The Relationship Between β-Cell Function and Glycated Hemoglobin

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Abstract
OBJECTIVE: The study objective was to assess the relationship between β-cell function and HbA1c. RESEARCH DESIGN AND METHODS: A total of 522 Mexican American subjects participated in this study. Each subject received a 75-g oral glucose tolerance test (OGTT) after a 10- to 12-h overnight fast. Insulin sensitivity was assessed with the Matsuda index. Insulin secretory rate was quantitated from deconvolution of the plasma C-peptide concentration. β-Cell function was assessed with the insulin secretion/insulin resistance (IS/IR) (disposition) index and was related to the level of HbA1c. RESULTS: At HbA1c levels 5.5%, there was a precipitous decrease in both the Matsuda index and the IS/IR index. Subjects with HbA1c = 6.0–6.4% had a 44 and 74% decrease in the Matsuda index and the IS/IR index, respectively, compared with subjects with HbA1c <5.5% (P < 0.01 for both indices). Subjects with normal glucose tolerance and HbA1c 6.0%, both insulin sensitivity and β-cell function decrease markedly. Performing an OGTT is pivotal for accurate identification of subjects with impaired β-cell function.