The relative contributions of insulin resistance and beta cell failure to the transition from normal to impaired glucose tolerance varies in different ethnic groups

Aim: To evaluate ethnic differences in the contribution of decline in insulin secretion and insulin sensitivity in impaired glucose tolerance (IGT). Methods: Seven hundred and eighteen subjects of Arab, Japanese and Mexican American decent received oral glucose tolerance test (OGTT) with plasma glucose and insulin measurement every 30 min. The Matsuda index of insulin sensitivity and the relation between incremental increase under plasma insulin to glucose curves during the OGTT (ΔI_0-120/ΔG_0-120) were calculated. Results: NGT Japanese subjects had highest insulin sensitivity index (7.1 ± 4.6) and lowest insulin secretion index ((ΔI_0-120/ΔG_0-120 = 1.1 ± 0.9). Mexican Americans and Arabs had lower insulin sensitivity (4.1 ± 2.8 and 3.5 ± 2.3, respectively) and higher insulin secretion indices (2.2 ± 2.0 and 2.5 ± 2.5). IGT subjects in all ethnic groups had reduced insulin sensitivity and insulin secretion compared to NTG subjects. However, the reduction in insulin sensitivity was the largest in Mexican American (30%), the smallest in Arabs (11.5%) and intermediate in Japanese (23%). Conversely, the decrease in insulin secretion was the greatest in Arabs (80%), the smallest in Mexican Americans (41%) and intermediate in Japanese (55%). In a multivariate regression analysis model, the decline in insulin secretion was a stronger determinant of 2-h plasma glucose in Arabs than the reduction in insulin sensitivity while the opposite was observed in Mexican Americans and Japanese. Conclusion: Differences in insulin sensitivity and insulin secretion are present amongst different ethnic groups. The relative contributions of reduced insulin action and impaired insulin secretion are likely to contribute differentially to progression from NGT to IGT (and diabetes) in different ethnic groups.