Acute p-synephrine ingestion increases fat oxidation rate during exercise.

p-Synephrine is a protoalkaloid widely used in dietary supplements for weight management because of its purported thermogenic effects. However, there is a lack of scientific information about its effectiveness to increase fat metabolism during exercise. The purpose of this investigation was to determine the effects of an acute ingestion of p-synephrine on fat oxidation at rest and during exercise.

In a double-blind, randomized and counterbalanced experimental design, 18 healthy subjects performed two acute experimental trials after the ingestion of p-synephrine (3 mg kg(-1) ) or after the ingestion of a placebo (cellulose). Energy expenditure and fat oxidation rates were measured by indirect calorimetry at rest and during a cycle ergometer ramp exercise test (increases of 25 W every 3 min) until volitional fatigue.

In comparison with the placebo, the ingestion of p-synephrine did not change energy consumption (1.6 ± 0.3 vs. 1.6 ± 0.3 kcal min(-1) ; P = 0.69) or fat oxidation rate at rest (0.08 ± 0.02 vs. 0.10 ± 0.04 g min(-1) ; P = 0.15). However, the intake of p-synephrine moved the fat oxidation-exercise intensity curve upwards during the incremental exercise (P < 0.05) without affecting energy expenditure. Moreover, p-synephrine increased maximal fat oxidation rate (0.29 ± 0.15 vs. 0.40 ± 0.18 g min(-1) ; P = 0.01) during exercise although it did not affect the intensity at which maximal fat oxidation was achieved (55.8 ± 7.7 vs. 56.7 ± 8.2% VO2peak ; P = 0.51).

The acute ingestion of p-synephrine increased the fat oxidation rate while it reduced the carbohydrate oxidation rate when exercising at low-to-moderate exercise intensities.