fruit, obtained on the US market, Avula and co-workers found synephrine levels ranging from ~4–60 mg/L; no synephrine was detected in juices from grapefruit, lime, or lemon. An analysis of the synephrine levels in a range of different citrus fruits, carried out on juices that had been extracted from fresh, peeled fruit, was reported by Uckoo and co-workers, with the following results: Marrs sweet orange (C. sinensis Tan.): ~85 mg/L; Nova tangerine (C. reticulata Tan.): ~78 mg/L; clementine (C. clementina Tan.): ~115 mg/L; Meyer lemon (C. limon Tan.) ~3 mg/kg; Ugli tangelo (C. reticulata × C. paradisi) ~47 mg/kg. No synephrine was detected in: Rio Red grapefruit (C. paradisi Macf.); Red-fleshed pummelo (C. grandis Tan.); or Wekiwa tangelo (C. reticulata × C. paradisi). Numerous additional comparable analyses of the synephrine content of Citrus fruits and products derived from them may be found in the research literature. In humans and other animals Low levels of synephrine have been found in normal human urine, as well as in other mammalian tissue. To reduce the likelihood that the synephrine detected in urine had a dietary origin, the subjects tested by Ibrahim and co-workers abstained from the consumption of any citrus products for 48 hours prior to providing urine samples. A 2006 study of synephrine in human blood platelets by D'Andrea and co-workers showed increased levels in platelets from patients suffering from aura-associated migraine (0.72 ng/108 platelets, compared to 0.33 ng/108 platelets in control subjects). Earlier, the same research group had reported a normal human blood plasma level of synephrine of 0.90–13.69 ng/mL. Stereoisomers Since synephrine exists as either of two enantiomers (see Chemistry section below for further discussion), which do not produce identical biological effects (see Pharmacology section below) some researchers have examined the stereoisomeric composition of synephrine extracted from natural sources. Although it seems clear that synephrine is found in those Citrus species which have been studied predominantly as the l-isomer, low levels of d-synephrine have been detected in juice and marmalade made from C. unshiu, and low levels (0.002%) have been reported in fresh fruit from C. aurantium. There are indications that some d-synephrine may be formed by the racemization of l-synephrine as a result of the processing of fresh fruit, although this matter has not been completely clarified. However, regardless of the situation in Citrus species, Ranieri and McLaughlin reported the isolation of racemic (i.e. a mixture of equal amounts