Vitamin C and Cardiovascular Diseases (Heart Disease and Stroke) - 7 out of 12 prospective studies, which examined large numbers of people (700 to 87,000) over a number of years (3 to 20), found a significant relationship between higher levels of vitamin C intake and a lower risk of heart disease and stroke. The remaining studies, which did not find a relationship between vitamin C intake and cardiovascular diseases, compared individuals who were already consuming close to 100 mg daily with those consuming higher amounts.
A careful experimental study at the NIH demonstrated that some human tissues (leukocytes) tend to become saturated with vitamin C at a dose of 100 mg/day. Thus, it is possible that once tissue saturation has been achieved, additional protective effects of vitamin C against cardiovascular diseases are small and therefore difficult to detect in prospective studies. Consistent with this possibility, at least 6 prospective studies have found low blood levels of vitamin C at baseline to be associated with a subsequent increase in the risk of heart disease or stroke.
Health Benefit of Vitamin C and Cancer
A large number of studies have shown that increased consumption of fresh fruits and vegetables is associated with a reduced risk for most types of cancer. Such studies are the basis for dietary guidelines endorsed by the U.S. Department of Agriculture and the National Cancer Institute, which recommend at least 5 servings of fruits and vegetables per day. A number of case-control studies have investigated the role of vitamin C in cancer prevention.
Most have shown that higher intakes of vitamin C are associated with decreased incidence of cancers of the mouth, throat and vocal chords, esophagus, stomach, colon-rectum, and lung. Because the possibility of bias is greater in case control studies, prospective studies are generally given more weight in the evaluation of the effect of nutrient intake on disease. In general, prospective studies in which the lowest intake group consumed more than 86 mg of vitamin C daily have not found differences in cancer risk, while studies finding significant cancer risk reductions found them in people consuming at least 80 to 110 mg of vitamin C daily.
A prospective study of 870 men over a period of 25 years found that those who consumed more than 83 mg of vitamin C daily had a striking 64% reduction in lung cancer compared with those who consumed less than 63 mg per day. Although most large prospective studies found no association between breast cancer and vitamin C intake, two recent studies found dietary vitamin C intake to be inversely associated with breast cancer risk in certain subgroups.
In the Nurses' Health Study, premenopausal women with a family history of breast cancer who consumed an average of 205 mg/day of vitamin C from foods had a 63% lower risk of breast cancer than those who consumed an average of 70 mg/day. In the Swedish Mammography Cohort, women who were overweight and consumed an average of 110 mg/day of vitamin C had a 39% lower risk of breast cancer compared to overweight women who consumed an average of 31 mg/day.
Vitamin C Health Benefit and Cataracts
Cataracts are a leading cause of blindness in the U.S. Cataracts occur more frequently and become more severe as people age. Decreased vitamin C levels in the lens of the eye have been associated with increased severity of cataracts in humans. Some, but not all, studies have observed increased dietary vitamin C intake and increased blood levels of vitamin C to be associated with decreased risk of cataracts.
Those studies that have found a relationship suggest that vitamin C intake may have to be higher than 300 mg/day for a number of years before a protective effect can be detected. Recently, a 7-year controlled intervention trial of a daily antioxidant supplement containing 500 mg of vitamin C, 400 IU of vitamin E, and 15 mg of b-carotene in 4,629 men and women found no difference between the antioxidant combination and a placebo on the development and progression of age-related cataracts. Therefore, the relationship between vitamin C intake and the development of cataracts requires further clarification before specific recommendations can be made.
Vitamin C and Cardiovascular Diseases
The ability of blood vessels to relax or dilate is compromised in individuals with atherosclerosis. The damage to the heart muscle caused by a heart attack and damage to the brain caused by a stroke is related, in part, to the inability of blood vessels to dilate enough to allow blood flow to the affected areas. The pain of angina pectoris is also related to insufficient dilation of the coronary arteries. Treatment with vitamin C has consistently resulted in improved dilation of blood vessels in individuals with atherosclerosis as well as those with angina pectoris, congestive heart failure, high cholesterol, and high blood pressure. Improved blood vessel dilation has been demonstrated at a dose of 500 mg of vitamin C daily.
Vitamin C and Common Cold
The work of Linus Pauling stimulated public interest in the use of large doses (greater than 1 gram/day) of vitamin C to prevent infection with the viruses responsible for the common cold. Reviews of the research conducted on this issue over the past 20 years conclude that, in general, large doses of vitamin C do not have a significant effect on the incidence of the common cold.
However, a few studies have indicated that certain susceptible groups (e.g., individuals with low dietary intake and marathoners) may be less susceptible to the common cold when taking supplemental vitamin C. Additionally, large doses of vitamin C have been found to decrease the duration and severity of colds, an effect that may be related to the antihistamine effects found to occur with large doses (2 grams) of vitamin C.