Vitamin C, widely recognized as L-ascorbic acid, is a vital water-soluble vitamin that our bodies require but cannot produce on their own. Naturally abundant in various foods, and also available in supplements and fortified products, understanding “What Is Vitamin C” and its role in maintaining health is crucial. Unlike most animals, humans must obtain vitamin C through their diet, making it an essential nutrient for overall well-being [[1](#en1)].
Vitamin C is fundamental for numerous bodily functions. It plays a key role in the biosynthesis of collagen, a protein vital for wound healing and connective tissue strength. It’s also necessary for the production of L-carnitine and certain neurotransmitters, and participates in protein metabolism [[1](#en1), 2]. Beyond these roles, vitamin C stands out as a potent physiological antioxidant [[3](#en3)]. This antioxidant property helps regenerate other antioxidants in the body, including vitamin E (alpha-tocopherol) [[4](#en4)].
Current research is deeply invested in exploring the potential of vitamin C to prevent or delay chronic diseases. By combating free radicals through its antioxidant actions, vitamin C may offer protection against conditions like cancer, cardiovascular disease, and other ailments where oxidative stress is a significant factor. Furthermore, vitamin C is integral to a healthy immune system [[4](#en4)] and enhances the body’s ability to absorb nonheme iron [[5](#en5)], the type of iron found in plant-based foods. A deficiency in vitamin C leads to scurvy, a condition marked by fatigue, widespread weakness in connective tissues, and fragile capillaries [[1](#en1), 2, 4, 6-9].
The body’s absorption of vitamin C in the intestines is carefully controlled by a dose-dependent active transporter [[4](#en4)]. Cells further accumulate vitamin C using another specific transport protein. Intriguingly, in vitro studies show that dehydroascorbic acid, the oxidized form of vitamin C, can enter cells via certain glucose transporters and is then converted back to ascorbic acid. However, the exact physiological importance of dehydroascorbic acid uptake and its contribution to the body’s overall vitamin C management remains an area of ongoing research.
When vitamin C is taken orally, the body tightly regulates its concentration in tissues and plasma. Absorption rates vary with intake levels: at moderate doses (30–180 mg/day), the body absorbs a high percentage (70%–90%). However, as doses increase beyond 1 gram per day, absorption efficiency drops to less than 50%, with excess, unmetabolized ascorbic acid being excreted in urine [[4](#en4)]. Pharmacokinetic studies indicate that a daily oral dose of 1.25 grams of ascorbic acid can lead to peak plasma vitamin C concentrations of about 135 micromol/L. This is approximately double the concentration achieved by consuming 200–300 mg/day from vitamin C-rich foods [[10](#en10)]. Predictive pharmacokinetic models suggest that even very high doses, such as 3 grams of ascorbic acid taken every four hours, would only elevate peak plasma concentrations to around 220 micromol/L [[10](#en10)].
The total amount of vitamin C in the human body can vary significantly, from about 300 mg in near-scurvy conditions to approximately 2 grams in a healthy individual [[4](#en4)]. High concentrations (in the millimolar range) are maintained within cells and tissues, particularly in leukocytes (white blood cells), the eyes, adrenal glands, pituitary gland, and brain. In contrast, extracellular fluids like plasma, red blood cells, and saliva contain much lower levels of vitamin C (in the micromolar range) [[4](#en4)].
Recommended Daily Intake of Vitamin C
Dietary Reference Intakes (DRIs) for vitamin C and other nutrients are established by the Food and Nutrition Board (FNB) at the Institute of Medicine (IOM) of the National Academies [[8](#en8)]. DRIs are a set of reference values used for planning and assessing nutrient intakes for healthy populations, varying by age and gender [[8](#en8)]. These include:
- Recommended Dietary Allowance (RDA): The average daily intake level sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals. RDAs are often used to plan nutritionally adequate diets for individuals.
- Adequate Intake (AI): Established when evidence is insufficient to develop an RDA, AI is an intake level assumed to ensure nutritional adequacy.
- Estimated Average Requirement (EAR): The average daily intake level estimated to meet the needs of 50% of healthy individuals. EARs are typically used to assess nutrient intakes of groups and plan adequate diets for them, and can also be used to assess individual intakes.
- Tolerable Upper Intake Level (UL): The maximum daily intake unlikely to cause adverse health effects.
Table 1 outlines the current RDAs for vitamin C [[8](#en8)]. These recommendations are based on vitamin C’s roles in white blood cell function and antioxidant activity and are considerably higher than the amount needed to prevent deficiency [[4](#en4), 8, 11]. For infants up to 12 months, the FNB has set an AI for vitamin C based on the average intake in healthy, breastfed infants.
Table 1: Recommended Dietary Allowances (RDAs) for Vitamin C [[8](#en8)]
| Age | Male | Female | Pregnancy | Lactation |
|---|---|---|---|---|
| 0–6 months | 40 mg* | 40 mg* | ||
| 7–12 months | 50 mg* | 50 mg* | ||
| 1–3 years | 15 mg | 15 mg | ||
| 4–8 years | 25 mg | 25 mg | ||
| 9–13 years | 45 mg | 45 mg | ||
| 14–18 years | 75 mg | 65 mg | 80 mg | 115 mg |
| 19+ years | 90 mg | 75 mg | 85 mg | 120 mg |
| Smokers | Individuals who smoke require 35 mg/day more vitamin C than nonsmokers. |
* Adequate Intake (AI)
Natural Sources of Vitamin C
Food Sources
Fruits and vegetables are the richest sources of vitamin C (see Table 2) [[12](#en12)]. In many diets, citrus fruits, tomatoes, tomato juice, and potatoes are significant contributors of vitamin C [[8](#en8)]. Other excellent food sources include bell peppers (red and green), kiwifruit, broccoli, strawberries, Brussels sprouts, and cantaloupe (see Table 2) [[8](#en8), 12]. Although vitamin C is not naturally present in grains, it is added to some fortified breakfast cereals.
Red bell pepper slices, showcasing a rich food source of Vitamin C.
The vitamin C content in food can diminish with prolonged storage and cooking because ascorbic acid is water-soluble and heat-sensitive [[6](#en6), 8]. Steaming or microwaving can help reduce cooking losses. Fortunately, many top vitamin C sources, like fruits and vegetables, are often consumed raw. Eating five varied servings of fruits and vegetables daily can easily provide over 200 mg of vitamin C.
Table 2: Vitamin C Content of Selected Foods [[12](#en12)]
| Food | Milligrams (mg) per serving | Percent (%) DV* |
|---|---|---|
| Red pepper, sweet, raw, ½ cup | 95 | 106 |
| Orange juice, ¾ cup | 93 | 103 |
| Orange, 1 medium | 70 | 78 |
| Grapefruit juice, ¾ cup | 70 | 78 |
| Kiwifruit, 1 medium | 64 | 71 |
| Green pepper, sweet, raw, ½ cup | 60 | 67 |
| Broccoli, cooked, ½ cup | 51 | 57 |
| Strawberries, fresh, sliced, ½ cup | 49 | 54 |
| Brussels sprouts, cooked, ½ cup | 48 | 53 |
| Grapefruit, ½ medium | 39 | 43 |
| Broccoli, raw, ½ cup | 39 | 43 |
| Tomato juice, ¾ cup | 33 | 37 |
| Cantaloupe, ½ cup | 29 | 32 |
| Cabbage, cooked, ½ cup | 28 | 31 |
| Cauliflower, raw, ½ cup | 26 | 29 |
| Potato, baked, 1 medium | 17 | 19 |
| Tomato, raw, 1 medium | 17 | 19 |
| Spinach, cooked, ½ cup | 9 | 10 |
| Green peas, frozen, cooked, ½ cup | 8 | 9 |
*DV = Daily Value. The U.S. Food and Drug Administration (FDA) has set a DV for vitamin C at 90 mg for adults and children aged 4 years and older [[13](#en13)]. Food labels are not required to list vitamin C content unless it has been added to the food. Foods providing 20% or more of the DV are considered high sources of vitamin C, but even foods with lower percentages contribute to a healthy diet.
For more detailed information on the nutrient content of various foods, including vitamin C, refer to the U.S. Department of Agriculture’s (USDA’s) FoodData Central, which offers comprehensive lists of foods containing vitamin C, searchable by nutrient content and by food name.
Vitamin C Supplements
Dietary supplements typically provide vitamin C in the form of ascorbic acid, which is just as bioavailable as naturally occurring ascorbic acid in foods like orange juice and broccoli [[14-16](#en14)]. Other supplement forms include sodium ascorbate, calcium ascorbate, mineral ascorbates, ascorbic acid with bioflavonoids, and combination products like Ester-C, which contains calcium ascorbate, dehydroascorbate, calcium threonate, xylonate, and lyxonate [[17](#en17)].
Research comparing the bioavailability of different vitamin C supplement forms has yielded mixed results. One study indicated that Ester-C and ascorbic acid resulted in similar plasma vitamin C concentrations, but Ester-C led to significantly higher vitamin C levels in leukocytes 24 hours post-ingestion [[18](#en18)]. However, another study found no significant differences in plasma vitamin C levels or urinary excretion among ascorbic acid, Ester-C, and ascorbic acid with bioflavonoids [[17](#en17)]. Considering these findings and the lower cost of ascorbic acid, many experts suggest that simple ascorbic acid is the most practical choice for vitamin C supplementation [[17](#en17)].
Vitamin C Intake and Status in Populations
According to the 2001–2002 National Health and Nutrition Examination Survey (NHANES), the average daily vitamin C intake is 105.2 mg for adult males and 83.6 mg for adult females in the U.S. [[19](#en19)]. These intakes meet the current RDA for most non-smoking adults. For children and adolescents aged 1–18 years, mean intakes range from 75.6 mg/day to 100 mg/day, also satisfying the RDA for these age groups [[19](#en19)]. While the 2001–2002 NHANES data did not include breastfed infants and toddlers, breast milk is generally considered an adequate source of vitamin C [[8](#en8), 14]. Vitamin C supplement use is also common, contributing to total vitamin C intake from both foods and supplements. NHANES data from 1999–2000 showed that about 35% of adults take multivitamin supplements (which often contain vitamin C), and 12% take a separate vitamin C supplement [[20](#en20)]. Data from 1999–2002 NHANES indicated that approximately 29% of children consume some form of dietary supplement containing vitamin C [[21](#en21)].
Vitamin C status is typically evaluated by measuring plasma vitamin C levels [[4](#en4), 14]. While other measures, such as leukocyte vitamin C concentration, might offer a more accurate reflection of tissue vitamin C levels, they are more complex to assess and may not always provide consistent results [[4](#en4), 9, 14].
Vitamin C Deficiency: Scurvy
Severe vitamin C deficiency leads to scurvy [[7](#en7), 8, 11]. The onset of scurvy varies depending on an individual’s vitamin C stores, but symptoms can emerge within a month of minimal or no vitamin C intake (below 10 mg/day) [[6](#en6), 7, 22, 23]. Early signs may include fatigue (possibly due to impaired carnitine production), general discomfort, and gum inflammation [[4](#en4), 11]. As deficiency progresses, collagen synthesis is compromised, weakening connective tissues and leading to symptoms like petechiae, ecchymoses, purpura, joint pain, poor wound healing, hyperkeratosis, and corkscrew hairs [[1](#en1), 2, 4, 6-8]. Other signs of scurvy include depression, swollen and bleeding gums, and tooth loosening or loss due to tissue and capillary fragility [[6](#en6), 8, 9]. Iron deficiency anemia can also occur as a result of increased bleeding and reduced nonheme iron absorption due to low vitamin C levels [[6](#en6), 11]. In children, bone disease can be present [[6](#en6)]. If left untreated, scurvy is fatal [[6](#en6), 9].
Historically, scurvy was rampant among sailors on long voyages who lacked access to fresh fruits and vegetables, a source of vitamin C. In the mid-1700s, Sir James Lind, a British Navy surgeon, demonstrated through experiments that citrus fruits could cure scurvy, although the identification of ascorbic acid as the active component did not occur until 1932 [[24-26](#en24)].
Today, vitamin C deficiency and scurvy are rare in developed nations [[8](#en8)]. Overt deficiency symptoms only appear when vitamin C intake remains below approximately 10 mg/day for several weeks [[5-8](#en5), 22, 23]. While uncommon in developed countries, vitamin C deficiency can still occur in populations with limited dietary diversity.
Groups at Increased Risk of Vitamin C Inadequacy
Vitamin C inadequacy, where intakes are below the RDA but above the level causing overt deficiency (around 10 mg/day), can still pose health risks. Certain groups are more susceptible to insufficient vitamin C levels.
Smokers and Secondhand Smoke Exposure
Consistent research shows that smokers have lower plasma and leukocyte vitamin C levels compared to nonsmokers, partly due to increased oxidative stress from smoking [[8](#en8)]. Consequently, the IOM recommends that smokers need an additional 35 mg of vitamin C per day compared to nonsmokers [[8](#en8)]. Exposure to secondhand smoke also reduces vitamin C levels. While the IOM has not set a specific vitamin C requirement for those regularly exposed to secondhand smoke, ensuring they meet the standard RDA is advised [[4](#en4), 8].
Infants Fed Evaporated or Boiled Milk
In developed countries, most infants are fed breast milk or infant formula, both of which provide adequate vitamin C [[8](#en8), 14]. However, feeding infants evaporated or boiled cow’s milk is discouraged for various reasons, including the risk of vitamin C deficiency. Cow’s milk is naturally low in vitamin C, and heat treatments like boiling can further destroy this nutrient [[6](#en6), 12].
Individuals with Limited Dietary Variety
While fruits and vegetables are the best sources of vitamin C, many other foods contain small amounts. A varied diet usually enables most individuals to meet the vitamin C RDA or at least avoid scurvy. However, those with limited food choices—including some elderly individuals, those with very low incomes who prepare their own food, people with alcohol or drug dependencies, restrictive dieters, individuals with mental illness, and sometimes children—may not get enough vitamin C [[4](#en4), 6-9, 11].
People with Malabsorption and Certain Chronic Diseases
Certain medical conditions can impair vitamin C absorption or increase the body’s need for it. Individuals with severe intestinal malabsorption, cachexia, and some cancer patients may be at higher risk of vitamin C inadequacy [[27](#en27)]. Low vitamin C levels can also be observed in patients with end-stage renal disease undergoing chronic hemodialysis [[28](#en28)].
Vitamin C and Its Impact on Health
Due to its antioxidant properties and role in immune function, vitamin C has been studied for its potential to prevent and treat various health conditions. This section explores the evidence for vitamin C’s role in four key areas: cancer (prevention and treatment), cardiovascular disease, age-related macular degeneration (AMD) and cataracts, and the common cold.
Vitamin C and Cancer Prevention
Epidemiological studies suggest that higher consumption of fruits and vegetables, rich in vitamin C and other nutrients, is linked to a lower risk of many types of cancer [[1](#en1), 2]. Vitamin C’s potential cancer-protective mechanisms include limiting the formation of carcinogens like nitrosamines [[2](#en2), 29], modulating immune responses [[2](#en2), 4], and reducing oxidative damage through its antioxidant action [[1](#en1)].
Case-control studies have often found an inverse relationship between dietary vitamin C intake and cancers of the lung, breast, colon, rectum, stomach, oral cavity, larynx, pharynx, and esophagus [[2](#en2), 4]. Plasma vitamin C concentrations are also typically lower in individuals with cancer compared to healthy controls [[2](#en2)].
However, prospective cohort studies have presented inconsistent findings, possibly due to variations in vitamin C intake across different populations studied. For instance, in the Nurses’ Health Study involving over 82,000 women, higher vitamin C intake from food (average 205 mg/day) was associated with a 63% lower risk of breast cancer among premenopausal women with a family history of the disease, compared to lower intake (average 70 mg/day) [[30](#en30)]. Conversely, another study found no significant reduction in breast cancer risk for postmenopausal women with high vitamin C intake [[31](#en31)]. A review by Carr and Frei noted that many prospective cohort studies not showing a significant cancer risk reduction involved participants with already relatively high vitamin C intakes, often exceeding 86 mg/day even in the lowest intake groups [[2](#en2)]. Studies that did report a lower cancer risk typically observed these associations in individuals with vitamin C intakes of at least 80–110 mg/day, levels approaching vitamin C tissue saturation [[2](#en2), 22, 32].
Most randomized clinical trials suggest that vitamin C supplementation, often combined with other micronutrients, does not significantly affect cancer risk. The SU.VI.MAX study, a large randomized trial, found that antioxidant supplementation including 120 mg ascorbic acid reduced total cancer incidence in men but not in women over a 7.5-year period [[33](#en33)]. However, other trials, such as the Physicians’ Health Study II and the Women’s Antioxidant Cardiovascular Study, found no reduction in cancer risk with vitamin C supplementation [[35](#en35), 36]. Similarly, a large intervention trial in Linxian, China, showed no significant impact on esophageal or gastric cancer risk with vitamin C and molybdenum supplements [[37](#en37)]. Reviews of vitamin C and antioxidant supplements for gastrointestinal cancer prevention have also found no compelling evidence of benefit [[39](#en39), 40].
Currently, the evidence regarding dietary vitamin C intake and cancer risk remains inconclusive. Most clinical trials indicate that modest vitamin C supplementation, alone or with other nutrients, does not offer a clear benefit in cancer prevention. A major limitation in interpreting many of these studies is the lack of baseline and post-supplementation vitamin C level measurements. Given the body’s tight control over vitamin C concentrations, if participants already had near-saturated levels, supplementation might have little to no effect on outcomes [[2](#en2), 10, 22, 31, 37, 41, 42].
Vitamin C in Cancer Treatment
In the 1970s, initial research by Cameron, Campbell, and Pauling suggested that high-dose vitamin C could improve quality of life and extend survival in terminal cancer patients [[43](#en43), 44]. However, subsequent studies, including a randomized, placebo-controlled trial by Moertel et al. at the Mayo Clinic, did not support these findings [[45](#en45)]. The Moertel study, using 10 g/day oral vitamin C in advanced colorectal cancer patients, found no benefit over placebo. A 2003 review concluded that vitamin C showed no significant mortality benefit for advanced cancer patients [[40](#en40)].
Recent research suggests that the method of vitamin C administration (intravenous [IV] vs. oral) may explain these conflicting results [[1](#en1), 46, 47]. Most negative trials, including Moertel’s, used oral administration, while Cameron’s positive studies used a combination of oral and IV routes. Oral vitamin C, even at high doses, achieves a maximum plasma concentration of only about 220 micromol/L, whereas IV administration can reach concentrations as high as 26,000 micromol/L [[47](#en47), 48]. These high concentrations have been shown to be selectively toxic to tumor cells in vitro [[1](#en1), 67]. Animal studies indicate that pharmacologic doses of IV vitamin C may be promising for treating tumors that are otherwise difficult to treat [[49](#en49)]. High concentrations of vitamin C may act as a pro-oxidant, generating hydrogen peroxide that selectively targets cancer cells [[49-51](#en49)]. Based on these findings and case reports of extended survival in advanced cancer patients treated with high-dose IV vitamin C, some researchers advocate for re-evaluating high-dose IV vitamin C as a cancer treatment [[3](#en3), 47, 49, 52].
It is important to note the uncertainty regarding interactions between supplemental vitamin C and other antioxidants with chemotherapy and radiation [[53](#en53)]. Therefore, individuals undergoing cancer treatment should consult their oncologist before taking vitamin C or other antioxidant supplements, particularly at high doses [[54](#en54)].
Vitamin C and Cardiovascular Disease
Extensive epidemiological evidence suggests that diets rich in fruits and vegetables are associated with a lower risk of cardiovascular disease [[1](#en1), 55, 56]. This association may be partly due to the antioxidant content of these foods, as oxidative damage is a major factor in cardiovascular disease [[1](#en1), 4, 56]. Beyond its antioxidant properties, vitamin C has demonstrated benefits in reducing monocyte adhesion to the endothelium, improving endothelium-dependent nitric oxide production and vasodilation, and reducing vascular smooth-muscle-cell apoptosis, all of which are relevant to preventing plaque instability in atherosclerosis [[2](#en2), 57].
Prospective studies examining the link between vitamin C intake and cardiovascular disease risk have yielded mixed results [[56](#en56)]. The Nurses’ Health Study, a 16-year study of over 85,000 women, found an inverse association between total vitamin C intake (dietary and supplemental) and coronary heart disease risk [[58](#en58)]. However, dietary vitamin C intake alone showed no significant association, suggesting that supplement users may be at lower risk. Conversely, a smaller study indicated that postmenopausal women with diabetes taking high-dose vitamin C supplements (≥300 mg/day) had increased cardiovascular disease mortality [[59](#en59)].
A prospective study in over 20,000 British adults found a 42% lower stroke risk in those with the highest baseline plasma vitamin C concentrations compared to the lowest [[60](#en60)]. However, in the Physicians’ Health Study, vitamin C supplementation in male physicians showed no significant decrease in cardiovascular disease or coronary heart disease mortality over a 5.5-year period [[61](#en61)]. A pooled analysis of nine prospective studies involving over 290,000 individuals showed a 25% lower risk of coronary heart disease incidence in those taking ≥700 mg/day of supplemental vitamin C compared to non-users [[62](#en62)]. A 2008 meta-analysis of prospective cohort studies concluded that dietary, but not supplemental, vitamin C intake is inversely associated with coronary heart disease risk [[55](#en55)].
Most clinical intervention trials have not demonstrated a beneficial effect of vitamin C supplementation on the primary or secondary prevention of cardiovascular disease. The Women’s Antioxidant Cardiovascular Study, a secondary prevention trial, found no overall effect of 500 mg/day vitamin C supplementation on cardiovascular events over a 9.4-year period [[63](#en63)]. Similarly, the Physicians’ Health Study II showed no effect of 500 mg/day vitamin C on major cardiovascular events over an 8-year follow-up [[64](#en64)].
Other trials have often examined combinations of vitamin C with other antioxidants, making it difficult to isolate vitamin C’s specific effects. The SU.VI.MAX study, using a combination of vitamin C, vitamin E, beta-carotene, selenium, and zinc, found no effect on ischemic cardiovascular disease in either men or women after 7.5 years [[33](#en33)]. The Women’s Angiographic Vitamin and Estrogen study even found that vitamin C and E supplementation increased all-cause mortality in postmenopausal women with coronary stenosis compared to placebo [[65](#en65)].
A 2006 meta-analysis concluded that antioxidant supplements, including vitamins C and E, do not affect atherosclerosis progression [[66](#en66)]. A systematic review of vitamin C’s effects on cardiovascular disease prevention and treatment also found no favorable effects [[67](#en67)]. However, follow-up data from the Linxian trial in China showed that daily vitamin C and molybdenum supplements significantly reduced cerebrovascular deaths by 8% over a 10-year post-intervention period [[38](#en38)].
While the Linxian trial suggests a possible benefit, overall, the evidence from most intervention trials does not strongly support vitamin C supplements for preventing or reducing cardiovascular disease morbidity or mortality. Similar to cancer prevention research, the limitations related to the body’s tight control of vitamin C levels may affect the interpretation of these trial results [[22](#en22), 23, 41, 42].
Vitamin C, Age-Related Macular Degeneration, and Cataracts
Age-related macular degeneration (AMD) and cataracts are leading causes of vision loss in older adults. Oxidative stress is considered a contributing factor to both conditions, leading to research into the role of antioxidants like vitamin C in their prevention and treatment.
A population-based study in the Netherlands linked high dietary intakes of vitamin C, beta-carotene, zinc, and vitamin E with a reduced risk of AMD in older adults [[68](#en68)]. However, most prospective studies have not confirmed these findings [[69](#en69)]. A 2007 systematic review and meta-analysis concluded that current evidence does not support a role for vitamin C or other antioxidants in the primary prevention of early AMD [[70](#en70)].
While antioxidants may not prevent AMD development, some evidence suggests they might slow its progression [[71](#en71)]. The Age-Related Eye Disease Study (AREDS), a large clinical trial, evaluated the effect of high doses of antioxidants including 500 mg vitamin C on AMD progression in older individuals [[72](#en72)]. After 6.3 years, participants at high risk of advanced AMD who received the antioxidant supplements had a 28% lower risk of progressing to advanced AMD compared to placebo. The follow-up AREDS2 study confirmed the value of this and similar supplement formulations in reducing AMD progression over 5 years [[73](#en73)].
Higher dietary vitamin C intakes and plasma ascorbate concentrations have been associated with a lower risk of cataract formation in some studies [[2](#en2), 4]. A 5-year prospective study in Japan found that higher dietary vitamin C intake was linked to a reduced risk of developing cataracts in adults aged 45–64 years [[74](#en74)]. Case-control studies suggest that vitamin C intakes over 300 mg/day may reduce cataract risk by 70%–75% [[2](#en2), 4]. However, vitamin C supplement use, particularly at high doses (around 1,000 mg/day), was associated with a 25% increased risk of age-related cataract extraction in a study of Swedish women [[75](#en75)]. This finding was specific to high-dose supplements and not multivitamins with lower vitamin C content.
Clinical trial data are limited. One study in Chinese adults found no significant reduction in cataract risk with daily 120 mg vitamin C plus molybdenum supplements [[76](#en76)]. However, another study showed that older adults (65–74 years) receiving 180 mg vitamin C with other nutrients in a multivitamin/mineral supplement had a 43% lower risk of developing nuclear cataracts [[76](#en76)]. The AREDS study found no significant reduction in cataract risk or progression with 500 mg vitamin C and other antioxidants over 6.3 years [[77](#en77)], and AREDS2 confirmed these findings [[78](#en78)].
Overall, current evidence does not indicate that vitamin C, alone or with other antioxidants, affects the risk of developing AMD, although AREDS formulations might slow AMD progression in high-risk individuals.
Vitamin C and the Common Cold
In the 1970s, Linus Pauling proposed that vitamin C could effectively treat and prevent the common cold [[79](#en79)]. Subsequent controlled studies have produced mixed results, leading to ongoing debate despite high public interest [[80](#en80), 81].
A 2007 Cochrane Review analyzed placebo-controlled trials of at least 200 mg/day vitamin C, used either preventatively or after cold symptoms started [[81](#en81)]. Prophylactic vitamin C did not significantly reduce cold risk in the general population. However, in trials involving marathon runners, skiers, and soldiers under extreme physical stress or in cold environments, preventative vitamin C (250 mg/day to 1 g/day) reduced cold incidence by 50%. In the general population, prophylactic vitamin C modestly reduced cold duration by 8% in adults and 14% in children. When taken after cold symptoms appeared, vitamin C did not affect duration or severity.
Current evidence suggests that regular vitamin C intakes of at least 200 mg/day do not reduce common cold incidence in the general population, but may be beneficial for those under extreme physical stress, in cold environments, or with marginal vitamin C status, such as the elderly and smokers [[81-83](#en81)]. Vitamin C supplements might slightly shorten cold duration and reduce symptom severity in the general population [[80](#en80), 83], possibly due to the antihistamine effect of high-dose vitamin C [[84](#en84)]. However, starting vitamin C after cold symptoms begin appears to be ineffective [[81](#en81)].
Potential Health Risks from Excessive Vitamin C Intake
Vitamin C is generally considered to have low toxicity and is not known to cause serious adverse effects even at high intakes [[8](#en8)]. The most common side effects are gastrointestinal issues like diarrhea, nausea, and abdominal cramps, resulting from the osmotic effect of unabsorbed vitamin C in the GI tract [[4](#en4), 8].
In postmenopausal women with diabetes in the Iowa Women’s Health Study, supplemental (but not dietary) vitamin C intake of at least 300 mg/day was linked to an increased risk of cardiovascular disease mortality [[59](#en59)]. The mechanism for this, if real, is unclear and this finding is from a subgroup in one epidemiological study. Other studies have not observed this association, making its significance uncertain. High vitamin C intakes may also increase urinary oxalate and uric acid excretion, potentially contributing to kidney stone formation, particularly in those with pre-existing renal disorders [[8](#en8)]. However, studies on vitamin C intake and urinary oxalate excretion have yielded conflicting results, and it is not clear if vitamin C plays a significant role in kidney stone development [[8](#en8), 85-87]. The strongest evidence for vitamin C contributing to kidney stones is in individuals with pre-existing hyperoxaluria [[23](#en23)].
Because vitamin C enhances nonheme iron absorption, there is a theoretical concern that high intakes could cause iron overload. In healthy individuals, this does not appear to be a problem [[8](#en8)]. However, in individuals with hereditary hemochromatosis, chronic high-dose vitamin C consumption could worsen iron overload and lead to tissue damage [[4](#en4), 8].
Under certain conditions, vitamin C can act as a pro-oxidant, potentially contributing to oxidative damage [[8](#en8)]. In vitro studies have suggested that high-dose supplemental oral vitamin C might cause chromosomal or DNA damage and possibly contribute to cancer development [[8](#en8), 88, 89]. However, other studies have not shown increased oxidative damage or cancer risk with high vitamin C intakes [[8](#en8), 90].
Other reported effects of high vitamin C intakes include reduced vitamin B12 and copper levels, increased ascorbic acid metabolism or excretion, dental enamel erosion, and allergic reactions [[8](#en8)]. However, some of these conclusions were due to assay artifacts, and subsequent studies have not confirmed these observations [[8](#en8)].
The FNB has established Tolerable Upper Intake Levels (ULs) for vitamin C, applicable to both food and supplement intakes (Table 3) [[8](#en8)]. Long-term vitamin C intakes exceeding the UL may increase the risk of adverse health effects. The ULs do not apply to individuals receiving vitamin C for medical treatment under physician supervision [[8](#en8)].
Table 3: Tolerable Upper Intake Levels (ULs) for Vitamin C [[8](#en8)]
| Age | Male | Female | Pregnancy | Lactation |
|---|---|---|---|---|
| 0–12 months | Not possible to establish* | Not possible to establish* | ||
| 1–3 years | 400 mg | 400 mg | ||
| 4–8 years | 650 mg | 650 mg | ||
| 9–13 years | 1,200 mg | 1,200 mg | ||
| 14–18 years | 1,800 mg | 1,800 mg | 1,800 mg | 1,800 mg |
| 19+ years | 2,000 mg | 2,000 mg | 2,000 mg | 2,000 mg |
*Formula and food should be the only sources of vitamin C for infants.
Potential Interactions of Vitamin C with Medications
Vitamin C supplements can interact with several types of medications. Individuals taking these medications regularly should discuss their vitamin C intakes with their healthcare providers. Some examples include:
Chemotherapy and Radiation
The safety and effectiveness of using vitamin C and other antioxidants during cancer treatment is debated [[53](#en53), 91, 92]. Some data suggest antioxidants might protect tumor cells from radiation therapy and chemotherapy agents like cyclophosphamide and doxorubicin [[54](#en54), 91, 93, 94]. However, some of this data has been criticized for study design flaws [[52](#en52)]. Other data suggest antioxidants might protect normal tissues from chemotherapy and radiation-induced damage [[91](#en91), 93] and potentially enhance conventional cancer treatment effectiveness [[95](#en95)]. Due to the body’s tight control of vitamin C levels, it’s unclear if oral supplements can significantly alter concentrations to produce these effects. Patients undergoing chemotherapy or radiation should consult their oncologist before taking vitamin C or other antioxidant supplements, especially in high doses [[54](#en54)].
3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins)
Vitamin C, combined with other antioxidants, may reduce the increase in high-density lipoprotein (HDL) levels from niacin–simvastatin (Zocor) therapy [[96](#en96), 97]. It’s unclear if this interaction occurs with other lipid-altering regimens [[54](#en54)]. Healthcare providers should monitor lipid levels in patients taking both statins and antioxidant supplements [[54](#en54)].
Vitamin C and Healthful Eating Patterns
The federal government’s 2020–2025 Dietary Guidelines for Americans emphasizes that “nutritional needs should be met primarily through foods… In some cases, fortified foods and dietary supplements are useful when it is not possible otherwise to meet needs for one or more nutrients.”
For detailed guidance on building a healthy dietary pattern, consult the Dietary Guidelines for Americans and the USDA’s MyPlate..
The Dietary Guidelines for Americans defines a healthy dietary pattern as one that:
- Includes a variety of vegetables and fruits, with fruits, especially citrus fruits and juices, and many vegetables being excellent sources of vitamin C.
- Includes grains, with at least half being whole grains.
- Includes fat-free and low-fat dairy, such as milk, yogurt, and cheese, and oils.
- Includes a variety of protein foods, such as lean meats, poultry, eggs, seafood, beans, peas, lentils, nuts, seeds, and soy products.
- Limits foods and beverages high in added sugars, saturated fat, and sodium.
- Limits alcoholic beverages.
- Stays within individual daily calorie needs.
References
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Disclaimer
This fact sheet from the National Institutes of Health (NIH) Office of Dietary Supplements (ODS) is for informational purposes only and does not substitute for medical advice. Consult your healthcare provider (doctor, registered dietitian, pharmacist, etc.) for personalized advice about dietary supplements. Mention of specific products or services in this publication does not imply endorsement by ODS.
Updated: March 26, 2021
