**1. Introduction**

Vitamin C (ascorbate) is an essential water-soluble micronutrient that is obtained through the diet primarily from fruits and vegetables [1]. The bioavailability of dietary vitamin C represents the proportion absorbed by the intestines and available for metabolic processes within the body. Vitamin C is actively transported into the body via two sodium-dependent vitamin C transporters, SVCT1 and SVCT2, which exhibit different tissue distributions and uptake kinetics [2,3]. SVCT1 is expressed in epithelial tissue and is primarily responsible for intestinal uptake and renal reabsorption of vitamin C, the latter maintaining whole body homeostasis [3]. SVCT2 is expressed in specialized and metabolically active tissues and is required for delivery of vitamin C to tissues with a high demand for the vitamin either for enzymatic reactions [4] and/or to help protect these tissues from oxidative stress [3].

Kiwifruit are rich in vitamin C [5] and we have previously used a genetically vitamin C-deficient mouse model (the *Gulo* mouse) to investigate the comparative bioavailability of synthetic *versus* kiwifruit-derived vitamin C [6]. Interestingly, we found that kiwifruit gel provided higher serum, leukocyte, heart, liver and kidney levels of ascorbate than equivalent amounts of purified vitamin C, suggesting a synergistic activity of the whole fruit in this model. Although synthetic and food-derived vitamin C are chemically identical, the bioavailability of vitamin C could potentially be affected by the numerous micronutrients and phytochemicals with antioxidant properties that are present in fruits and vegetables [7,8]. For example, kiwifruit contain reasonable amounts of vitamin E [9], which has been shown to spare vitamin C in an animal model [10]. Kiwifruit also contain numerous different flavonoids [9,11], some of which can inhibit the *in vitro* oxidation of vitamin C via direct scavenging of free radicals and/or chelation of redox-active metal ions [7,8].

To determine if the results of our animal study [6] are translatable to humans, we carried out a randomized human study comparing the bioavailability of vitamin C from gold kiwifruit (*Actinidia Chinensis* var. *Hort 16A*) with a tablet of equivalent dosage. We have previously shown that consumption of half a gold kiwifruit per day results in a significant increase in plasma ascorbate in individuals with low initial levels (<23 μmol/L) [12]. For this study, we chose a dose of half a kiwifruit per day and the equivalent 50 mg/day vitamin C since this dose lies on the steeply rising portion of the sigmoidal plasma bioavailability curve [13]. This enhances the likelihood of detecting a difference between the two interventions compared with doses >100 mg/day where plasma saturation is approached [13].

Several previous studies have tested the comparative bioavailability of synthetic *versus* food-derived vitamin C utilizing plasma and/or urine levels [14–18]. However, only one has investigated the comparative bioavailability of vitamin C in leukocytes [19]. Therefore, in addition to plasma and urine, we have monitored the bioavailability of vitamin C in peripheral blood mononuclear cells and neutrophils, seminal fluid and skeletal muscle tissue [20] before and after the six week intervention. We also monitored the participants' dietary intake of vitamin C using seven day food and beverage records.
