Vitamin C benefits

Is the role in the body the same as the benefit of supplementation?

No. Scurvy develops in vitamin C deficiency; this shows the vitamin is necessary. However, according to the NIH, the body tightly controls tissue and plasma levels: at intakes of 100 mg per day and above, cells are seen to reach saturation, and at 200 mg or more plasma levels rise only slightly. Because baseline levels were not measured in many studies, no difference from supplementation is expected if people are already close to saturation. ^[1]

The depletion–repletion study by Levine and colleagues, conducted in hospital with seven healthy volunteers over 4–6 months, shows this curve: plasma level rises steeply between 30–100 mg/day, the first dose beyond the sigmoid portion was 200 mg/day, and full plasma saturation occurred at 1,000 mg/day; neutrophils, monocytes and lymphocytes reached saturation at 100 mg/day. ^[4] It is a small physiology study and did not measure clinical outcomes (disease).

Does vitamin C prevent or shorten the common cold?

The Cochrane review (Hemilä and Chalker, 2013) examined placebo-controlled trials in which at least 0.2 g of vitamin C per day was used; trials without placebo were excluded. ^[2]

No preventive effect was shown in the general population. This does not mean “definitely ineffective”: the upper limit of the confidence interval is 1.00, so a very small effect cannot be entirely excluded. The nearly halved reduction observed in groups under extreme physical stress is based on five small trials and cannot be carried over to the general population. The shortening of duration was seen only in regular-use trials; it was not replicated in treatment trials. The Cochrane authors write that routine supplementation cannot be justified, that it may be useful in people exposed to brief periods of severe exercise, and that, given the consistent duration effect in regular-use trials and the low cost and safety, people with colds may try therapeutic vitamin C for themselves to see whether it works. ^[2] The NIH summarizes the same results. ^[1]

Is vitamin C beneficial for skin and collagen?

Collagen synthesis by skin fibroblasts is absolutely dependent on vitamin C; this has been shown in cell cultures and animal studies. Vitamin C is present at high concentration in normal skin, and keratinocytes accumulate it. ^[3] In deficiency, impaired collagen synthesis is the cause of the skin and gum findings of scurvy. ^[1]

However, being “necessary in the body” does not mean that additional oral supplementation changes skin structure in a person with adequate levels. The review by Pullar and colleagues states the following: vitamin C applied to the skin has difficulty reaching the inside of the skin, and most formulas that appear effective in human studies contain vitamins C and E together and with a carrier; there is an association between adequate fruit and vegetable intake and skin health but the responsible component is unknown; a number of studies have reported that signs of aging were reduced, but measuring skin changes is difficult. One of the review’s aims is to compare topical application with dietary intake, and the authors write that the efficacy of topical application is poorly understood compared with improving dietary intake. ^[3] Part of the review was funded by the kiwifruit producer Zespri; the authors report that the funder did not influence topic selection or interpretation.

In summary: the requirement for collagen is established; the claim that supplementation “rejuvenates the skin” is not certain on the basis of this review.

Does vitamin C increase iron absorption?

The NIH reports that vitamin C increases the absorption of non-heme iron found in plant foods. In deficiency, iron-deficiency anemia due to bleeding and reduced non-heme iron absorption can be seen. ^[1] This information is a physiological explanation for food combinations (for example, a plant iron source with a food containing vitamin C); what amount would measurably change hemoglobin level, and in whom, cannot be determined from the data cited here. In people with iron overload, the same effect is in an unwanted direction: in hereditary hemochromatosis, chronic high-dose vitamin C can worsen iron accumulation. ^[1] Using vitamin C together with iron medication is a separate question: the 2021 guideline of the British Society of Gastroenterology (BSG) does not recommend adding vitamin C to iron therapy, noting that a large randomized trial showed it did not increase the hematologic response or the rate of replenishing stores and did not reduce side effects. ^[7] Details are on the iron medication page.

Is there evidence for cancer and cardiovascular disease?

Cancer. In case-control studies, high dietary vitamin C intake is associated with lower risk of some cancers; prospective cohorts are inconsistent, and according to the NIH, in most randomized trials supplementation (alone or with other nutrients) did not affect cancer risk. Examples: the Physicians’ Health Study II (500 mg/day vitamin C plus 400 IU vitamin E every other day, mean 8 years, middle-aged and older men) did not reduce prostate or total cancer; in the Women’s Antioxidant Cardiovascular Study, 500 mg/day for a mean of 9.4 years did not affect total cancer incidence or death. ^[1] Observational associations may be influenced by all components of fruit and vegetable intake and do not show causality.

Cardiovascular. According to the NIH, prospective studies are contradictory, and most intervention trials have not shown protection from vitamin C supplementation. In a trial of 8,171 women with a history of cardiovascular disease, 500 mg/day vitamin C did not reduce events over a mean of 9.4 years; in 423 postmenopausal women, the combination of vitamins C and E provided no benefit and significantly increased all-cause mortality. ^[1] In the Linxian (China) population intervention trial, in the group given 120 mg per day of vitamin C plus 30 µg of molybdenum for 5–6 years, an 8% reduction in cerebrovascular deaths was reported in the 10-year follow-up after the intervention ended; the NIH records this as a possible benefit but states that the overall picture is not convincing. ^[1]

Eye. According to the NIH, current evidence does not show that vitamin C, alone or with other antioxidants, affects the risk of developing age-related macular degeneration (AMD); clinical trial data for cataract are limited and mixed (in AREDS, cataract risk was not significantly reduced). The AREDS formulation (500 mg C, 400 IU E, 15 mg beta-carotene, 80 mg zinc, 2 mg copper) reduced the risk of progression to advanced stage by 28% over a mean of 6.3 years in people at high risk of developing advanced AMD (intermediate AMD or advanced disease in one eye). This result belongs to the multi-component product and cannot be attributed to vitamin C alone. ^[1]

In whom is extra supplementation more likely to provide a benefit?

According to the NIH’s interpretation, regular vitamin C intake may be useful for cold frequency in people exposed to extreme exercise or cold, or those with marginal vitamin C status (for example, older adults and regular smokers). Plasma and leukocyte vitamin C levels are lower in smokers; for this reason the NIH/FNB adds 35 mg to the daily reference. ^[1] In the EPIC-Norfolk cross-sectional study, vitamin C deficiency (plasma <11 µmol/L) was associated with aging, male sex, low physical activity, smoking and socioeconomic deprivation; low plasma vitamin C level was also associated with poor self-reported functional health, but this cross-sectional design does not show cause and effect. ^[5]

What limits should be kept in mind when reading benefit claims?

In observational studies, high vitamin C levels occur together with a healthy lifestyle; plasma level can also be a marker of fruit and vegetable intake. Intervention trials, in turn, were mostly conducted in low-risk, relatively well-nourished people. The harms of supplementation must also be taken into account: digestive complaints at high doses, and a higher kidney stone risk in men in observational studies (in Ferraro et al. 2016, HR 1.43 for total intake of ≥1,000 mg per day in 40,536 men; not significant in women). ^[6] Details are on the excess page.

This guide does not recommend personal doses.