Age-related macular degeneration (AMD) and cataracts
AMD and cataracts are two of the leading causes of vision loss in older individuals. Oxidative stress might contribute to the etiology of both conditions. Thus, researchers have hypothesized that vitamin C and other antioxidants play a role in the development and/or treatment of these diseases.
A population-based cohort study in the Netherlands found that adults aged 55 years or older who had high dietary intakes of vitamin C as well as beta-carotene, zinc, and vitamin E had a reduced risk of AMD. However, most prospective studies do not support these findings. The authors of a 2007 systematic review and meta-analysis of prospective cohort studies and randomized clinical trials concluded that the current evidence does not support a role for vitamin C and other antioxidants, including antioxidant supplements, in the primary prevention of early AMD.
Although research has not shown that antioxidants play a role in AMD development, some evidence suggests that they might help slow AMD progression. The Age-Related Eye Disease Study (AREDS), a large, randomized, placebo-controlled clinical trial, evaluated the effect of high doses of selected antioxidants (500 mg vitamin C, 400 IU vitamin E, 15 mg beta-carotene, 80 mg zinc, and 2 mg copper) on the development of advanced AMD in 3,597 older individuals with varying degrees of AMD. After an average follow-up period of 6.3 years, participants at high risk of developing advanced AMD (i.e., those with intermediate AMD or those with advanced AMD in one eye) who received the antioxidant supplements had a 28% lower risk of progression to advanced AMD than participants who received a placebo. A follow-up AREDS2 study confirmed the value of this and similar supplement formulations in reducing the progression of AMD over a median follow-up period of 5 years.
High dietary intakes of vitamin C and higher plasma ascorbate concentrations have been associated with a lower risk of cataract formation in some studies. In a 5-year prospective cohort study conducted in Japan, higher dietary vitamin C intake was associated with a reduced risk of developing cataracts in a cohort of more than 30,000 adults aged 45–64 years. Results from two case-control studies indicate that vitamin C intakes greater than 300 mg/day reduce the risk of cataract formation by 70%–75%. Use of vitamin C supplements, on the other hand, was associated with a 25% higher risk of age-related cataract extraction among a cohort of 24,593 Swedish women aged 49–83 years. These findings applied to study participants who took relatively high-dose vitamin C supplements (approximately 1,000 mg/day) and not to those who took multivitamins containing substantially less vitamin C (approximately 60 mg/day). Data from clinical trials are limited. In one study, Chinese adults who took daily supplements of 120 mg vitamin C plus 30 mcg molybdenum for 5 years did not have a significantly lower cataract risk. However, adults aged 65–74 years who received 180 mg vitamin C plus 30 mcg molybdenum combined with other nutrients in a multivitamin/mineral supplement had a 43% significantly lower risk of developing nuclear cataracts than those who received a placebo. In the AREDS study, older individuals who received supplements of 500 mg vitamin C, 400 IU vitamin E, and 15 mg beta-carotene for an average of 6.3 years did not have a significantly lower risk of developing cataracts or of cataract progression than those who received a placebo. The AREDS2 study, which also tested formulations containing 500 mg of vitamin C, confirmed these findings.
Overall, the currently available evidence does not indicate that vitamin C, taken alone or with other antioxidants, affects the risk of developing AMD, although some evidence indicates that the AREDS formulations might slow AMD progression in people at high risk of developing advanced AMD.
The common cold
In the 1970s Linus Pauling suggested that vitamin C could successfully treat and/or prevent the common cold. Results of subsequent controlled studies have been inconsistent, resulting in confusion and controversy, although public interest in the subject remains high.
A 2007 Cochrane review examined placebo-controlled trials involving the use of at least 200 mg/day vitamin C taken either continuously as a prophylactic treatment or after the onset of cold symptoms. Prophylactic use of vitamin C did not significantly reduce the risk of developing a cold in the general population. However, in trials involving marathon runners, skiers, and soldiers exposed to extreme physical exercise and/or cold environments, prophylactic use of vitamin C in doses ranging from 250 mg/day to 1 g/day reduced cold incidence by 50%. In the general population, use of prophylactic vitamin C modestly reduced cold duration by 8% in adults and 14% in children. When taken after the onset of cold symptoms, vitamin C did not affect cold duration or symptom severity.
Overall, the evidence to date suggests that regular intakes of vitamin C at doses of at least 200 mg/day do not reduce the incidence of the common cold in the general population, but such intakes might be helpful in people exposed to extreme physical exercise or cold environments and those with marginal vitamin C status, such as the elderly and chronic smokers. The use of vitamin C supplements might shorten the duration of the common cold and ameliorate symptom severity in the general population, possibly due to the anti-histamine effect of high-dose vitamin C. However, taking vitamin C after the onset of cold symptoms does not appear to be beneficial.