Interrelationships between Fasting Insulin, Insulin Resistance, and Pancreatic β-Cell Function in Type 2 Diabetes Patients

Background: Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) and Homeostatic Model Assessment for β-cell function (HOMA-β) are validated surrogate indices used to evaluate insulin sensitivity and pancreatic β-cell function. However, evidence describing the interrelationships among fasting insulin, HOMA-derived indices, and glycated haemoglobin (HbA1c) in Nigerian adults remains limited. Objective: This study investigated the relationships between fasting insulin concentration, insulin resistance, pancreatic β-cell function, and glycaemic control among adults with T2DM and apparently healthy non-diabetic individuals in Calabar, Nigeria. Methods: A hospital-based comparative cross-sectional study was conducted. Fasting plasma glucose (FPG), fasting serum insulin, and glycated haemoglobin (HbA1c) were determined using standardised biochemical methods. HOMA-IR and HOMA-β were calculated using the original Matthews equations. Pearson's correlation analysis was used to evaluate relationships among HbA1c, fasting insulin, HOMA-IR, and HOMA-β, with statistical significance set at p < 0.05. Results: Fasting insulin demonstrated a very strong positive correlation with HOMA-IR (r = 0.9721, p < 0.05) and a strong positive correlation with HOMA-β (r = 0.8745, p < 0.05) among diabetic participants, while HOMA-IR was strongly correlated with HOMA-β (r = 0.7640, p < 0.05). In contrast, HbA1c exhibited weak, non-significant positive correlations with fasting insulin, HOMA-IR, and HOMA-β. Conversely, among non-diabetic participants, fasting insulin correlated strongly with HOMA-IR (r = 0.9642, p < 0.05) and HOMA-β (r = 0.9084, p < 0.05), while HOMA-IR showed a strong positive correlation with HOMA-β (r = 0.8778, p < 0.05). HbA1c demonstrated weak, non-significant negative correlations with fasting insulin, HOMA-IR, and HOMA-β. Conclusion: Fasting insulin, HOMA-IR, and HOMA-β exhibited strong interrelationships in both diabetic and non-diabetic participants, highlighting their complementary roles in evaluating insulin sensitivity and pancreatic β-cell function. Conversely, HbA1c showed no significant association with these metabolic indices, suggesting that long-term glycaemic control reflects broader physiological influences beyond fasting insulin dynamics and β-cell function.