Synephrine, the active ingredient in Citrus aurantium had a moment as "the" successor to Ephedra sinica – until it became clear that its metabolism boosting and adrenergic effect was almost nil. American researchers [funded, by the way, by supplement manufacturer Nutratech] discovered that the addition of two flavonoids turn synephrine into a an active substance.
The researchers did an experiment with 50 test subjects, who were divided among 5 groups.
The first group [in the graph below represented by the first bar; in the table below labelled Placebo] were given a placebo.
The second group [second bar; T2] were given 50 mg synephrine [structural formula shown above] in the form of Advantra Z supplement. Nutratech, the manufacturer of this supplement, is also the employer of one of the researchers. Two other researchers have also worked as a consultant for Nutratech. In the second group, the amount of energy burned while resting increased by 35 calories in a period of 75 minutes.
The third group [third bar; T2] were given 50 mg synephrine plus 600 mg naringin [structural formula shown below]. The amount of energy burned while resting increased by 99 calories in a period of 75 minutes.
The fourth group [fourth bar; T4] were given 50 mg synephrine plus 600 mg naringin plus 100 mg hesperidin [structural formula shown on the left]. The amount of energy burned while resting increased by 153 calories in a period of 75 minutes.
The fifth group [fifth bar; T5] were given 50 synephrine plus 600 mg naringin plus 1000 mg hesperidin. The amount of energy burned while resting only increased by 49 calories in a period of 75 minutes.
During the 75-minute period the researchers also measured the subjects' blood pressure and heart rate. It seems that while the addition of naringin and hesperidin boosts the metabolism-raising properties of synephrine, but not its cardiovascular side effects.
Hesperidin and naringin are both found in citrus fruit, as is synephrine. The researchers speculate that hesperidin and naringin activate the fat hormone adiponectin. Adiponectin makes muscle cells more sensitive to insulin.
"The data suggests the combination of the flavonoids naringin and hesperidin with p-synephrine may assist weight management", the researchers conclude. "Further studies are required to determine optimal doses as well as safety and efficacy associated with long term use."
Int. J. Med. Sci. 2011; 8(4):295-301.