**4. Discussion**

In direct contrast to our previous *Gulo* knockout mouse study, which showed clear differences in comparative vitamin C bioavailability [6], this investigation has shown no differences in the steady-state bioavailability of synthetic *versus* kiwifruit-derived vitamin C in plasma, semen, peripheral blood leukocytes and skeletal muscle of humans. Other comparative bioavailability studies in guinea pigs have shown similar results to our animal study [23–27]. Enhanced uptake of vitamin C into specific organs of guinea pigs (e.g., adrenals and spleen) was observed when administered with flavonoid-rich juices/extracts or purified plant flavonoids [23–27], although comparable vitamin C accumulation was observed in some organs (e.g., liver) in several studies [23–25,28], suggesting either tissue specific differences or effects of differences in study design.

In agreement with our current human study, others have shown little difference in steady-state plasma and/or urine bioavailability of synthetic vitamin C and that found in different fruits, fruit juices and vegetables [14–19]. Only one previous study has investigated the comparative bioavailability of synthetic *versus* natural vitamin C in leukocytes [19]. Although neutrophils express SVCT2 [29], when their respiratory burst is activated they primarily transport the oxidized form of ascorbate (dehydroascorbic acid) via the glucose transporters GLUT 1 and GLUT 3, followed by intracellular reduction [30]. In support of our observations with neutrophils and mononuclear cells, Pelletier *et al.* [19] found no difference in leukocyte ascorbate uptake between synthetic vitamin C (in the presence or absence of rutin) and that from orange juice. Interestingly, an *in vitro* study showed that the flavonoids myricetin and quercitin inhibited the uptake of both ascorbate and dehydroascorbic acid into monocytic (HL-60 and U937) and lymphocytic (Jurkat) cells [31]. Whether this occurs *in vivo* is, however, uncertain due to the low plasma bioavailability of flavonoids [32].

Although leukocyte ascorbate status is often used as an indicator of whole body status, whether this is an accurate model for other tissues and organs is uncertain. This premise is supported by our animal study which indicated that different organs exhibited maximal uptake at varying doses of vitamin C [6]. Therefore, we also investigated the previously unreported effects of synthetic *versus* natural vitamin C on skeletal muscle ascorbate status. Ascorbate is transported into muscle cells via SVCT2 [33,34]. In our study muscle tissue exhibited a greater relative uptake of ascorbate than leukocytes, however, there was again no difference in bioavailability between the synthetic and fruit-derived vitamin C.

We also investigated seminal fluid ascorbate status. Although the baseline seminal fluid ascorbate was lower than vitamin C replete and healthy non-smoking men [35–37], little change was observed with the low dose vitamin C tablet or half kiwifruit per day dose. Early studies have shown that vitamin C intakes of up to 250 mg/day are required to return depleted seminal ascorbate to normal levels [35,36,38], indicating that higher intakes of vitamin C, e.g., those that result in plasma saturation, are required to increase seminal fluid levels [39].
