Low birth weight (LBW) is associated with increased risk of developing type 2 diabetes later in life. Progression from normal to impaired glucose tolerance and overt diabetes may depend, to some extent, on elevation of plasma free fatty acids (FFAs). We undertook this study to elucidate whether a prolonged physiological lipid load could unmask or augment existing metabolic defects in otherwise healthy young LBW subjects. Forty 19-year-old men (LBW [n = 20], controls [normal birth weight, NBW] [n = 20]) without a family history of diabetes underwent an intravenous glucose tolerance test (0.3 g kg(-1)), followed by 2-step hyperinsulinemic-euglycemic clamps (2 x 120 minutes: 10 and 40 mU m(-2) min(-1)) in combination with [3-3H]-glucose and indirect calorimetry. The tests were preceded, in randomized order, by a 24-hour continuous intralipid (20%, 0.4 mg mL(-1) h(-1)) or saline infusion. Estimates of cellular glucose metabolism were obtained and a disposition index calculated. Clamp FFA concentrations were 4- to ten-fold higher during lipid infusion. Both groups experienced a similar decrease in insulin-stimulated glucose disposal in response to lipid infusion (approximately 15%; P < .05), which was mainly accounted for by reduced glucose oxidation (approximately 30%; P < .001). Glycolysis, glucose storage, and glucose production were not significantly altered by lipid infusion. Nevertheless, the LBW group had significantly lower insulin-stimulated glycolysis during lipid infusion (approximately 27%; P < .05) than the NBW group. An appropriate increase in insulin secretion matched the decline in insulin sensitivity in both groups. A 24-hour low-grade intralipid infusion has similar effects on whole-body glucose metabolism and first-phase insulin secretion in 19-year-old, healthy, lean, LBW men with normal glucose tolerance and in NBW controls. We reproduced our previous finding of lower insulin-stimulated glycolysis in this population.