Nearly all studies show that smokers have decreased plasma and leukocyte ascorbate levels compared to nonsmokers. Part of this difference may be attributable to a lower intake of fruits and vegetables among smokers than among nonsmokers (Dallongeville et al., 1998; Marangon et al., 1998). However, studies that have adjusted for differences in vitamin C intake (Marangon et al., 1998) and those which have assessed populations with similar fruit and vegetable intakes (Lykkesfeldt et al., 2000) still find that smokers have lower plasma vitamin C concentrations than nonsmokers. This indicates that smoking per se predisposes to lower vitamin C status.

The mechanism by which smoking compromises vitamin C status has not been well established. A radioisotope-labeled ascorbic acid dilution study showed that the metabolic turnover of the vitamin in smokers averaged about double that of nonsmokers: 70.0 versus 35.7 mg/day (Kallner et al., 1981). Increased ascorbate turnover in smokers is likely due to the increased oxidative stress from substances in smoke that are directly oxidizing or that stimulate oxidizing inflammatory responses (Elneihoum et al., 1997; Lehr et al., 1997; Pryor, 1997). This hypothesis is supported by the finding that the ratio of dehydroascorbic acid (DHA) to ascorbate in plasma of smokers is increased compared to that in nonsmokers (Lykkesfeldt et al., 1997).

Most studies have found that smokers suffer increased in vivo oxidation of susceptible biological molecules, including lipids (Morrow et al., 1995; Reilly et al., 1996), lipoproteins (Sasaki et al., 1997), and deoxyribonucleic acid (DNA) (Asami et al., 1997; Panayiotidis and Collins, 1997). In many but not all of these studies, intervention with administration of vitamin C or cessation of smoking decreased the oxidant damage measured. Supplementation of smokers with vitamin C (2 g/day) reduced elevated levels of urinary isoprostanes, a measure of in vivo lipid peroxidation (Reilly et al., 1996). This is consistent with earlier findings that either endogenous or in vitro added ascorbic acid uniquely protected plasma lipids against oxidative damage caused by cigarette smoke (Frei et al., 1991). Large doses of vitamin C (1 g/day or more) provided protection against lymphocyte DNA strand break damage induced ex vivo by radiation with H₂O₂ (hydrogen peroxide) (Green et al., 1994;