Health Risks from Excessive Vitamin C
Vitamin C has low toxicity and is not believed to cause serious adverse effects at high intakes. The most common complaints are diarrhea, nausea, abdominal cramps, and other gastrointestinal disturbances due to the osmotic effect of unabsorbed vitamin C in the gastrointestinal tract.
In postmenopausal women with diabetes who participated in the Iowa Women’s Health Study, supplemental (but not dietary) vitamin C intake (at least 300 mg/day) was significantly associated with an increased risk of cardiovascular disease mortality. The mechanism for this effect, if real, is not clear and this finding is from a subgroup of patients in an epidemiological study. No such association has been observed in any other epidemiological study, so the significance of this finding is uncertain. High vitamin C intakes also have the potential to increase urinary oxalate and uric acid excretion, which could contribute to the formation of kidney stones, especially in individuals with renal disorders. However, studies evaluating the effects on urinary oxalate excretion of vitamin C intakes ranging from 30 mg to 10 g/day have had conflicting results, so it is not clear whether vitamin C actually plays a role in the development of kidney stones. The best evidence that vitamin C contributes to kidney stone formation is in patients with pre-existing hyperoxaluria.
Due to the enhancement of nonheme iron absorption by vitamin C, a theoretical concern is that high vitamin C intakes might cause excess iron absorption. In healthy individuals, this does not appear to be a concern. However, in individuals with hereditary hemochromatosis, chronic consumption of high doses of vitamin C could exacerbate iron overload and result in tissue damage.
Under certain conditions, vitamin C can act as a pro-oxidant, potentially contributing to oxidative damage. A few studies in vitro have suggested that by acting as a pro-oxidant, supplemental oral vitamin C could cause chromosomal and/or DNA damage and possibly contribute to the development of cancer. However, other studies have not shown increased oxidative damage or increased cancer risk with high intakes of vitamin C.
Other reported effects of high intakes of vitamin C include reduced vitamin B12 and copper levels, accelerated metabolism or excretion of ascorbic acid, erosion of dental enamel, and allergic responses. However, at least some of these conclusions were a consequence of assay artifact, and additional studies have not confirmed these observations.
The FNB has established ULs for vitamin C that apply to both food and supplement intakes (Table 3). Long-term intakes of vitamin C above the UL may increase the risk of adverse health effects. The ULs do not apply to individuals receiving vitamin C for medical treatment, but such individuals should be under the care of a physician.
Table 3: Tolerable Upper Intake Levels (ULs) for Vitamin C
Age Male Female
Pregnancy
Lactation
0–12 months
Not possible to establish*
1–3 years 400 mg 400 mg 4–8 years 650 mg 650 mg
9–13 years
14–18 years
1,200 mg 1,200 mg
1,800 mg 1,800 mg 1,800 mg
19+ years 2,000 mg 2,000 mg 2,000 mg
*Formula and food should be the only sources of vitamin C for infants. Interactions with Medications
Vitamin C supplements have the potential to interact with several types of medications. A few examples are provided below. Individuals taking these medications on a regular basis should discuss their vitamin C intakes with their healthcare providers.
Chemotherapy and radiation
The safety and efficacy of the use of vitamin C and other antioxidants during cancer treatment is controversial. Some data indicate that antioxidants might protect tumor cells from the action of radiation therapy and chemotherapeutic agents, such as cyclophosphamide, chlorambucil, carmustine, busulfan, thiotepa, and doxorubicin. At least some of these data have been criticized because of poor study design. Other data suggest that antioxidants might protect normal tissues from chemotherapy- and radiation-induced damage and/or enhance the effectiveness of conventional cancer treatment. However, due to the physiologically tight control of vitamin C, it is unclear whether oral vitamin C supplements could alter vitamin C concentrations enough to produce the suggested effects. Individuals undergoing chemotherapy or radiation should consult with their oncologist prior to taking vitamin C or other antioxidant supplements, especially in high doses.
3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins)
Vitamin C, in combination with other antioxidants, may attenuate the increase in high-density lipoprotein levels resulting from combination niacin–simvastatin (Zocor®) therapy. It is not known whether this interaction occurs with other lipid-altering regimens. Healthcare providers should monitor lipid levels in individuals taking both statins and antioxidant supplements.
Vitamin C and Healthful Diets
The federal government’s 2015-2020 Dietary Guidelines for Americans notes that “Nutritional needs should be met primarily from foods. … Foods in nutrient-dense forms contain essential vitamins and minerals and also dietary fiber and other naturally occurring substances that may have positive health effects. In some cases, fortified foods and dietary supplements may be useful in providing one or more nutrients that otherwise may be consumed in less-than-recommended amounts.”
The Dietary Guidelines for Americans describes a healthy eating pattern as one that: ● Includes a variety of vegetables, fruits, whole grains, fat-free or low-fat milk and milk products, and oils. Fruits, particularly citrus fruits, fruit juices, and many vegetables are excellent sources of vitamin C. Some ready-to-eat breakfast cereals are fortified with vitamin C.
● Includes a variety of protein foods, including seafood, lean meats and poultry, eggs, legumes (beans and peas), nuts, seeds, and soy products.
● Limits saturated and trans fats, added sugars, and sodium.
● Stays within your daily calorie needs.
Source: https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/#en8