Effects of p-Synephrine and Caffeine Ingestion on Substrate Oxidation during Exercise

Purpose Caffeine and p-synephrine are substances usually included in commercially available products for weight loss because of their purported thermogenic effects. However, scientific information is lacking about the effects of combining these substances on substrate oxidation during exercise. The purpose of this investigation was to determine the isolated and combined effects of p-synephrine and caffeine on fat oxidation rate during exercise. Methods In a double-blind randomized experiment, 13 healthy subjects participated in four experimental trials after the ingestion of a capsule containing a placebo, 3 mg·kg−1 of caffeine, 3 mg·kg−1 of p-synephrine, or the combination of these doses of caffeine and p-synephrine. Energy expenditure and substrate oxidation rates were measured by indirect calorimetry during a cycle ergometer ramp test from 30% to 90% of V˙O2max. Results In comparison with the placebo, the ingestion of caffeine, p-synephrine, or p-synephrine + caffeine did not alter total energy expenditure or heart rate during the whole exercise test. However, the ingestion of caffeine (0.44 ± 0.15 g·min−1, P = 0.03), p-synephrine (0.43 ± 0.19 g·min−1, P

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