Vitamin C: Benefits, Dosage, Contraindications
Scientific name(s)
Ascorbic Acid
Family or group:
Vitamins
Indications
Rating methodology
EFSA approval.
Vitamin C deficiency ✪✪✪✪✪
Vitamin C deficiency is common in at-risk populations (single men, the elderly, the homeless, psychiatric disorders) and under-evaluated in the general population. Scurvy is a disease that occurs after three months of complete deficiency in ascorbic acid, which comes primarily from fresh fruits and vegetables. Clinical manifestations include asthenia, myalgias, arthralgias, vascular purpura, hemorrhagic syndrome, and later dental manifestations: gingival bleeding and tooth loss. Biological signs are nonspecific: anemia, hypocholesterolemia, hypoalbuminemia. Treatment of scurvy involves administering 1 g of vitamin C divided into multiple daily doses (intestinal absorption and renal excretion are saturable from 100 mg of intake), usually orally for 15 days, parenterally in case of malabsorption. The hemorrhagic syndrome disappears within 48 hours and overall improvement occurs within 15 days.
Posologie
Orally
95 mg
15 - days
Immune boosting ✪✪✪✪✪
Vitamin C seems to increase T lymphocyte activity, phagocyte function, leukocyte mobility, and possibly antibody and interferon production. Additionally, some researchers believe that vitamin C levels in white blood cells decrease at the onset of a cold and that increasing vitamin C intake may be beneficial. EFSA acknowledges that 200 mg of vitamin C in addition to the recommended daily allowance (100 mg per day) contributes to the normal functioning of the immune system during and after intense physical exertion.
Posologie
Orally
200 - 3000 mg
Ascorbic acid and the immune response
Vitamin C and SARS coronavirus
Nutritional aspects of ascorbic acid: uses and abuses
T lymphocyte mobility: defects and effects of ascorbic acid, histamine and complexed IgG
Fatigue ✪✪✪✪✪
EFSA acknowledges the contribution of vitamin C to the reduction of fatigue. Vitamin C plays a crucial role in various physiological functions such as tyrosine metabolism, synthesis of several neurotransmitters (norepinephrine, dopamine), and iron metabolism regulation. Its antioxidant role and importance in maintaining immunity are well established. A deficiency in vitamin C can lead to fatigue, personality changes, and decreased psychomotor performance, often going undiagnosed due to the non-specificity of symptoms. Vitamin C is therefore essential for combating fatigue and supporting many vital body functions.
Posologie
Orally
100 mg
Daily Vitamin C Consumption and Fatigability
Vitamin C status and perception of effort during exercise in obese adults adhering to a calorie-reduced diet
Intravenous Vitamin C Administration Reduces Fatigue in Office Workers: A Double-Blind Randomized Controlled Trial
Skin health ✪✪✪✪✪
Topical application of vitamin C has been shown to increase collagen production and improve collagen organization. European health authorities (EFSA, European Food Safety Authority, and the European Commission) have determined that products containing vitamin C (ascorbic acid) may claim to contribute to normal collagen formation necessary for the normal function of blood vessels, bones, cartilage, skin, teeth, and gums.
Posologie
Orally
95 mg
Use of topical ascorbic acid and its effects on photodamaged skin topography
Clinical, biometric and structural evaluation of the long-term effects of a topical treatment with ascorbic acid and madecassoside in photoaged human skin
Topical activity of ascorbic acid: from in vitro optimization to in vivo efficacy
DIETARY SUPPLEMENT: VITAMIN C
Bone health ✪✪✪✪✪
While the relationship between calcium, vitamin D, and bone mineral density is well known, other nutrients such as vitamin C are also essential for bone development, repair, and maintenance. European health authorities (EFSA, European Food Safety Authority, and the European Commission) have determined that products containing vitamin C (ascorbic acid) may claim to contribute to normal collagen formation necessary for the normal function of blood vessels, bones, cartilage, skin, teeth, and gums.
Posologie
Orally
95 mg
Scientific Opinion on the substantiation of health claims related to vitamin C and protection of DNA, proteins and lipids from oxidative damage (ID 129, 138, 143, 148), antioxidant function of lutein (ID 146), maintenance of vision (ID 141, 142), collagen formation (ID 130, 131, 136, 137, 149), function of the nervous system (ID 133), function of the immune system (ID 134), function of the immune system during and after extreme physical exercise (ID 144), non-haem iron absorption (ID 132, 147), energy-yielding metabolism (ID 135), and relief in case of irritation in the upper respiratory tract (ID 1714, 1715) pursuant to Article 13(1) of Regulation (EC) No 1924/2006
Favorable effect of dietary vitamin C on bone mineral density in postmenopausal women (KNHANES IV, 2009): discrepancies regarding skeletal sites, age, and vitamin D status
The relation of dietary vitamin C intake to bone mineral density: results from the PEPI study
DIETARY SUPPLEMENT: VITAMIN C
Emotional balance ✪✪✪✪✪
Ascorbate is involved in the synthesis of neurotransmitters. It is a necessary cofactor in the biosynthesis of norepinephrine from dopamine and in the hydroxylation of tryptophan to produce serotonin. It also acts as a modulator of glutamatergic, cholinergic, and GABAergic transmission. The European health authorities (EFSA, European Food Safety Authority, and the European Commission) have determined that products containing vitamin C (ascorbic acid) can claim to contribute to the normal functioning of the nervous system and normal mental functions.
Posologie
Orally
95 mg
Impaired adrenal catecholamine system function in mice with deficiency of the ascorbic acid transporter (SVCT2)
Vitamin and mineral requirements in human nutrition
Vitamin C function in the brain: vital role of the ascorbate transporter SVCT2
DIETARY SUPPLEMENT: VITAMIN C
Oxidative Stress ✪✪✪✪✪
At physiological concentrations, vitamin C is the most effective aqueous antioxidant in plasma, interstitial fluids, and soluble phases of cells. Vitamin C acts as a powerful free radical scavenger in plasma, protecting cells against oxidative damage caused by reactive oxygen species (ROS). It scavenges free radicals of oxygen and nitrogen such as superoxide, hydroxyl, peroxyl, and nitroxide radicals, as well as non-radical reactive species such as singlet oxygen, peroxynitrite, and hypochlorite. In addition to its direct antioxidant function, it also indirectly increases free radical scavenging by regenerating vitamin E and maintaining glutathione in a reduced form. Vitamin C decreases oxidants in gastric juice, reduces lipid peroxidation, and reduces oxidative damage to DNA and proteins.
Posologie
Orally
95 mg
Influence of vitamin C on markers of oxidative stress in the earliest period of ischemic stroke
Supplementation of ascorbic acid and alpha-tocopherol is useful to preventing bone loss linked to oxidative stress in elderly
Effect of vitamin C supplementation on postprandial oxidative stress and lipid profile in type 2 diabetic patients
Does vitamin C supplementation influence the levels of circulating oxidized LDL, sICAM-1, sVCAM-1 and vWF-antigen in healthy male smokers?
High dosage of ascorbic acid and alpha-tocopherol is not useful for diminishing oxidative stress and DNA damage in healthy elderly adults
UV Exposure ✪✪✪✪✪
According to a study, the topical application of vitamins E and C, combined with melatonin, showed a modest photoprotective effect when used before UV exposure, but had no effect when used during or after UV exposure.
Posologie
Topically
Modulation of UV-light-induced skin inflammation by D-alpha-tocopherol and L-ascorbic acid: a clinical study using solar simulated radiation.
Effect of topical antioxidants on UV-induced erythema formation when administered after exposure.
Topical melatonin in combination with vitamins E and C protects skin from ultraviolet-induced erythema: a human study in vivo.
Gout ✪✪✪✪✪
Clinical studies report that the daily intake of 500 to 1500 mg of vitamin C is associated with a reduction in the risk of gout by 17% to 34% compared to those consuming less than 250 mg per day. Additionally, people who consume more than 500 mg of vitamin C per day have serum uric acid levels 0.5 to 0.6 mg/dL lower than those consuming less than 90 mg per day. However, taking 500 mg of vitamin C daily for 8 weeks does not lower serum uric acid levels in patients with gout. Thus, vitamin C may help prevent the development of gout, but cannot help treat the disease.
Posologie
Orally
500 - 1500 mg
Cold ✪✪✪✪✪
There is considerable controversy over the effectiveness of vitamin C for treating the common cold. The majority of evidence shows that taking high doses of vitamin C orally can reduce the duration of cold symptoms by 1 to 1.5 days for some patients. Other studies have found no effect with doses up to 3 grams per day. Some clinical data suggest that vitamin C supplementation decreases the incidence of colds among individuals exposed to physical stress but not in the general population. A dose of 1 to 3 grams per day has been used for colds, although doses of 2 g and above appear to be more effective than lower doses.
Posologie
Orally
2 - 3 g
Effect of Vitamin C on Common Cold: Randomized Controlled Trial
Vitamin C supplementation and common cold symptoms: problems with inaccurate reviews.
The effectiveness of vitamin C in preventing and relieving the symptoms of virus-induced respiratory infections.
Vitamin C, the placebo effect, and the common cold: a case study of how preconceptions influence the analysis of results.
Ascorbic acid and the common cold: reviewing the evidence.
Vitamin C for preventing and treating the common cold.
Hypercholesterolemia ✪✪✪✪✪
Analysis of clinical research suggests that taking 500 mg of vitamin C daily for at least 4 weeks reduces low-density lipoprotein (LDL) cholesterol by about 8 mg/dL and reduces triglycerides by about 20 mg/dL in patients with hypercholesterolemia. Furthermore, vitamin C does not appear to reduce cholesterol levels in patients with normal cholesterol levels.
Posologie
Orally
500 mg
4 - weeks
Long-term vitamin C supplementation has no markedly favourable effect on serum lipids in middle-aged Japanese subjects
Vitamin C supplementation lowers serum low-density lipoprotein cholesterol and triglycerides: a meta-analysis of 13 randomized controlled trials.
High Blood Pressure ✪✪✪✪✪
Taking 500 mg of vitamin C orally with standard antihypertensive medications seems to further lower systolic blood pressure, but effects on diastolic pressure are contradictory. However, taking 500 mg of vitamin C daily without antihypertensive treatment does not appear to reduce systolic or diastolic blood pressure.
Posologie
Orally
500 mg
Vitamin C: update on physiology and pharmacology
Role of vitamin C in the function of the vascular endothelium.May JM1, Harrison FE.
Treatment of hypertension with ascorbic acid.
Is vitamin C an effective antihypertensive supplement? A review and analysis of the literature.
Ascorbic acid reduces blood pressure and arterial stiffness in type 2 diabetes.
The combination of vitamin C and grape-seed polyphenols increases blood pressure: a randomized, double-blind, placebo-controlled trial.
AMD ✪✪✪✪✪
Supplementation with antioxidants including vitamins A (15 mg), C (500 mg), E (400 IU) combined with Zinc (80 mg) reduces the risk of vision loss by 27% and slows the progression of the disease in patients at high risk of developing age-related macular degeneration (AMD).
Posologie
Orally
500 mg
10 - years
Synergies
Long-term effects of vitamins C and E, β-carotene, and zinc on age-related macular degeneration: AREDS report no. 35.
Age-Related Eye Disease Study Research Group. Potential public health impact of age-related eye disease study results: AREDS report no. 11.
Wrinkles ✪✪✪✪✪
Topical preparations containing 5% to 10% vitamin C seem to improve wrinkles. In a clinical trial, a topical preparation containing 10% vitamin C as L-ascorbic acid with acetyl tyrosine, zinc sulfate, sodium hyaluronate, and bioflavonoids applied to the face for 3 months improved wrinkles and skin tone compared to placebo. There is also evidence that 3% vitamin C applied for 12 weeks can reduce facial wrinkles. In another clinical study, vitamin C was also applied topically as a patch containing 5.6% vitamin C every 4 days for 12 weeks on the crow's feet area of one side of the face. The patch reduced wrinkles compared to the control treatment applied to the contralateral side of the face. Finally, a recent study conducted on women aged 30 to 65 years showed that applying a liquid containing 20% vitamin C, 0.0005% raspberry leaf cell culture extract, and 1% vitamin E to the face every evening for 8 weeks improves skin color, elasticity, radiance, smoothness, and the appearance of wrinkles compared to usual face products used alone.
Posologie
Topically, orally
3 - months
vitamin C-based cream
Evaluation of the anti-wrinkle effect of a lipophilic pro-vitamin C derivative, tetra-isopalmitoyl ascorbic acid
Anti-aging and brightening effects of a topical treatment containing vitamin C, vitamin E, and raspberry leaf cell culture extract: A split-face, randomized controlled trial
Use of topical ascorbic acid and its effects on photodamaged skin topography.
Topical activity of ascorbic acid: from in vitro optimization to in vivo efficacy.
Skin Healing ✪✪✪✪✪
Vitamin C is important for effective wound healing, as a deficiency contributes to fragility of granulation tissue and thus impedes the healing process. In vitro studies with skin graft samples have shown that vitamin C prolongs cell viability, promotes the formation of an epidermal barrier, and aids grafting. Thus, vitamin C is used to improve wound healing before surgical intervention.
Posologie
Orally
Vitamin C and human wound healing
Vitamin C Regulates Keratinocyte Viability, Epidermal Barrier, and Basement Membrane In Vitro, and Reduces Wound Contraction After Grafting of Cultured Skin Substitutes
The importance of patients' nutritional status in wound healing
Osteoarthritis ✪✪✪✪✪
Dietary vitamin C consumption by subjects suffering from osteoarthritis seems to slow disease progression. Clinical research also suggests that taking vitamin C as calcium ascorbate at a dose of 1 gram per day for 2 weeks significantly reduces the pain and severity of osteoarthritis compared to placebo, although the effects are less than those observed with nonsteroidal anti-inflammatory drugs (NSAIDs).
Posologie
Orally
1 g
2 - weeks
calcium ascorbate
Synergies
Allergic Rhinitis ✪✪✪✪✪
The epidemiological study shows that high plasma vitamin C levels are not associated with a reduced risk of allergic rhinitis. However, clinical research shows that the nasal form of vitamin C administered three times a day for 2 weeks reduces nasal discharge, obstruction, and nasal edema in 74% of patients with allergic rhinitis.
Posologie
Nasally
2 - weeks
Spray
Osteoporosis ✪✪✪✪✪
Some data suggest that vitamin C intake could be related to bone mineral density in premenopausal women. In postmenopausal women who consume estrogens and smoke, higher vitamin C levels could reduce fracture risk. However, in menopausal women without a history of smoking or estrogen use, higher serum vitamin C levels have been associated with lower bone mineral density. Currently, there is not enough information to make recommendations about vitamin C and bone density.
Posologie
Orally
1 g
Cancer ✪✪✪✪✪
The relationship between vitamin C and cancer prevention as well as the treatment has been the subject of extensive research, highlighting its potential interest. Historically, increased consumption of fruits and vegetables, rich in vitamin C, has been linked to a decreased risk of several non-hormone-dependent cancers. Epidemiological studies have shown a significant protective effect of vitamin C, notably against cancers of the digestive system, lung, and in some cases, breast and prostate. Research by Linus Pauling on the administration of high doses of vitamin C intravenously marked a turning point, suggesting potential therapeutic use in the treatment of advanced cancers. These works suggest that vitamin C could improve quality of life and some objective markers in patients with terminal cancer. However, the absence of randomized trial principles in their studies has raised questions about the validity of their results. More recent studies have provided contradictory evidence on the effectiveness of vitamin C in reducing cancer exacerbations, with results varying depending on the route of administration (oral vs. intravenous) and the initial vitamin C levels of patients. Although high concentrations of vitamin C obtained by intravenous administration have shown selective cytotoxicity towards cancer cell lines, clinical trials have not always confirmed these observations. The debate on the use of vitamin C in oncology continues, with some studies suggesting benefits as a complement to standard treatments, while others caution against the potential interactions with certain treatments, such as the inactivation of bortezomib (a drug used in the treatment of certain types of cancer). Consumption of at least 80-110 mg of vitamin C daily over the long term is associated with significant reductions in the risk of certain cancers. Supplementation for patients with terminal cancer: 10 g/day of oral vitamin C (even if it has not shown a significant difference in patients' outcome). Intravenous administration of 150-710 mg/kg/day of vitamin C for up to 8 weeks.
Posologie
Orally, intravenously
80 - 110 mg
Properties
Antioxidant
Vitamin C is one of the four dietary antioxidants, along with vitamin E, beta-carotene, and selenium, and it participates in the breakdown of oxygen free radicals, providing protection against toxic agents for the cell. Thus, it reduces oxidants in gastric juice, lipid peroxidation, and oxidative damage to DNA and proteins.
Usages associés
Essential
Vitamin C is involved in many biochemical reactions through a hydroxylation mechanism. Its role in collagen synthesis is important. Vitamin C deficiency induces degradation of collagen structure, explaining the clinical manifestations of scurvy (a disease due to vitamin C deficiency): alteration of dentin formation and tooth loss, vascular wall damage and purpura with hemorrhagic syndrome, edema, and skin alterations due to keratin involvement and bone remodeling. Furthermore, vitamin C acts as a cofactor in the synthesis of catecholamines, notably in the conversion of dopamine to norepinephrine, which may explain the behavior and mood disturbances observed during scurvy. Other actions include involvement in the catabolism of phenylalanine and tyrosine (amino acids), in the transformation of cholesterol to bile acids, increase in metabolism of toxins and carcinogens by hepatic cytochrome P450, and absorption of non-heme iron (not incorporated in the heme structure).
Usages associés
Neurological
Ascorbate is involved in neurotransmitter synthesis. It is a cofactor required for the biosynthesis of norepinephrine from dopamine and the hydroxylation of tryptophan to produce serotonin. It also acts as a modulator of glutaminergic, cholinergic, and GABAergic transmission. Moreover, it is involved in neural maturation and acts as a neuroprotective agent.
Bioenergetic
Vitamin C seems to be easily absorbed and stored in skeletal muscle tissues, where it can provide antioxidant protection and may support the biosynthesis of carnitine and collagen. Additionally, a reduction in muscle damage biomarkers has been observed.
Usages associés
Dermatological Effect
Vitamin C is thought to play a role in neutralizing free radicals generated in the skin due to exposure to ultraviolet rays, which causes photoaging of the skin. Topical application of vitamin C before UV exposure is believed to prevent skin damage due to the antioxidant effects of vitamin C. Furthermore, topical application of vitamin C has been shown to increase collagen production and improve its organization.
Usages associés
Cardiovascular
In people with chronic heart failure, vitamin C appears to improve endothelial dysfunction and arterial dilation by preventing inactivation of nitric oxide (NO)-mediated vasodilation. Vitamin C also seems to improve endothelial function and vascular resistance in patients with chronic renal failure. Some researchers believe vitamin C may prevent or slow atherosclerosis by inhibiting low-density lipoprotein (LDL) cholesterol, degrading oxygen-free radicals in vascular cells, and limiting cellular responses to oxidized LDL, such as NO production derived from the endothelium. There is evidence that vitamin C reduces LDL cholesterol levels in patients with hypercholesterolemia; however, taking vitamin C does not seem to decrease serum lipid levels in people with normal lipid levels.
Usages associés
Anti-inflammatory
Some research suggests that vitamin C may reduce the level of C-reactive protein (CRP) (a protein produced in response to inflammation) in subjects actively or passively exposed to tobacco smoke.
Usages associés
Immunostimulant
Vitamin C appears to increase T lymphocyte activity, phagocyte function, leucocyte mobility, and possibly the production of antibodies and interferon. Additionally, some researchers believe that vitamin C levels in white blood cells decrease at the onset of a cold and that increasing vitamin C intake may be beneficial.
Usages associés
Uricosuric Effect
Vitamin C is thought to decrease serum uric acid levels by a uricosuric effect. Some research shows that uric acid clearance in healthy subjects who consume vitamin C increases by more than 200% within 2 to 6 hours. Indeed, vitamin C may compete with uric acid during renal reabsorption.
Usages associés
Remineralizing
While the relationship between calcium, vitamin D, and bone mineral density is well-known, other nutrients such as vitamin C are also essential for bone development, repair, and maintenance. Epidemiological studies have demonstrated a positive association between bone density and vitamin C intake. Low vitamin C intake has been associated with decreased mineral density, particularly in the femoral neck and hip. Data from 13,080 adults from 1988 to 1994 identified a relationship between dietary and serum ascorbic acid, bone mineral density, and bone fracture.
Usages associés
Anticancer
Vitamin C is selectively cytotoxic to many cancer cell lines and has anticancer activity in vivo. Importantly, pharmacological concentrations of ascorbic acid (0.3-20 mmol/L) are required to find evidence of cytotoxicity in vitro and in vivo, whereas physiological concentrations of ascorbic acid (0.1 mmol/L) have no effect on tumor or normal cells. The most reliable method to achieve these high doses is intravenous (IV) administration of vitamin C rather than oral, which has limited absorption. The effect appears dose-proportional and is exerted through multiple mechanisms, such as immunomodulation, inhibition of cell division and growth, gene regulation, and apoptosis induction. The mechanism of cytotoxicity demonstrated in several models is the ability of ascorbate at pharmacological concentrations to exert a pro-oxidant activity, generating hydrogen peroxide-dependent cytotoxicity towards a variety of cancer cells in vitro and in vivo without negatively affecting normal cells. One possible explanation for the preferential targeting of tumor cells concerns their overexpression of GLUTs that transport dehydroascorbic acid. The latter accumulates in tumor cells and leads to an increase in intracellular hydrogen peroxide levels.
Usages associés
Anti-aging
Free radicals are generated in the skin after exposure to ultraviolet (UV) rays and cause photo-induced aging. Vitamin C in the skin is thought to play a key role in neutralizing free radicals and reducing UV-induced skin damage, and thus topical application of vitamin C prevents skin damage when applied before UV exposure. Topical preparations are also believed to help treat photoaged and wrinkled skin by increasing collagen production and improving collagen organization. Topical formulations containing 10% vitamin C may be most effective at increasing vitamin C concentrations in the skin.
Usages associés
Antiallergenic
Some evidence suggests that low levels of vitamin C are associated with higher plasma levels of histamine. Theoretically, people with low levels of vitamin C may have more severe symptoms of allergic rhinitis. There is also evidence that vitamin C may have mild antihistamine properties. Indeed, ascorbic acid is involved in histamine metabolism, acting with Cu2+ to inhibit its release and enhance its breakdown.
Usages associés
Safety dosage
Adult from 18 years: 95 mg
The EFSA has defined a level of intake likely to meet the daily needs of almost all individuals in an apparently healthy population. The maximum chronic daily intake is not set by the EFSA.
Child from 1 to 3 years: 20 mg
Pregnant woman from 18 years: 105 mg
Infant from 7 to 11 months: 20 mg
Child from 11 to 14 years: 70 mg
Child from 15 to 17 years: 90 mg
Lactating woman from 18 years: 155 mg
Child from 4 to 6 years: 30 mg
Child from 7 to 10 years: 45 mg
Interactions
Médicaments
Anticancer drugs: moderate interaction
The use of antioxidants like vitamin C is controversial. Indeed, they can reduce the effect of chemotherapy by producing free radicals. However, some researchers believe that antioxidants can make chemotherapy more effective by inducing oxidative stress, which can interfere with dead or cancerous cells.
Estrogens: moderate interaction
The combination of vitamin C with oral contraceptives can increase estrogen levels by 55%. It is believed that vitamin C prevents the oxidation of estrogens in tissues, regenerates oxidized estrogens, and reduces the conjugation of estrogens to sulfate in the intestinal wall. These effects are not observed when tissue levels of vitamin C are high.
Fluphenazine: moderate interaction
A case of significant reduction in fluphenazine levels was noted during supplementation with vitamin C (500 mg twice a day). The mechanism is not yet known.
Indinavir: moderate interaction
The intake of oral doses of vitamin C in association with indinavir can probably reduce the levels of the latter. The mechanism of this interaction is not yet known.
Warfarin: moderate interaction
High doses of vitamin C can reduce the response to warfarin by causing diarrhea or decreasing the absorption of warfarin.
Plantes ou autres actifs
Vitamin C: moderate interaction
Vitamin C can increase the absorbed portion of aluminum. It is believed that vitamin C is an aluminum chelate that keeps it in solution, which promotes its absorption. In people with normal kidney function, urinary excretion of aluminum is probably increased, which reduces aluminum toxicity. However, people with kidney failure who take aluminum-containing compounds chronically should avoid high vitamin C supplementation.
Vitamin C: moderate interaction
Vitamin C increases the absorption of iron.
Precautions
Alzheimer's Disease: use with caution
Clinical research has shown that combining 500 mg of vitamin C, 800 mg of vitamin E, 900 mg of lipoic acid may worsen cognitive decline compared to placebo in patients with mild to moderate Alzheimer's disease.
Cancer: use with caution
Cancer cells accumulate high concentrations of vitamin C. They absorb the oxidized form (dehydroascorbic acid) and then reconvert it to vitamin C. The effect of this phenomenon on cancer cells is still unknown. However, cancer patients should not use high doses of vitamin C before consulting their oncologists.
Type 2 Diabetes: use with caution
Vitamin C may affect glycogenolysis by increasing blood glucose. However, this effect is controversial. In postmenopausal women with diabetes, supplementation of 300 mg/day of vitamin C is associated with an increased risk of cardiovascular mortality.
Transplant: use with caution
Chronic high-dose vitamin C use before renal transplantation may increase the risk of graft loss or delay its function.
Kidney Stones: avoid
Vitamin C can cause precipitation of urate, cystine, or oxalate stones.
Hemochromatosis: avoid
Vitamin C supplementation may exacerbate iron toxicity by mobilizing iron stores.
G6PD Deficiency: avoid
Large amounts of vitamin C can cause hemolysis in people with a glucose-6-phosphate dehydrogenase (G6PD) deficiency.
