Synephrine is the name of a chemical compound found in the peel of the unripe Citrus aurantium fruit. The form of synephrine in Citrus aurantium is p-synephrine. Synephrine is sold as a weight loss supplement on account of its ability to activate adrenergic receptors, some of which might be involved in accelerating fat oxidation rates in cells. Synephrine is often compared to ephedrine (an alkaloid from the Ephedra sinica plant), a known central nervous system stimulant that has been shown to have significant benefits at causing weight loss in humans and animals. Pharmacologically, p-Synephrine, does not have a high affinity for the α-1 and α-2 adrenergic receptors, and nor does it have a high affinity for the β-1 and β-2 adrenergic receptors. This explains the lack of cardiovascular effects in p-synephrine. However, p-synephrine may activate the β-3 adrenergic receptors, which are involved in the upregulation of lipolysis in brown and white adipose tissue. The pharmacology of p-synephrine appears to be related to its hydroxyl group that is present on the para-position of the phenol ring. Further studies suggest that noradrenaline and adrenaline levels are not increased through normal use of p-synephrine. In contrast to p-synephrine, m-synephrine (phenylephrine) possesses a hydroxyl group in the meta-position of the phenol ring. Commercially available m-synephrine is synthesised artificially and is not present in citrus fruit. Ephedrine acts on α-1, β-1, and β-2 adrenergic receptors to produce cardiovascular effects, while interacting with β-3 adrenergic receptors to promote thermogenesis. Phenylephrine (m-synephrine) exerts cardiovascular effects similar to ephedrine as well as bronchodilation suggesting its receptor binding is more similar to ephedrine than to p-synephrine.
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