Vitamin C (ascorbic acid) is an essential nutrient, which is involved in a number of cellular processes. However, unlike most mammals, humans are unable to synthesize vitamin C and it must therefore be acquired from the diet. This is thought to be achieved by two transporters, SVCT1 and SVCT2, recently cloned from rat (Tsukaguchi et al. 1999) and human kidney (Daruwala et al. 1999). Tissue distribution suggests that SVCT1 is the predominant transporter in the intestine (Tsukaguchi et al. 1999), and subsequent studies have shown that SVCT1 is functionally expressed in the human intestinal Caco-2 cell line (Wang et al. 1999). Vitamin C supplements are becoming increasingly common, adding greatly to the dietary load. However, little is known regarding the effects of vitamin supplements on intestinal physiology. There are reports that taking high doses of vitamin C can reduce the percentage absorption of the vitamin and that the unabsorbed fraction is retained within the intestine, producing mild diarrhoea (reviewed in Bates, 1997). The aim of our study was to investigate the effect of high doses of ascorbate on SVCT1 expression, using the Caco-2 cell model of small intestinal enterocytes.

Caco-2 cells were cultured for 21 days on Anapore inserts and incubated for the final 24 h in the presence of ascorbic acid (4.5 mg ml^-1 of culture medium). L-[¹⁴C] ascorbic acid uptake (100 µM) was measured by scintillation counting. The effects of ascorbate on SVCT1 expression were determined by RT-PCR using transporter-specific primers. Parallel experiments measured β-actin mRNA as a control. Band densities were analysed using Scion Image software and data expressed as a ratio of SVCT1 : β-actin for each sample. All data are the means ± S.E.M. of four experiments. Statistical analysis was performed using Student’s unpaired t test.

Ascorbic acid uptake by Caco-2 cells showed typical exponential kinetics and had a half-time of 2.1 min. Following incubation with ascorbate for 24 h, uptake was decreased significantly (control, 0.37 ± 0.03 nmol cm^-2 h^-1; ascorbate, 0.18 ± 0.01 nmol cm^-2 h^-1, P < 0.05). Interestingly, expression of SVCT1 was also significantly reduced by exposure to elevated levels of ascorbate for 24 h (control, 71.8 ± 12.6 a.u.; ascorbate, 16.9 ± 5.0 a.u., P < 0.01), whereas β-actin levels were unaffected. Taken together, these data suggest that multi-gram supplements might not be the most appropriate way of increasing the body pool of vitamin C.

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Daruwala, R., Song, J., Koh, W.S., Rumsey, S.C. & Levine, M. (1999). FEBS Lett. 460, 480-484.

Tsukaguchi, H., Toki, T., Mackenzie, B., Berger, U.V., Chen, X.-Z., Wang, Y., Brubaker, R.F. & Hediger, M.A (1999). Nature 399, 70-75.

Wang, H., Dutta, B., Huang, W., Devoe, L.D., Leibach, F.H., Ganapathy, V. & Prasad, P.D. (1999). Biochim. Biophys. Acta 1461, 1-9.