Significance of High HbA₁ Levels in Normal Glucose Tolerance

The significance of high hemoglobin A₁ (HbA₁) levels (≥8.0%) found in 12.1% of 648 individuals with normal glucose tolerance constituting a part of a representative population sample was examined. Measurement error in HbA₁ and/or glucose-tolerance levels was precluded by HbA₁ remaining in the same range over 3.5 yr in 89.7% of 29 individuals with initially high and 68.1% of 22 individuals with initially low (<6.5%) HbA₁. Rate of deterioration to glucose intolerance (6.9%) in the high group during that period resembled the rate (11.8%) in a control group (n = 279). Fasting plasma glucose significantly accounted for only 2.4% of total HbA₁ - population variance. No correlation of HbA₁ was found with other correlates of glucose tolerance or with daily caloric intake and physical activity. A small but significant increment in HbA₁ was associated with smoking (7.1 vs. 6.8%, P < .01) and with clinically overt atherosclerosis (7.3 vs. 6.9%, P < .01). We conclude that factors unrelated to glucose metabolism are the main determinants of HbA₁ level in normal glucose tolerance and play an important role in diabetes as well. These factors have bearing on evaluation of diabetic control by HbA₁ and possibly on risk for diabetic complications.