cardiovascular disease and mortality. An increase in hemoglobin A1c of 1 percentage point was associated with a relative risk for death from any cause of 1.24 (95% CI, 1.14 to 1.34; P < 0.001) in men and with a relative risk of 1.28 (CI, 1.06 to 1.32; P < 0.001) in women. These relative risks were independent of age, body mass index, waist-to-hip ratio, systolic blood pressure, serum cholesterol concentration, cigarette smoking, and history of cardiovascular disease. When persons with known diabetes, hemoglobin A(1c) concentrations of 7% or greater, or a history of cardiovascular disease were excluded, the result was similar (adjusted relative risk, 1.26 [CI, 1.04 to 1.52]; P = 0.02). Fifteen percent (68 of 521) of the deaths in the sample occurred in persons with diabetes (4% of the sample), but 72% (375 of 521) occurred in persons with HbA1c concentrations between 5% and 6.9%. LIMITATIONS: Whether HbA1c concentrations and cardiovascular disease are causally related cannot be concluded from an observational study; intervention studies are needed to determine whether decreasing HbA1c concentrations would reduce cardiovascular disease.
CONCLUSIONS: The risk for cardiovascular disease and total mortality associated with hemoglobin A1c concentrations increased continuously through the sample distribution. Most of the events in the sample occurred in persons with moderately elevated HbA1c concentrations. These findings support the need for randomized trials of interventions to reduce hemoglobin A1c concentrations in persons without diabetes.