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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
more years. With a cohort of 247 nurses from the above study, vitamin C supplement use, in amounts ranging from less than 400 mg/day to greater than 700 mg/day for 10 years or more, was associated with a statistically significant protective effect on lens opacities (Jacques et al., 1997). Women who consumed vitamin C supplements for less than 10 years were not protected.
Although many of the above studies suggest a protective effect of vitamin C against cataracts, the data are not consistent or specific enough to estimate the vitamin C requirement based on cataracts.
Asthma and Obstructive Pulmonary Disease
It is suspected that vitamin C may decrease the risk of asthma and other related pulmonary conditions (Hatch, 1995). Two cross-sectional studies suggest that high plasma vitamin C concentrations or intakes protect or perhaps enhance respiratory function in men but not in women (Ness et al., 1996) and in both men and women (Britton et al., 1995). Similarly, dietary vitamin C intake was positively associated with enhanced pulmonary function in 2526 adult men and women participants in the First National Health and Nutrition Survey Epidemiological Follow-up Study (Schwartz and Weiss, 1994). In another study, 20 middle-aged men and women patients with mild asthma had decreased ascorbate and α-tocopherol concentrations in lung lining fluid, while blood levels were normal (Kelly et al., 1999). These findings and the presence of increased oxidized glutathione in the airways indicate an increased oxidative stress in asthma patients.
A series of small, clinical experiments reported that vitamin C supplementation of 2 g/day may be protective against airway responsiveness to viral infections, allergens, and irritants (Bucca et al., 1992). In contrast, a clinical experiment testing the blocking effect of 2 g/day vitamin C against exercise-induced asthma found little evidence of such an effect (Cohen et al., 1997).
Although many of the above studies suggest a protective effect of vitamin C against asthma and obstructive pulmonary disease, the data are not consistent or specific enough to estimate the vitamin C requirement based on asthma or pulmonary disease.
There has been a great deal of interest in the use of vitamin C to protect against the common cold, much of this research stimulated by the views put forth by the late Linus Pauling (Hemila and Her-