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Synephrine is a common ingredient in many thermogenic and energy supplements. Chemically, synephrine is both an a-adrenergic and "-adrenergic agonist. Studies have shown synephrine specifically stimulates lipolysis, an effect that is attributed primarily to stimulation of b3 adrenergic receptors, whereas stimulation of '1 and '2 adrenergic receptors are linked with cardiac effects. Thus synephrine is expected to be associated with less stimulation of heart rate and blood pressure while still promoting fat breakdown and thermogenesis. There are only a few studies examining the effects of synephrine in subjects exercising. In a recent study, physiologic measurements were made in healthy subjects after ingestion of a supplement containing 21 mg synephrine and 304 mg caffeine at rest and when exercise was performed one hour after intake. The syephrine/caffeine supplement did not alter the body temperature, fluid loss, oxygen consumption, heart rate or systolic blood pressure response to exercise, but diastolic blood pressure was slightly elevated by about 12% compared to placebo. Supplement had a positive effect on subjective ratings of perceived exertion. Subjects rated the exercise session easier when they ingested the synephrine/caffeine supplement. The study provides evidence for the general safety of synephrine and caffeine and further show they can act centrally by reducing the perception of effort during exercise.

Haller CA, Duan M, Jacob P 3rd, Benowitz N. Human pharmacology of a performance-enhancing dietary supplement under resting and exercise conditions. Br J Clin Pharmacol. 2008 Jun;65(6):833-40. Epub 2008 Mar 13.