Q&A: Can vitamin C supplements boost your health beyond a balanced diet?

Takeaway messages 🔑

Introduction

Scurvy was a deadly risk for sailors on long voyages due to lack of fresh produce. The discovery of vitamin C's role in preventing scurvy, particularly through James Lind's 1747 experiment with sailors and citrus fruits, highlighted the critical link between diet and health (1). This breakthrough eventually led to the British Navy adopting citrus fruits to prevent scurvy, resulting in a dramatic improvement in sailors' health during long sea voyages. However, the mechanism by which citrus fruits promoted recovery remained unknown at the time. Nearly two centuries later, ascorbic acid (vitamin C) was identified and isolated by scientists such as Albert Szent-Györgyi, whose pioneering work earned him the Nobel Prize in Physiology or Medicine in 1937. These discoveries advanced the field of nutritional science and informed public health initiatives promoting balanced diets rich in essential vitamins. More broadly, this understanding emphasised the importance of nutrients in preventing chronic diseases and maintaining overall health, shaping modern dietary guidelines.

The healing compound in citrus was first called antiscorbutic factor—the original name given to vitamin C. Scorbutic is an adjective that refers to scurvy or conditions related to scurvy. Today, vitamin C is known as ascorbic acid, L-ascorbic acid, ascorbate, dehydroascorbic acid (DHAA C). Vitamin C is water-soluble. Vitamin C is a 6-carbon compound similar to glucose with the chemical formula C₆H₈O₆. Vitamin C exists in several chemical forms, with the major forms thereof summarised in Table 1.

Table 1: Major chemical forms of vitamin C

Vitamin C is an essential vitamin for humans and a few other species because they cannot synthesise it and must obtain it through diet or supplements (2). It is crucial for normal body functioning, growth and development. Unlike most animals, which can synthesise vitamin C from glucose, humans, primates, guinea pigs, certain fruit-eating bats and some bird species lack the necessary enzyme to produce it endogenously i.e. inside the body (2, 3).

Functions of vitamin C🛡️🧬🩸

Vitamin C is not a coenzyme (a small organic molecule that binds to an enzyme and directly participates in the catalytic reaction, often by transferring chemical groups) such as B vitamins. Instead, vitamin C acts independently as an antioxidant, donating electrons or hydrogen ions to other substances to inhibit oxidation. Vitamin C is involved in various redox reactions, easily accepting and donating electrons. This antioxidant function protects cells from free radicals, which are highly reactive and unstable molecules produced during normal cellular metabolism. Specifically, ascorbic acid donates two hydrogen atoms (with electrons) to neutralise free radicals, forming DHAA. This reaction is reversible, allowing DHAA to regain hydrogen and become ascorbic acid again. This redox cycle is key to vitamin C’s antioxidant function, protecting cells from oxidative damage. Factors such as intense sunlight, certain drugs and toxic substances for example smog and cigarette smoke can increase free radical production. Antioxidants neutralise free radicals by donating electrons, preventing oxidative damage to lipids, DNA and proteins, which can lead to diseases such as cancer.

Vitamin C is a non-enzymatic cofactor (4). Vitamin C enhances the absorption of essential minerals i.e. iron, copper and chromium by preventing their oxidation and converting them into more bioavailable forms. Specifically, vitamin C improves the absorption of non-haeme iron from plant-based foods by reducing it to ferrous iron (Fe²⁺), a more absorbable form. It also counteracts compounds that inhibit iron absorption, such as phytates and tannins by forming a chelate complex (a strong chemical bond where a nutrient is tightly held by another molecule to protect it and improve absorption). A systematic review and meta-analysis of 11 studies involving 1,930 patients with iron deficiency anaemia found that adding vitamin C to oral iron supplementation produced statistically significant but small increases in haemoglobin and ferritin compared with iron alone (5). Regular consumption of vitamin C-rich foods with iron-rich meals improves iron status and helps prevent iron deficiency anaemia, particularly in at-risk populations such as vegetarians, pregnant women and individuals with chronic illnesses. However, in individuals with conditions that cause excess iron in the body, such as haemochromatosis, high vitamin C intake could be problematic, as it may exacerbate iron overload and lead to tissue damage. Beyond acting as an antioxidant and enhancing mineral absorption, vitamin C plays numerous other crucial roles in the body (see Table 2).

Table 2: Functions of vitamin C in the body

* The terms proline and lysine refer to the amino acids themselves, whereas prolyl and lysyl refer to those amino acids when they are incorporated into a protein or peptide chain.

Although bleeding gums, easy bruising, and other haemorrhagic manifestations are hallmark symptoms of scurvy, vitamin C does not directly participate in the coagulation cascade responsible for blood clot formation. Rather, it plays a critical role in collagen synthesis and the maintenance of vascular integrity, thereby supporting healthy blood vessels and helping to prevent bleeding associated with fragile capillaries and impaired connective tissue. The connection between vitamin C and fibrinogen, the abundant clotting factor in the blood, is indirectly mediated through its anti-inflammatory effects (27-29). Supporting this, combined high-dose vitamin C (2 g/day) and vitamin E (400–800 IU/day) supplementation improved endothelial function and reduced plasma levels of plasminogen activator inhibitor-1 (PAI-1), von Willebrand factor (vWF) and the PAI-1/tPA ratio in chronic smokers (30). Many fruits and vegetables rich in vitamin C also contain high levels of dietary fibre. Dietary fibre is known for its anti-inflammatory properties, which can enhance gut health and lower blood levels of C-reactive protein, a marker of inflammation (31). Therefore, the consumption of foods high in both fibre and vitamin C may contribute to reduced inflammation, indirectly influencing blood coagulation positively and impacting blood clot formation.

Daily vitamin C requirements

Body weight is an important determinant of vitamin C requirements because larger individuals have a greater volume of distribution, resulting in lower plasma vitamin C concentrations for a given intake (32). Infants under 12 months typically need 40–50 mg/day. Maternal dietary intake (especially from fruits and vegetables) directly influences breast milk vitamin C levels (33). Children need approximately 15–45 mg of vitamin C daily, while adolescents require 65–75 mg. Until adolescence, there are no gender-specific dosage differences for vitamin C.

Adult men and women older than 19 years require 90 mg/day and 75 mg/day, respectively due to differences in body size and composition rather than sex itself, with smokers and passive smokers needing an additional 35 to 40 mg/day more than their non-smoking counterparts (11, 32). Overweight and obese individuals may require higher vitamin C intakes to achieve adequate status, with estimates suggesting an additional 10–20 mg/day for every 10 kg increase in body weight above 60 kg (32). Increased inflammation and oxidative stress associated with obesity may further elevate vitamin C requirements (32).

Vitamin C deficiency 📉

Deficiency symptoms include anaemia (microcytic/small cell type), atherosclerotic plaques/pinpoint haemorrhages, bone fragility, poor wound healing, bleeding gums and loose teeth, muscle degeneration, rough skin and blotchy bruises. Vitamin C deficiency may contribute to depression, emotional instability and cognitive symptoms by disrupting neurotransmitter function, even in the absence of scurvy or psychosis (13). The deficiency disease of vitamin C is called scurvy and only appears when vitamin C intake is less than 1 mg/day (34). Certain individuals might be at risk of vitamin C inadequacy or deficiency (see Table 3).

Table 3: Individuals and conditions associated with an increased risk of vitamin C inadequacy or deficiency

Upper limit and potential risks of vitamin C intake🚽

The Upper Level of Intake (UL) for vitamin C is set at 2000 mg/day for adults (11). Exceeding this amount can lead to adverse effects such as diarrhoea, nausea and abdominal cramps, although high doses are also excreted quickly. Additionally, the beneficial effects of vitamin C on iron absorption can become harmful when exceeding the tolerable UL, as excessive free iron can cause cellular damage similar to that inflicted by free radicals (42). High-dose vitamin C may influence platelet aggregation, which could theoretically affect blood clotting and interact with anticoagulant medications. Individuals with kidney disorders, iron overload conditions and gastrointestinal conditions are at higher risk for adverse effects from exceeding the UL and should avoid using vitamin C supplements.

Dietary sources of vitamin C🍊🍋🌶️🥦

Although citrus fruits are well-known sources of vitamin C, many other fruits, including quava, kiwi,  strawberries and papayas, often contain comparable or even higher concentrations of the vitamin. Fruit juice can be a convenient source of vitamin C, particularly for individuals who struggle to consume sufficient whole fruit; but should not replace more than half of daily fruit servings, as it may not provide the same health benefits as whole fruits and has been associated with a modestly increased risk of breast cancer (43). Vegetables including cabbage varieties, dark greens such as green peppers and broccoli, lettuce and tomatoes. Despite their moderate vitamin C content, potatoes significantly contribute to overall intake due to their widespread consumption. A medium-sized potato provides about 20% of the recommended daily intake, making it a key source of vitamin C, though its content decreases as potatoes age. Regular cuts of beef, pork, lamb, poultry and fish contain only trace amounts of vitamin C and cooking further depletes these levels, rendering the vitamin C content negligible.

Table 4: Vitamin C content in various food sources

Note: These values provide a general guide and can vary slightly depending on the source and preparation method.

Because vitamin C is easily destroyed by heat and oxygen, consuming whole raw fruits and some raw or lightly steamed vegetables is recommended (see Table 5).

Table 5: Best practices for preserving vitamin C during food preparation and cooking

Assessment of vitamin C status

Vitamin C concentrations may be measured in individuals with clinical suspicion of scurvy or chronic low intake or in research studies. Because vitamin C is highly unstable—sensitive to light, temperature, pH and oxidation—strict pre-analytical handling is essential. Samples must be rapidly processed, protected from light, acidified and stored at ultra-low temperatures to prevent degradation.

Plasma is the liquid portion of unclotted blood and contains clotting factors; serum is the liquid remaining after blood has clotted and lacks clotting factors; and the buffy coat is the thin layer between the plasma and red blood cells after centrifugation that contains most of the leukocytes (white blood cells) and platelets. Vitamin C status is typically assessed using plasma concentrations, although leukocyte levels may reflect tissue stores better but are less practical due to higher sample requirements and variability. Plasma vitamin C concentrations >50 µmol/L are considered adequate, while concentrations <23 µmol/L indicate hypovitaminosis C and concentrations <11 µmol/L indicate deficiency (32). Measuring vitamin C in serum should be avoided because the clotting process delays sample separation and exposes vitamin C to oxygen and metal ions released from platelets and blood cells, which accelerates oxidation of ascorbic acid and can lead to falsely low concentrations. Plasma measurement is therefore the preferred method (37).

Vitamin C exists as AA and its oxidised form DHAA, both of which are measured as total vitamin C. Analytically, high-performance liquid chromatography (HPLC) is the standard method for quantifying AA, DHAA or total vitamin C, often using reduction steps to convert DHAA to AA. Emerging alternatives include point-of-care devices measuring oxidation–reduction potential (sORP), which provide a rapid indirect estimate of vitamin C status (37).

There is a moderate positive correlation between dietary vitamin C intake (measured via food frequency questionnaire and dietary recalls) and plasma vitamin C concentrations (44). With bioavailability, absorption conditions, genetic make-up, stress and food processing influencing this relationship (38, 44). Dietary vitamin C is absorbed in the intestine (absorption) and kidney (reabsorption), with transport into the bloodstream mediated by the sodium-dependent vitamin C transporter 1 (SVCT1; gene: SLC23A1) (https://www.omim.org/entry/603790?search=SVCT1). This transporter has limited capacity and becomes saturated at high intakes. Genetic variation in SLC23A1 can influence vitamin C status by altering transport efficiency. For example, the minor (A) allele of the missense variant rs33972313 has been associated with a reduction in circulating ascorbate of approximately 6 µmol/L per allele in a cohort of more than 15,000 individuals (38).

Global vitamin C status: Vitamin C deficiency as a global public health concern

Vitamin C status varies considerably worldwide, with evidence indicating that low vitamin C status remains a public health concern, particularly in low- and middle-income countries (45). There is high prevalence rates of deficiency in older adults in India (45). Similarly, deficiencies have been recorded among Black African populations in South Africa, Nigeria and Uganda, although comprehensive national prevalence estimates are lacking (45). Average vitamin C status is generally better in high-income countries, deficiency and insufficiency still occur, especially among at risk individuals (45). The available evidence also highlights a shortage of high-quality epidemiological studies, underscoring the need for further research to determine the true global burden of vitamin C deficiency.

Research on dietary vitamin C and health

Research has explored the association of dietary vitamin C with various diseases. Meta-analyses indicate that dietary vitamin C intake is inversely associated with oesophageal cancer (16) and lung cancer (17), but not with ovarian cancer (46) or pancreatic cancer (47). A systematic review and meta-analysis by Zhong et al. (48) demonstrated that dietary vitamin C intake reduces the risk of digestive system cancers, particularly oral, pharyngeal and oesophageal cancers, gastric cancer and colon cancer. The most substantial protective effect was observed for oesophageal cancers at an intake of 250 mg/day. While higher plasma vitamin C levels were inversely associated with gastric cancer risk, no association was found with other digestive system cancers. The study also highlighted that vitamin C intake reduces the risk of colon cancer but does not impact rectal cancer risk. Higher dietary vitamin C intake was associated with a reduced risk of both cervical and prostate cancer, with an 8% reduction in cervical cancer risk for every 50 mg/day increase in intake  (49) and a 9% reduction in prostate cancer risk for every 150 mg/day increase in intake  (50). The apparent protective effect of dietary vitamin C against cancer may reflect the combined actions of vitamin C and other beneficial constituents of fruits and vegetables, rather than the effects of vitamin C in isolation (51).

Dietary vitamin C consumption has several benefits related to heart health, with deficiencies linked to a higher risk of mortality from cardiovascular disease (18). In a meta-analysis of 16 prospective studies, the risk of stroke was inversely associated with dietary and circulating vitamin C levels, although supplements showed no clear preventive effect (52). A meta-analysis of 28 observational studies found that higher dietary and circulating (serum and plasma) vitamin C levels were inversely associated with metabolic syndrome, with higher levels linked to a lower risk of the condition (19). Higher dietary intakes of antioxidant-rich foods, including vitamin C, vitamin E, β-carotene and anthocyanins, were associated with a lower risk of Parkinson’s disease, with dose–response analyses showing a 6% reduction in risk for every 50 mg/day increase in vitamin C intake (53). A genetic variant in the SLC23A1 gene, used as a proxy for lifelong plasma vitamin C concentrations, supports a causal association between lower vitamin C status and an increased risk of age-related cataracts (54). Vitamin C intake was positively associated with handgrip strength in older Korean adults, indicating a potential benefit for muscle function and physical performance (55).

Insights and research on health benefits and risks of vitamin C supplements💊

Vitamin C is one of the most popular dietary supplements worldwide (12). Granger and Eck (56) initially argued that supplementation in populations with sufficient vitamin C intake from dietary sources would not provide additional benefits in disease prevention. The effectiveness of vitamin C supplementation, traditionally believed to be limited by the saturation of the SVCT1 transporter at high doses (57), appears to be enhanced through regular high-dose intake (58). Increased dosage has been shown to potentially upregulate the expression of SVCT2, another transporter, which could compensate for SVCT1's limitations, thus improving vitamin C uptake in skeletal muscle (58).

Various forms of vitamin C supplementation are available including tablets, capsules, powders, chewable gummies and innovative ones such as liposomal vitamin C and buffered vitamin C and Ester C (refer to Table 6 below). The liposomal form of vitamin C is recognised for its enhanced bioavailability, which may allow for more effective absorption and utilisation at higher doses. This enables higher concentrations of vitamin C to be absorbed without being hindered by transporter saturation. Yet, conclusive scientific evidence confirming the superiority of liposomal vitamin C over traditional forms is still developing.

Ester-C has shown potential effectiveness in treating scurvy (59) and in reducing oxalate levels (60), which are metabolites of vitamin C. Although reports suggest that Ester-C may alleviate common cold symptoms, these claims lack comparison with standard vitamin C (61) and have not been widely replicated. Furthermore, while studies supporting Ester-C's benefits have been funded by Zila Nutraceuticals, it's important to note that its advantages over traditional vitamin C are not conclusively proven. Ester-C could be beneficial for individuals who are sensitive to acidic foods, but further evidence is needed (62).

Table 6: Vitamin C supplements forms

Notes: To make liposomal vitamin C at home, blend 1 tbsp of sunflower/soya lecithin with 1 cup of water until dissolved. Separately, dissolve 1 tbsp of L-ascorbic acid powder in 1/2 cup of water. Combine both solutions and blend well. Store in the fridge and shake before use. Use within 2 weeks.

Table 7 provides various benefits including improvements in endothelial function, glucose levels in diabetes, risk factors for stroke and metabolic syndrome, blood pressure reduction, and other health outcomes associated with vitamin C supplementation. Note that the evidence from systematic reviews and meta-analyses, which compile data from multiple studies, generally carries more weight than findings from single studies.

Table 7: Summary of the benefits of vitamin C supplementation

While vitamin C supplementation is generally safe at recommended doses, taking high doses (3,000–10,000 mg) can lead to diarrhoea, increasing discomfort and potentially leading to dehydration. There is also an increased risk of dental erosion when using chewable vitamin C tablets, which could compromise tooth enamel over time (90). While liposomal vitamin C in liquid form offers enhanced absorption, it might also pose a risk of dental erosion due to its acidity. It is recommended to consume it through a straw to minimise direct contact with the teeth and protect enamel. Although rare, there is a concerning possibility of nephrotoxicity—kidney damage—following high-dose vitamin C administration which increases the formation of oxalate, which may lead to kidney stones or kidney damage (91, 92). Two large prospective cohort studies found that vitamin C supplementation was associated with an increased risk of kidney stones, particularly in men (93), with one Swedish cohort reporting approximately a twofold higher risk among supplement users compared with non-users (94). People with certain inherited blood disorders—such as Thalassemia, glucose-6-phosphate dehydrogenase deficiency, sickle cell disease and haemochromatosis—should use high-dose vitamin C supplements with caution and only under medical supervision (37). In people with G6PD deficiency, extremely high intravenous doses (more than 60 g) may trigger the breakdown of red blood cells (haemolysis), whereas lower doses (up to 6 g/day) may be safe and could even offer some protective effects. Severe side effects from typical oral vitamin C supplementation are uncommon, but exceeding recommended doses may increase health risks. Obtaining vitamin C from food is generally the safest approach, although supplementation can be beneficial when dietary intake is inadequate or when specific evidence-based health benefits are expected.

Interference of high-dose vitamin C with diagnostic tests

Interference with diabetes diagnostic tests

High-dose vitamin C supplementation can interfere with point-of-care glucose testing, leading to inaccurate results. It may cause false-negative urine glucose readings by neutralising dipstick reagents, and false-positive blood glucose readings by reacting with electrochemical test strips. For accurate diagnosis and monitoring, especially in clinical or research settings, lab-based testing is recommended (95).

Interference with faecal occult blood testing

High-dose vitamin C supplementation may cause false-negative results in guaiac-based faecal occult blood tests (gFOBTs). As a potent antioxidant, vitamin C inhibits the oxidation reaction required for colour development in the test, potentially masking the presence of blood in the stool and reducing test sensitivity (96).

Conclusion

Vitamin C is essential for numerous bodily functions. While a balanced diet rich in fruits and vegetables typically provides adequate vitamin C, certain individuals, such as those with limited food variety, certain medical conditions or increased physiological needs, may benefit from supplementation. Individuals with inadequate intake, smokers, people under intense physical stress, those overweight or obese, type 2 diabetes, hypertension or certain clinical conditions may experience measurable improvements in blood pressure, endothelial function, glucose control or recovery outcomes. However, higher doses are not without risk and can cause diarrhoea, dental erosion, interfere with glucose testing, and in rare cases contribute to kidney damage. The best strategy is to prioritise a nutrient-rich diet and use supplements selectively when there is a clear nutritional need or evidence-based therapeutic indication.

Reflections🤔💭

1.           Assess your diet or consider a blood test: Reflect on your daily fruit and vegetable intake. Are you consuming at least five varied servings of fruits and vegetables each day? Have you ever tested your plasma circulating vitamin C status?

2.           Evaluate your health needs: Consider any specific health conditions you have that might increase your need for vitamin C, such as a compromised immune system or chronic illness.

3.           Consider supplementation if you are not at risk: Based on your dietary intake or plasma levels and health needs, determine if vitamin C supplementation might be beneficial for you. Discuss with a healthcare provider if necessary. If necessary, experiment with traditional vitamin C supplements and liposomal vitamin C (see recipe under Table 6).

4.           Monitor your intake: If you decide to supplement, keep track of your vitamin C intake to ensure it does not exceed the Upper Level of Intake (2000 mg/day for adults) to avoid potential adverse effects.

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