Context: Low serum IGF-I levels have been linked to increased risk for development of type 2 diabetes. However, the physiological role of IGF-I in glucose metabolism is not well characterised. Objective: To explore glucose and lipid metabolism associated with variations in serum IGF-I levels. Design, Setting and Participants: IGF-I levels were measured in healthy, non-obese male volunteers aged 18-50 years from a biobank (n=275) to select 24 subjects (age 34.8±8.9 yr), 12 each in the lowest (Low-IGF) and highest (High-IGF) quartiles of age specific IGF-I standard deviation scores. Evaluations were undertaken after a 24hr fast, and included glucose and glycerol turnover rates using tracers, intravenous glucose tolerance test to estimate peripheral insulin sensitivity (IS) and acute insulin and C-peptide responses (indices of insulin secretion), magnetic resonance spectroscopy to measure intramyocellular lipids (IMCL), calorimetry and gene expression studies in a muscle biopsy. Main outcome measures: Acute insulin and C-peptide responses, IS, and glucose and glycerol appearance rates (Ra). Results: Fasting insulin and C-peptide levels, and glucose Ra were reduced (all p<0.05) in Low-IGF compared with High-IGF subjects indicating increased hepatic IS. Acute insulin and C-peptide responses were lower (both p<0.05), but similar peripheral IS resulted in reduced insulin secretion adjusted for IS in Low-IGF subjects (p=0.044). Low-IGF subjects had higher overnight levels of free fatty acids (p=0.028) and β-hydroxybutyrate (p=0.014), increased accumulation of IMCL in tibialis anterior muscle (p=0.008) and a tendency for elevated fat oxidation rates (p=0.058), however, glycerol Ra were similar. Gene expression of the fatty acid metabolism pathway (p=0.0014) was upregulated, whereas the GLUT1 receptor gene was downregulated (p=0.005) in the skeletal muscle in Low-IGF subjects. Conclusions: These data suggest that serum IGF-I levels could be an important marker of β-cell function and glucose as well as lipid metabolic responses during fasting.