Effects of acute (-)-hydroxycitrate supplementation on substrate metabolism at rest and during exercise in humans

Background: (-)-Hydroxycitrate (HCA), a competitive inhibitor of ATP-citrat e lyase, should reduce the extramitochondrial acetyl-CoA pool. It has been hypothesized that HCA ingestion can reduce malonyl-CoA concentrations and c onsequently increase fatty acid oxidation in vivo. Objective: This study investigated the acute effects of HCA supplementation on substrate utilization at rest and during exercise in endurance-trained humans. Design: Ten cyclists [((x) over bar +/- SD) age: 24 +/- 2 y, weight: 73 +/- 2 kg, maximal oxygen uptake: 4.95 +/- 0.11 L/min, maximal work output (Wma x): 408 +/- 8 W] were studied at rest and during 2 h of exercise at 50% Wma x on 2 occasions. Both 45 and 15 min before exercise and 30 and 60 min afte r the start of exercise, 3.1 mL/kg body wt of an HCA solution (19 g/L) or p lacebo was ingested. Total fat and carbohydrate oxidation rates were assess ed. Blood samples were collected at 15-min intervals at rest and every 30 m in during exercise. Results: Plasma HCA concentrations increased after HCA ingestion up to 0.39 +/- 8.02 mmol/L (82.0 +/- 4.8 mg/L). However, no significant differences i n total fat and carbohydrate oxidation rates were observed between trials. Accordingly, plasma glucose, glycerol, and fatty acid concentrations did no t differ between trials. Plasma lactate concentrations were significantly l ower in the HCA than in the placebo trial after 30 min of exercise but at t he end of the exercise period they did not differ between trials. Conclusion: HCA, even when provided in large quantities, does not increase total fat oxidation in vivo in endurance-trained humans.