Failure of carbohydrate to spare leucine oxidation in obese subjects

Leucine kinetics were studied in six obese subjects (W/H² = 39 ± 4) and six normal subjects (W/H² = 21 ± 3) before and after an oral load of 150 g glucose. An intravenous infusion of l-¹³C leucine was given to the fasting subjects for 450 min: a steady state of plasma leucine enrichment was established 90 min after the start of the infusion, and the glucose load was given 220 min after the start of the infusion. Compared with the lean controls these obese subjects showed a greater area under the curve of blood glucose after the glucose load (P < 0.025) and higher insulin and glucagon levels both before and after the meal (P < 0.05), thus indicating the well-known insulin insensitivity of obese (but not diabetic) subjects with respect to glucose metabolism. After the glucose load the lean subjects showed a significant and sustained decrease in leucine oxidation (from 20.0 ± 2.2 to 13.3 ± 1.5 μmol/kg LBM/h:P < 0.01). This response is similar to that observed when insulin-dependent diabetic subjects are given insulin. However, the obese subjects showed no decrease in leucine oxidation after the glucose meal (20.3 ± 1.9 before, and 21.2 ± 3.6 after). This indicates that obese subjects show insensitivity to the action of insulin with respect to protein metabolism as well as carbohydrate metabolism.