Any dietary or drug treatment with high doses of micronutrients may override the body's own control mechanisms; therefore, micronutrient therapies may be associated with potential side effects and toxicities. High-dosed micronutrients should not be used without medical supervision.
In individuals with atherosclerosis the ability of blood vessels to relax or dilate is compromised. The damage to the heart muscle caused by a heart attack (myocardial infarction) 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 heart (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 vitamin C daily (26, 75, 76).
Individuals with high blood pressure (hypertension) are at increased risk of developing cardiovascular diseases. Several studies have demonstrated decreased blood pressure as an effect of vitamin C supplementation. One study of individuals with high blood pressure found that a daily supplement of 500 mg vitamin C resulted in an average drop in the peak (systolic) blood pressure in the arteries of 9% after 4 weeks (27). It should be noted that those participants who were taking antihypertensive medication continued to do so throughout the 4-week study. With the same dose in Type 2 Diabetes patient the blood pressure and arterial stiffness could be improved (77).
As oxidative stress has been associated with hypertension, research has suggested that the antioxidant properties of vitamin C and vitamin E might be linked with a decrease in blood pressure.
In a randomized controlled trial, patients with hypertension were treated with 1 g vitamin C and 400 mg vitamin E. After 8 weeks of treatment the systolic, diastolic and mean blood pressure was significantly lower compared to placebo-treated patients (28).
Because the findings regarding vitamin C and high blood pressure have not yet been replicated in larger studies, it is important for individuals with significantly high blood pressure to continue current therapy (medication, lifestyle changes, etc.) in consultation with their treating physician.
While research is underway to determine whether combinations of antioxidant vitamins might be beneficial as an additional treatment to conventional cancer therapy, definitive conclusions are not yet possible (29).
However, vitamin C should not be used in place of therapy that has been demonstrated effective in the treatment of a particular type of cancer, for example, chemotherapy or radiation therapy. If an individual with cancer chooses to take vitamin supplements, it is important that the clinician coordinating his or her treatment is aware of the type and dose of each supplement.
Currently, there are no results from controlled clinical trials indicating that vitamin C would adversely affect the survival of cancer patients.
Cardiovascular diseases (heart disease and stroke) are the leading cause of death in individuals with diabetes. Evidence that diabetes is a condition of increased oxidative stress led to the hypothesis that higher intakes of antioxidant nutrients, such as vitamin C, could help decrease cardiovascular disease risk in diabetic individuals.
In support of this hypothesis, a 16-year study of 85,000 women, 2% of whom were diabetic, found that vitamin C supplement use (400 mg/day or more) was associated with significant reductions in the risk of fatal and non-fatal coronary heart disease in the entire group as well as those with diabetes (30).
In contrast, a 15-year study of postmenopausal women found that diabetic women who reported taking at least 300 mg/day of vitamin C from supplements when the study began were at significantly higher risk of death from coronary heart disease and stroke than those who did not take vitamin C supplements (31). Vitamin C supplement use was not associated with a significant increase in cardiovascular disease mortality in the group as a whole.
Although a number of observational studies have found that higher dietary intakes of vitamin C are associated with lower cardiovascular disease risk, randomized controlled trials have not found antioxidant supplementation that included vitamin C to reduce the risk of cardiovascular disease in diabetic or other high-risk individuals (32, 33).
It is possible that genetic differences may influence the effect of vitamin C supplementation on cardiovascular disease. Some findings suggest that there may be a subpopulation of people with diabetes who will benefit from antioxidant therapy while others may not benefit or could actually be harmed (34).
A systematic review and meta-analysis observed in patients with diabetes and with more prolonged supplementation a greater reduction in glucose concentrations. This indicates that personalised interventions with vitamin C may represent a feasible future strategy to enhance benefits and efficacy of interventions (83).
Authored by Dr Peter Engel in 2010, reviewed and revised by Dr. Volker Elste on 22.05.2017