Racial Differences in the Correlation of HbA1c to Early and Late Postprandial Glycemic Responses in Adults with Prediabetes

Fasting plasma glucose (FPG), 2-hour oral glucose tolerance test (OGTT), and hemoglobin A1c (HbA1c) are used to screen for prediabetes and diabetes. While HbA1c testing offers the convenience of a single blood draw without fasting, it is known to have confounders such as age, race, and red blood cell turnover rate to name a few. Furthermore, dynamic tests such as the OGTT and mixed-meal tolerance test (MMTT) are usually better at detecting β-cell dysfunction and insulin resistance when insulin is measured. We sought to determine how well HbA1c values correlated with glucose levels during a MMTT and whether the correlation varied by race. Baseline data from a clinical trial of 59 adults [27 white and 32 African American (AA)] with prediabetes defined as either FPG of 100-125 mg/dL and/or HbA1c of 5.7-6.4% were used. In this cohort, we correlated HbA1c to glucose levels at 30 and 120 mins after ingesting a standardized meal (smoothie) of 400 kcal (20% protein, 40% fat, and 40% carbohydrate). Mean FPG was higher in white participants (108±10 mg/dL) compared to AA participants (102±10 mg/dL) (p<0.05), while HbA1c was marginally higher in AA participants (5.7±0.2%) compared to white participants (5.6±0.3%) (p<0.01). In the white participants, HbA1c correlated with glucose 30-mins post-meal (r=0.46, p<0.03) but did not in AA participants (r=0.11, p>0.53). In AA participants, HbA1c correlated with glucose 120-mins post-meal (r=0.48, p<0.01) but did not in white participants (r=0.22, p=0.28). Thus, in this cohort, white adults with prediabetes showed greater impairment of fasting glucose than AA adults, despite lower HbA1c levels. Moreover, a higher HbA1c correlated with an impaired early glycemic response in white adults; whereas, a higher HbA1c correlated with impaired glucose tolerance in AA adults. These results may suggest racial differences in early vs. late postprandial responses that should be accounted for when screening for prediabetes and diabetes.