Physical inactivity is a known risk factor for type 2 diabetes. We studied whole body and forearm insulin sensitivity in subjects at increased risk for type 2 diabetes [persons with low birth weight (LBW group; n = 20) and first-degree relatives to type 2 diabetic patients (FDR group; n = 13)] as well as a control (CON) group (n = 20) matched for body mass index, age, and physical activity levels before and after 10 days of bedrest. Subjects were studied by hyperinsulinemic isoglycemic clamp combined with arterial and deep venous catheterization of the forearm. Forearm blood flow (FBF) was measured by venous occlusion plethysmography. All groups responded with a decrease in whole body insulin sensitivity in response to bedrest [CON group: 6.8 +/- 0.5 to 4.3 +/- 0.3 mg x min(-1) x kg(-1) (P <0.0001), LBW group: 6.2 +/- 0.5 to 4.3 +/- 0.3 mg x min(-1) x kg(-1) (P <0.0001), and FDR group: 4.3 +/- 0.7 to 3.1 +/- 0.3 mg x min(-1) x kg(-1) (P = 0.068)]. The percent decrease was significantly greater in the CON group compared with the FDR group (CON group: 34 +/- 4%, LBW group: 27 +/- 4%, and FDR group: 10 +/- 13%). Forearm insulin-stimulated glucose clearance decreased significantly in the CON and LBW groups in response to bedrest; in the FDR group, clearance was very low before bedrest and no change was observed. Before bedrest, the CON and LBW groups demonstrated a significant increase in FBF during hyperinsulinemia; after bedrest, an increase in FBF was observed only in the CON group. In conclusion, bedrest induced a pronounced reduction in whole body, skeletal muscle, and vascular insulin sensitivity in the CON and LBW groups. The changes were most pronounced in the CON group. In the FDR group, insulin resistance was already present before bedrest, but even this group displayed a high sensitivity to changes in daily physical activity.