OBJECTIVE Our previous studies showed that administration of dexamethasone plus food increased serum leptin levels 100% more than dexamethasone alone. We hypothesized that this increase in leptin from the meal could result directly from the provision of fuel metabolites rather than from the meal-induced rise in insulin. In the current study, we tested the effect of an i.v. lipid fuel source (Intralipid 20%/heparin) that would incur only a modest increase in insulin. This study was undertaken because the role of lipid in the regulation of human leptin levels has been controversial, with differing effects reported: stimulatory, inhibitory, or no effect at all. METHODS In order to evaluate how lipids affect serum leptin in humans, we administered the following to seven lean, healthy, fasting subjects: (i) Intralipid 20% at 0.83 ml/kg.h plus heparin (800 IE/h) infused i.v. for 7 h (LIPID), (ii) LIPID with one initial pulse of insulin (0.09 U/kg) given s.c. (LIPID+INS), (iii) LIPID with dexamethasone (2 mg i.v. push) given at the start of the infusion (LIPID+DEX), and (iv) LIPID with insulin plus dexamethasone (LIPID+INS+DEX). Control trials in another 14 subjects matched hormonal conditions but lacked the LIPID infusion. Blood levels were collected over 8 h for determination of free fatty acids (FFA), glucose, insulin, and leptin under each experimental condition. RESULTS Over the 420 min of LIPID infusion, FFA levels rose four-fold from 0.28+/-0.05 mmol/l to 0.99+/-0.05 mmol/l. Serum leptin levels were suppressed by 10-20% in the LIPID condition as compared with control (no LIPID) between 90 min (P=0.008) and 360 min (P=0.045). LIPID+DEX did not increase leptin. A pulse of insulin (INS) increased serum insulin levels to 49.9+/-6.1 U/ml at 90 min and increased serum leptin by 21.3+/-6.6% at 480 min (P=0.054). LIPID decreased leptin in the face of this insulin-induced increase (LIPID+INS), between 360 min (P=0.017) and 420 min (P=0.003), with a 23% suppressive effect at 420 min. LIPID+DEX elevated leptin levels by 112.5+/-35.8% at 480 min (P=0.037), however, the Intralipid/heparin infusion did not blunt the rise of leptin under these conditions. CONCLUSIONS These data showed that Intralipid/heparin: (i) are not sufficient to trigger the effect of dexamethasone on leptin, (ii) have an acute inhibitory effect on both fasting and insulin-stimulated leptin levels, and (iii) that this inhibitory effect cannot reverse the strong stimulatory effect of dexamethasone and insulin on serum leptin.