1. The effect of euglycaemic hyperinsulinaemia on the recovery of 13 C in expired CO 2 has been assessed in six normal subjects. Each was studied on three occasions: once with a 6 h primed constant infusion of NaH 13 CO 3 combined with a euglycaemic hyperinsulinaemic clamp for the last 3 h (study 1), once with a 6h primed constant infusion of NaH 13 CO 3 alone (study 2) and once with a 6 h infusion of normal saline combined with a hyperinsulinaemic clamp for the last 3 h (study 3). Measurements of 13 C enrichment of expired CO 2 were made in the third and sixth hour of each infusion. 2. There was no significant increase in enrichment during study 3 (3 h 0.00047 ± 0.00016 versus 6 h 0.00069 ± 0.00028 atom per cent excess) with potato-starch-derived D-glucose used to maintain eugly-caemia. 13 C recovery increased in the sixth hour of both study 1 and 2 (study 1: 3 h 74.4 ± 2.0 versus 6 h 85.5 ± 2.6%, P < 0.01; study 2: 3 h 72.1 ± 2.4 versus 6 h 81.7 ± 1.4%, P < 0.01). There was no significant difference in recovery between studies 1 and 2. 3. These results suggest that increased recovery during a sequential euglycaemic clamp is predominantly time-dependent. Studies which use this technique to examine the effect of insulin on substrate oxidation should take this into account.

1. Experimental elevation of plasma non-esterified fatty acid concentrations has been postulated to decrease insulin-stimulated glucose oxidation and storage rates. Possible mechanisms were examined by measuring skeletal muscle glycogen synthase activity and muscle glycogen content before and during hyperinsulinaemia while fasting plasma non-esterified fatty acid levels were maintained. 2. Fasting plasma non-esterified fatty acid levels were maintained in seven healthy male subjects by infusion of 20% (w/v) Intralipid (1 ml/min) for 120 min before and during a 240 min hyperinsulinaemic euglycaemic clamp (100 m-units h −1 kg −1 ) combined with indirect calorimetry. On the control day, 0.154 mol/l NaCl was infused. Vastus lateralis muscle biopsy was performed before and at the end of the insulin infusion. 3. On the Intralipid study day serum triacylglycerol (2.24 ± 0.20 versus 0.67 ± 0.10 mmol/l), plasma non-esterified fatty acid (395 ± 13 versus 51 ± 1 μmol/l), blood glycerol (152 ± 2 versus 11 ± 1 μmol/l) and blood 3-hydroxybutyrate clamp levels [mean (95% confidence interval)] [81 (64–104) versus 4 (3–5) μmol/l] were all significantly higher (all P < 0.001) than on the control study day. Lipid oxidation rates were also elevated (1.07 ± 0.07 versus 0.27 ± 0.08 mg min −1 kg −1 , P < 0.001). During the clamp with Intralipid infusion, insulin-stimulated whole-body glucose disposal decreased by 28% (from 8.53 ± 0.77 to 6.17 ± 0.71 mg min −1 kg −1 , P < 0.005). This was the result of a 48% decrease in glucose oxidation (3.77 ± 0.32 to 1.95 ± 0.21 mg min −1 kg −1 , P <0.001), with no significant change in nonoxidative glucose disposal (4.76 ± 0.49 to 4.22 ± 0.57 mg min −1 kg −1 , not significant). 4. Basal and insulin-stimulated glycogen synthase activities (13.1 ± 1.9 versus 11.4 ± 2.3% and 30.8 ± 2.3 versus 27.6 ± 4.5%, respectively) were unaffected by the increased plasma non-esterified fatty acid levels. Similarly, basal (36.1 ± 2.7 versus 37.2 ± 1.4 μmol/g) and stimulated (40.0 ± 0.6 versus 37.6 ± 4.4 μmol/g) muscle glycogen levels were unaltered. Insulin-stimulated hexokinase activity was also not affected (0.52 ± 0.08 versus 0.60 ± 0.08 units/g wet weight). 5. Maintenance of plasma non-esterified fatty acid levels at fasting values resulted in an increase in lipid oxidation and was associated with a decrease in insulin-stimulated whole-body glucose uptake and glucose oxidation rates, but no change in non-oxidative glucose disposal. Increased plasma non-esterified fatty acid levels did not appear to have a direct inhibitory effect on glycogen synthase activity or storage of glucose as glycogen at these insulin levels.

1. The importance of circulating non-esterified fatty acids as a substrate during and after low-grade exercise has been examined by using a nicotinic acid analogue to inhibit lipolysis. Seven healthy men received acipimox or placebo on separate occasions. After 90 min, bicycle exercise was performed for 45 min (40% of pre-determined maximum oxygen uptake), followed by a 60 min recovery period. 2. The plasma concentration of non-esterified fatty acids increased during exercise after placebo (320 ± 80 to 630 ± 110 μmol/l) and remained elevated in the post-exercise period. Basal concentrations were lower after acipimox (100 ± 10 μmol/l; P < 0.05); they declined to 60 ± 10 μmol/l during exercise and remained at this level for the rest of the study. 3. Lipid oxidation increased from 0.8 ± 0.1 to 4.2 ± 0.5 mg min −1 kg −1 during exercise after placebo ( P < 0.001) and remained elevated in the post-exercise period (1.2 ± 0.1 mg min −1 kg −1 ). It was lower after acipimox, but still increased from 0.3 ± 0.1 to 2.3 ± 0.2 mg min −1 kg −1 with exercise. Carbohydrate oxidation was increased after acipimox compared with after placebo, but only reached significance during the post-exercise period ( P < 0.05). 4. Although acipimox abolished the rise in the plasma concentration of non-esterified fatty acids during exercise, there was only a 50% decrease in the rate of lipid oxidation. This suggests that an alternative source of non-esterified fatty acids makes an important contribution to the supply of lipid for oxidation during exercise. The elevated plasma concentration of non-esterified fatty acids and lipid oxidation after exercise serve to limit the further oxidation of carbohydrate.