Aims/hypothesis: The impact of strategies for prevention of type 2 diabetes in isolated impaired fasting glycaemia (i-IFG) vs isolated impaired glucose tolerance (i-IGT) may differ depending on the underlying pathophysiology. We examined insulin secretion during OGTTs and IVGTTs, hepatic and peripheral insulin action, and glucagon and incretin hormone secretion in individuals with i-IFG (n=18), i-IGT (n=28) and normal glucose tolerance (NGT, n=20). Methods: Glucose tolerance status was confirmed by a repeated OGTT, during which circulating insulin, glucagon, glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) levels were measured. A euglycaemic-hyperinsulinaemic clamp with [3-3H]glucose preceded by an IVGTT was performed. Results: Absolute first-phase insulin secretion during IVGTT was decreased in i-IFG (p=0.026), but not in i-IGT (p=0.892) compared with NGT. Hepatic insulin sensitivity was normal in i-IFG and i-IGT individuals (p≥0.179). Individuals with i-IGT had peripheral insulin resistance (p=0.003 vs NGT), and consequently the disposition index (DI; insulin secretion×insulin sensitivity) during IVGTT (DIIVGTT)) was reduced in both i-IFG and i-IGT (p<0.005 vs NGT). In contrast, the DI during OGTT (DIOGTT) was decreased only in i-IGT (p<0.001), but not in i-IFG (p=0.143) compared with NGT. Decreased levels of GIP in i-IGT (p=0.045 vs NGT) vs increased levels of GLP-1 in i-IFG (p=0.013 vs NGT) during the OGTT may partially explain these discrepancies. Basal and post-load glucagon levels were significantly increased in both i-IFG and i-IGT individuals (p≤0.001 vs NGT). Conclusions/interpretation: We propose that differentiated preventive initiatives in prediabetic individuals should be tested, targeting the specific underlying metabolic defects.