man, 1995). Reviews of numerous studies generally conclude that vitamin C megadoses have no significant effect on incidence of the common cold, but do provide a moderate benefit in terms of the duration and severity of episodes in some groups (Chalmers, 1975; Jariwalla and Harakeh, 1996; Ludvigsson et al., 1977). The often-reported improvement in severity of colds after vitamin C ingestion may be due to the antihistaminic action of the vitamin at pharmacological doses (Johnston et al., 1992). One early study comparing 44 school-aged twins in vulnerability to colds found no significant overall treatment effect of vitamin C intakes at doses of 500 to 1,000 mg/day (Miller et al., 1977). Other trials came to similar conclusions (Coulehan et al., 1976; Ludvigsson et al., 1977). Some reviews have stated that any impact of vitamin C is slight or that it is protective only among some subgroups of people (Hemila, 1996, 1997). Others view the accumulated results as so incomplete and flawed as to offer no evidence of protective effects (Herbert, 1995). Thus, the data are not consistent or specific enough to estimate the vitamin C requirement based on the common cold.
Although vitamin C's role as an antioxidant and cofactor for catecholamine biosynthesis might suggest that it protects cognitive function, there is little valid evidence that it does. One study found no association between cognitive function and vitamin C intake (range 84 to 147 mg/day) in 5,182 Dutch residents aged 55 to 95 years (Jama et al., 1996). Another study of 442 men and women, aged 65 to 94 years, reported that higher plasma ascorbate levels were associated with better memory performance (Perrig et al., 1997).
Although several studies have reported an inverse correlation between vitamin C intake and cardiovascular disease, some types of cancer, and cataracts, others have failed to do so. Very little variation in risk is seen based on the intake of vitamin C for chronic obstructive pulmonary disease, cold or infectious disease, or cognitive function and memory. Also it is important that, for all their power, human-based observational or epidemiological studies imply but do not prove cause and effect. Such studies do not rule out the impact of unidentified factors. In a recent review of epidemiological studies, Gey (1998) suggested that plasma vitamin C concentrations as low as 50 µmol/L (1.0 mg/dL) provide the optimal ben-