[Evaluation of islet beta cell function in subjects with normal glucose tolerance, impaired glucose regulation, and type 2 diabetes mellitus].

Objective To investigate the function of islet beta cells in subjects with normal glucose tolerance (NGT),impaired glucose regulation (IGR),and type 2 diabetes mellitus (T2DM).Different indexes of insulin secretion,including△I_(30)/△G_(30),AIR or HOMAβ,were compared.Methods 178 subjects without overt diabetes were classified into three groups according to the results of 75g oral glucose tolerance test (OGTr):NGT group (n=68),IGR group (n=75),and T2DM group (n=35).Intravenous glucose tolerance test (IVGTT) was carried out 1～3 days later in all participants,with measurement of plasma insulin.The ratio of insulin-to-glucose levels increment during the first 30 min of OGTT(△I_(30)/△G_(30)) and the acute insulin response (AIR) in IVGTT were used as indexes of early insulin secretion.Homeostasis model assessment of insulin secretion (HOMAβ) was another indicator of insulin secretion.The fasting plasma proinsulin level (FPI) was measured and the ratio fasting proinsulin to fasting insulin (PI/I) was calculated.HOMA insulin resistance index (HOMA IR) was used to assess the insulin resistance.Results The △I_(30)/△G_(30) and AIR of the IGR group,adjusted by age,sex,and BMI,were both significantly lower than those of the NGT group.But the HOMAβof the IGR group was only slightly lower than that of the NGT group.Compared with the NGT subjects,the T2DM patients had a very severe islet beta cell dysfunction (84% decrease in AIR,70% decrease in △I_(30)/△G_(30) and 62% decrease in HOMA β).When the △I_(30)/ △G_(30) was adjusted by HOMR IR,the extent of impairment in early insulin response was similar to that of AIR (87% versus 84% lower than that of the NGT group).The FPI and ratio of proinsulin to insulin were higher in the T2DM subjects compared with the NGT subjects (24.4 pmol/L±18.0 pmol/L vs 10.9 pmol/ L±6.7 pmol/L;and 14.7%±10.5% vs.10.0%±6.5%,both P0.05).There was a significantly positive correlation between △I_(30)/△G_(30) and AIR (r=0.75,P0.001).Conclusion In the stage of IGR, an evident deficit in phasic insulin secretion after glucose load and a decreasing HOMAβare exhibited.In addition to this,qualitative decrease of insulin secretion appears in the DM stage.AIR is a reliable index of isletβcell function.△I_(30)/△G_(30) is an alternative one in the subjects with NGT and IGR.But it should be adjusted by IR in diabetic patients.