OBJECTIVE: (-)-Hydroxycitric acid ((-)-HCA) is available as a herbal supplement, and promoted as a weight loss agent. It is hypothesized that (-)-HCA can increase fat oxidation by inhibiting citrate lyase, an enzyme which plays a crucial role in energy metabolism during de novo lipogenesis. The indirect inhibition of the cytosolic pool of citrate by [-)-HCA and the subsequent reduction in acetyl coenzyme A and oxaloacetate alters steps in the citric acid cycle that promote fat oxidation. The objective of this study was to determine the effect of (-)-HCA on marker substrates of altered metabolism, as well as on respiratory quotient (RO) and energy expenditure (EE) in humans, following an overnight fast and during a bout of exercise. HYPOTHESIS OF STUDY: We hypothesized that supplementation with (-)-HCA would result in an increase in fat oxidation and metabolic rate, reflected by an increase in β-hydroxybutyrate and EE and/or a decrease in RQ. Furthermore, during moderately intense exercise, we hypothesized that (-)-HCA supplementation would increase the rate of lactate conversion to glucose in the liver, with a subsequent reduction of circulating lactate and an elevation of circulating ketone bodies due to the increased partial oxidation of fatty acids (FA) in mitochondria. Studies have examined the fat regulating action of (-)-HCA on steps of the citric acid cycle in rodents showing reductions in body weight and food intake. No studies have investigated the effects of (-)-HCA supplementation in conjunction with a typical daily dietary composition (that is approx 30-35% fat) on metabolic processes which could influence body weight regulation in humans. DESIGN: This was a double blind, placebo controlled, randomized, crossover study involving three days of (-)-HCA (3.0 g/d) or placebo supplementation. The effects of (-)-HCA supplementation on metabolic parameters with or without moderately intense exercise was studied over four laboratory visits. SUBJECTS: Sedentary adult male subjects (n = 10, age: 22-38 y, body mass index (BMI) 22.4-37.6 kg/m2). MEASUREMENTS: Two of the four visits involved no exercise (Protocol A) with and without (-)-HCA treatment, while the remaining two visits included a moderately intense exercise bout [Protocol B; 30 min at 40% maximal aerobic fitness (V2max) and 15 min at 60% VO2max) with and without (-)-HCA treatment. EE (by indirect calorimetry) and RQ were measured for 150 min following an overnight fast. Blood samples were collected for the determination of glucose, insulin, glucagon, lactate, and β-hydroxybutyrate concentrations. RESULTS: In a fasted state and following 3 d of (-)-HCA treatment, RQ was not significantly lowered during rest (Protocol A) nor during exercise (Protocol B) compared with the placebo treatment. Treatment with (-)-HCA did not affect EE, either during rest or during moderately intense exercise. Furthermore, the blood substrates measured were not significantly different between treatment groups under the fasting conditions of this study. CONCLUSION: These results do not support the hypothesis that (-)-HCA alters the short-term rate of fat oxidation in the fasting state during rest or moderate exercise, with doses likely to be achieved in humans while subjects maintain a typical Western diet (approx 30-35% total calories as fat).