Vitamin C (also called ascorbic acid) is an essential water-soluble vitamin with one of the broadest profiles of authorised functions among all vitamins. It contributes to normal collagen formation, to the normal function of the immune system, to the protection of cells from oxidative damage, to the reduction of tiredness and fatigue, to increasing iron absorption and to another 4 physiological functions officially authorised at the European level. This page covers how it works, its food sources, the commercial forms (ascorbic acid, ascorbate, liposomal vitamin C), the studied doses and where it fits within the Pleniage portfolio.
What is vitamin C?
Vitamin C, chemically ascorbic acid (its name refers to its historical ability to prevent scurvy, the disease caused by severe vitamin C deficiency), is a water-soluble vitamin essential for humans. "Essential" means that the body cannot synthesise it and must obtain it through the diet or through food supplements. Most mammals synthesise their own vitamin C in the liver, but humans, along with other primates and a few other species, evolutionarily lost that ability and depend on external intake.
It is one of the nutrients with the broadest profile of biological functions: it acts as an enzyme cofactor for several key enzymes (especially those involved in the synthesis of collagen and of certain neurotransmitters) and as a water-soluble antioxidant in the aqueous cellular compartments. We explain these mechanisms in detail further on.
What is it for? Functions authorised at the European level
At the European level, the EFSA (European Food Safety Authority) has evaluated the physiological functions of vitamin C and has officially authorised nine health claims for this vitamin. It is one of the vitamins with the highest number of authorised green EFSA claims, reflecting the diversity of biological functions in which it participates.
Normal collagen formation
Vitamin C contributes to normal collagen formation for the normal function of bones, cartilage, gums, skin and blood vessels (authorised EFSA claim, ID 129). This function reflects its role as a cofactor of the enzymes prolyl hydroxylase and lysyl hydroxylase, which stabilise the triple helix of collagen.
Normal function of the immune system
Vitamin C contributes to the normal function of the immune system (authorised EFSA claim, ID 134). It participates in the function of several immune cells (neutrophils, lymphocytes) and in the antioxidant response associated with immune activity.
Protection against oxidative damage
Vitamin C contributes to the protection of cells from oxidative damage (authorised EFSA claim, ID 136). As a water-soluble antioxidant it neutralises reactive oxygen species in the aqueous cellular compartments.
Reduction of tiredness and fatigue
Vitamin C contributes to the reduction of tiredness and fatigue (authorised EFSA claim, ID 138). This function is linked to its role in energy metabolism and in the synthesis of carnitine (a transporter of fatty acids into the mitochondria).
Increased iron absorption
Vitamin C contributes to increasing iron absorption (authorised EFSA claim, ID 140). It reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), the form absorbed more efficiently in the intestine. This effect is especially relevant for iron of plant origin (non-heme).
Other authorised functions
Four additional green EFSA claims complete the profile:
- Normal energy metabolism (authorised EFSA claim, ID 130).
- Normal function of the nervous system (authorised EFSA claim, ID 131).
- Normal psychological function (authorised EFSA claim, ID 133).
- Regeneration of the reduced form of vitamin E (authorised EFSA claim, ID 139): vitamin C participates in the recycling of oxidised vitamin E, a biochemical synergy between the two antioxidants.
How it works: enzyme cofactor and water-soluble antioxidant
Vitamin C has two distinct but complementary biochemical roles:
Cofactor of hydroxylase enzymes
Vitamin C is an enzyme cofactor for several hydroxylase enzymes, enzymes that add hydroxyl groups (—OH) to their substrate molecules. The most biologically relevant are:
- Prolyl-4-hydroxylase and lysyl hydroxylase: they stabilise the triple helix of collagen, an essential step in collagen synthesis. Without vitamin C, the collagen produced is defective and manifests clinically as scurvy in severe deficiencies.
- Dopamine β-hydroxylase: it catalyses the conversion of dopamine to noradrenaline (a neurotransmitter).
- Trimethyllysine hydroxylase: a key step in the endogenous synthesis of carnitine, a transporter of fatty acids into the mitochondrion.
The main water-soluble antioxidant of plasma
Vitamin C is one of the main water-soluble antioxidants of human plasma. It neutralises reactive oxygen species (hydroxyl radical, singlet oxygen, superoxide anion) by donating electrons from its enediol group. Its oxidised form (dehydroascorbate) can be reduced back to active vitamin C by cellular enzyme systems and by glutathione. In addition, vitamin C participates in the regeneration of oxidised vitamin E in cell membranes (green EFSA claim ID 139), an example of synergy between water-soluble and fat-soluble antioxidants.
Food sources and daily requirements
Vitamin C is present above all in fresh fruits and vegetables. The sources with the highest concentration are:
- Raw red peppers: one of the densest sources (≈140 mg/100g, almost three times more than an orange).
- Citrus fruits (orange, mandarin, lemon, grapefruit).
- Kiwi and other tropical fruits (guava, papaya).
- Berries (strawberries, raspberries, currants).
- Cruciferous vegetables (broccoli, Brussels sprouts, cauliflower).
- Tomato, spinach and other leafy vegetables.
- Potato cooked with its skin: provides a relevant amount.
The nutrient reference value (NRV) in the European Union for vitamin C is 80 mg/day (EU Regulation 1169/2011 on food information). International recommendations for adults range between 75 mg/day (women) and 90 mg/day (men) according to the US National Academies; smokers require additional amounts (~35 mg/day more) because of the greater oxidative stress associated with tobacco.
Vitamin C is heat-labile (it degrades with heat) and is partially lost during prolonged cooking or in foods stored for a long time. Consuming fresh or briefly cooked fruits and vegetables (steaming, quick sautéing) better preserves the content.
Studied doses: from the NRV to megadoses
A peculiar characteristic of vitamin C is the non-linear dose-plasma relationship. Classic pharmacokinetic studies published by Levine and colleagues demonstrated that the plasma concentration of vitamin C saturates at oral doses of around 200-400 mg/day. Above these doses, intestinal absorption decreases proportionally and the excess is eliminated in urine; megadoses (1-3 g/day and even higher) do NOT translate into proportionally higher plasma concentrations.
A review published in Nutrients (Carr and Maggini, 2017) synthesised the evidence on the role of vitamin C in the immune system and explicitly stated that doses of 100-200 mg/day are sufficient to maintain optimal plasma concentrations in healthy people; higher therapeutic doses (1-3 g/day) have been studied in specific clinical contexts (acute infections, hospital sepsis) and always under medical supervision.
The practical conclusion is that for general maintenance supplementation in people with a varied diet, doses of 100-500 mg/day of vitamin C are adequate; megadoses do not provide documented additional benefit in people with adequate nutritional status, and at very high doses they can increase the risk of gastrointestinal discomfort and of kidney stones in predisposed people.
Commercial forms: ascorbic acid, ascorbate and liposomal
| Form | Characteristics | Notes |
|---|---|---|
| Ascorbic acid | Natural form of the vitamin, acidic taste | The standard and most studied form. Usual on nutritional labelling. |
| Ascorbate (sodium, calcium, magnesium) | Salt of ascorbic acid, neutral taste | "Buffered" form — gentler on the stomach. Useful for people with digestive sensitivity. |
| Liposomal vitamin C | Encapsulated in liposomes (lipid vesicles) | Superior bioavailability documented in some studies. More expensive. A recent commercial trend. |
| Ester-C® (calcium ascorbate with metabolites) | Patented form with vitamin C metabolites | Registered trademark. Buffered calcium ascorbate. |
Vitamin C and the common cold: nuanced evidence
This section deserves explicit honesty given the very strong cultural association between vitamin C and prevention of the common cold. The most complete Cochrane review on vitamin C and the common cold (Hemilä and Chalker, periodically updated) has analysed dozens of clinical trials over decades and has established nuanced conclusions:
- Regular maintenance supplementation with vitamin C in the general population does NOT prevent the onset of the common cold.
- In people who already take vitamin C as maintenance, there is some evidence that supplementation may slightly reduce the duration and severity of symptoms when a cold appears.
- In people undergoing extreme physical effort (marathon runners, skiers in conditions of high cold exposure) pre-event supplementation may have some preventive effect.
In other words: the cultural association of "vitamin C to avoid catching a cold" is not fully supported by clinical evidence. The reasonable use of vitamin C is as an essential nutrient with multiple physiological functions (green EFSA claims) in general maintenance supplementation, not as a "cold preventive".
Safety and interactions
Vitamin C has a very favourable safety profile at the usual doses. EFSA has established a tolerable upper intake level (UL) of 1 g/day for vitamin C from food supplements, based on the prevention of gastrointestinal discomfort (osmotic diarrhoea) that appears at higher doses in sensitive people.
Adverse effects at high doses
- Gastrointestinal discomfort: diarrhoea, abdominal pain, nausea (at doses >1-2 g/day in sensitive people).
- Oxalate kidney stones: vitamin C is partially metabolised to oxalate. In people with a history of calcium oxalate kidney stones, high doses (≥1 g/day over prolonged periods) may increase the risk. Consult your doctor if you have this predisposition.
- Haemochromatosis (hereditary iron overload): vitamin C increases iron absorption and should be avoided at high doses in these people.
Drug interactions
- Oral anticoagulants: very high doses may interfere with warfarin (check the INR if the dose is changed).
- Aluminium (antacids): vitamin C may increase aluminium absorption; space out the doses.
- Chemotherapy and radiotherapy: people undergoing oncological treatment should consult their oncologist before starting supplementation with antioxidants.
How to choose a vitamin C supplement
- Dose consistent with the goal: 100-500 mg/day covers the NRV and the authorised physiological functions. Megadoses are not recommended for general maintenance.
- Form according to tolerance: standard ascorbic acid for general use; ascorbate (salts) if there is gastric sensitivity; liposomal if superior bioavailability is sought and the cost is accepted.
- Split doses: dividing the dose into 2-3 intakes takes better advantage of plasma saturation than a single high intake.
- Purity and traceability: third-party certificates of analysis when available.
Vitamin C in the Pleniage portfolio
In the formulation of PLENIAGE® ENERGY PRO, vitamin C is incorporated together with the complete B complex (B1, B2, B3, B5, B6, B12), magnesium citrate, alpha-lipoic acid (175 mg), vitamin E and biotin. This combination is consistent with the synergistic role of vitamin C in the regeneration of oxidised vitamin E (EFSA claim ID 139) and with its contribution to energy metabolism in synergy with the B-group vitamins. Each ingredient has individual scientific research and, in the case of vitamin C, nine officially authorised claims at the European level; the specific combination of this formula has not been the subject of its own clinical trial.
This page is part of the Energy and performance cluster. To explore other related components in more depth, see the Magnesium citrate page and the Alpha-lipoic acid page (both complementary cofactors in the formula).
Frequently asked questions about vitamin C
Which is better: ascorbic acid, ascorbate or liposomal vitamin C?
It depends on the goal. Ascorbic acid is the most studied natural form and the standard one for general supplementation. Ascorbates (sodium, calcium, magnesium salts) are buffered forms that are gentler on the stomach, useful for people with gastric sensitivity. Liposomal vitamin C (encapsulated in lipid vesicles) has superior bioavailability documented in some studies and is a recent commercial trend, although it is more expensive. For general maintenance the three forms are valid at the usual doses.
How much vitamin C do I need per day?
The nutrient reference value (NRV) in the European Union is 80 mg/day. International recommendations for adults are between 75 and 90 mg/day. For general maintenance supplementation, doses of 100-500 mg/day are adequate. Classic pharmacokinetic studies (Levine et al.) show that plasma saturates at around 200 mg/day; higher doses do not proportionally increase plasma concentrations.
Does vitamin C prevent colds?
The most complete Cochrane review on the topic (Hemilä and Chalker) concluded that regular supplementation with vitamin C does NOT prevent the onset of the common cold in the general population. There is some evidence that, in people who already take vitamin C as maintenance, supplementation may slightly reduce the duration and severity of symptoms. The cultural association of "vitamin C to avoid catching a cold" is not fully supported by clinical evidence.
Are megadoses of vitamin C useful?
Not for general maintenance supplementation. The plasma concentration of vitamin C saturates at around 200-400 mg/day; megadoses (1-3 g/day) do NOT translate into proportionally higher plasma concentrations. At very high doses gastrointestinal discomfort may appear and, in predisposed people, an increased risk of oxalate kidney stones. High therapeutic doses have been studied in specific clinical contexts under medical supervision.
Is vitamin C destroyed by cooking?
Yes, partly. Vitamin C is heat-labile and degrades with heat; prolonged cooking at high temperatures (especially in water) can destroy a significant fraction. Brief cooking with little water (steaming, quick sautéing) better preserves the content. Fruits and vegetables eaten raw retain all of their original vitamin C. Prolonged storage also reduces the content.
Does it have contraindications or interactions?
Yes, several that are relevant at high doses. People with a history of calcium oxalate kidney stones should avoid high doses (≥1 g/day over prolonged periods). People with haemochromatosis (hereditary iron overload) should avoid high doses because vitamin C increases iron absorption. People undergoing oncological treatment (chemotherapy or radiotherapy) should consult their oncologist before any supplementation with antioxidants. There is potential interaction with oral anticoagulants at very high doses.
Why do smokers need more vitamin C?
Tobacco smoke increases systemic oxidative stress and depletes antioxidant reserves more quickly, including vitamin C. Recommendations for smokers in the US add 35 mg/day to the NRV. It is an additional (public health) argument for quitting smoking; supplementation does NOT compensate for the overall harm of tobacco.
Vitamin C is an essential water-soluble vitamin with one of the broadest profiles of authorised functions among all vitamins (nine green EFSA claims). Its central role is twofold: enzyme cofactor (synthesis of collagen, neurotransmitters and carnitine) and the main water-soluble antioxidant of human plasma. Its strongest clinical evidence corresponds to the officially authorised physiological functions; the cultural association of "vitamin C for colds" is qualified by the evidence (Cochrane review). For general maintenance supplementation, doses of 100-500 mg/day are adequate; megadoses do not provide documented additional benefit in people with a varied diet.
At PLENIAGE® we publish scientific content on evidence-based supplementation. You can explore the Energy and performance cluster for more related pages and articles.
References
The statements in this article are based on the available scientific literature and on the health claims officially authorised by EFSA.
- EFSA Health Claims Register — vitamin C: authorised claims ID 129 (normal collagen formation), 130 (energy metabolism), 131 (nervous system), 133 (psychological function), 134 (immune system), 136 (oxidative protection), 138 (tiredness and fatigue), 139 (vitamin E regeneration), 140 (iron absorption). Official source: EU Register of Nutrition and Health Claims.
- EU Regulation 1169/2011 on the provision of food information to consumers — Annex XIII (NRV for vitamin C: 80 mg/day). EFSA UL for supplements: 1 g/day.
- Carr AC, Maggini S. Vitamin C and Immune Function. Nutrients. 2017;9(11):1211. PMID: 29099763.
- Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;(1):CD000980. PMID: 23440782.
- Levine M, Conry-Cantilena C, Wang Y, et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci U S A. 1996;93(8):3704-3709. PMID: 8623000.