Established and potential functions of vitamin C

Vitamin C (ascorbic acid) is an essential micronutrient required for normal metabolic func-tioning of the body. The vitamin acts as a cofactor for several enzymes involved in the biosynthesis of colla-gen, carnitine, and neurotransmitters. In addition to enzyme activation, oxidative stress reduction and immune function are thought to be the key functions of vitamin C, suggesting a potential preventive efficacy in cardiovascular disease, cancer, age-related cognitive decline and common cold. There is increasing evidence that the vitamin does not only act as a simple antioxidant but is able to affect reduction/oxidation-sensitive signaling pathways, modulate gene expression and regulate cell differentiation.

Data from intervention studies investigating efficacy of vitamin C supplementation in the prevention of multifactorial diseases are inconsistent. Lack of consideration of some specific features of vitamin C metabolism has led to studies in which clas-sification of participants according to vitamin C status is inaccurate. It seems that beneficial effects are most relevant to people with low blood levels of vitamin C, like smokers or people living in conditions associated with high levels of oxidative stress. Recent intake surveys have shown that there are large portions of popu-lations in developed countries that show insufficient intakes of vitamin C. More studies will be needed to determine the exact role of the vitamin, for example, in the reduction of oxidative DNA damage and in the modulation of gene expression and cellular function.

Metabolism and deficiency

Though most animals are able to synthesize large quantities of vitamin C, primates lost this capability as a result of a series of inactivating mutations of a gene encoding a key enzyme in the vitamin C biosynthetic pathway about 40 million years ago (1). The richest natural sources of vitamin C are fruits and vegetables, and it is also present in some cuts of meat, especially liver. Vitamin C is absorbed in its reduced state (ascorbic acid) and its oxidized form (dehydroascorbic acid) along the entire length of the human small intestine via a sodium-dependent active transporter (SVCT1) and facilitated diffusion, respectively (2). After absorption, the vitamin circulates mainly unbound and is available as a reducing agent in blood and interstitial fluids. It is directly transported into tissues via a sodium-dependent transporter (SVCT2). The amount of vitamin C that can be absorbed at any given time is limited by the function of SVCT1. Therefore, doses over approximately 400 mg/day result in plasma saturation and urinary excretion of excess absorbed vitamin C (3), particularly if combined with a healthy diet and/or taken more than once per day. At a single 100 mg/day dose, tissue saturation is achieved; however, higher intakes (500 mg/day) are required to achieve plasma saturation and to maximize antioxidant protection (4). A plasma level of 40–60 micromoles of vitamin C is considered to be healthy, but this can be increased further following doses of 250 mg per day or higher (3). Plasma levels under approximately 28 micromoles are considered depleted or marginally deficient and levels below 11 micromoles are considered deficient. The current US recommended intake for vitamin C is 75 mg/day in adult females, 90 mg/day in males, and 125 mg/day for smokers to account for increased oxidative stress and vitamin C turnover (5). However, many experts believe that the recommen-ded intakes for vitamin C are several orders of magnitude too low to support optimal vitamin C functionality (6).

A deficiency of vitamin C can result in physical symptoms, such as swelling of the lower extremities, blee-ding gums, fatigue, and hemorrhaging, as well as psychological problems, including depression, hysteria, and social introversion, which are all characteristics of scurvy (7). As symptoms of the disease are often non-specific or masquerade as other diseases (e.g., cellulitis, vasculitis or arthritis), there is misperception that scurvy is largely a historical disease and rarely observed in developed nations. However, there is still a large portion of the population that shows insufficient intake of vitamin C without overt symptoms. For in-stance vitamin C depletion has been observed in up to 30% of presumed healthy population samples (8-12). Those at risk of developing vitamin C deficiency typically have diets lacking in fresh fruit and vegetables (often associated with poor diet choices or imposed restrictive diet plans). Also, cigarette smokers exhibit decreased plasma vitamin C concentrations despite adequate dietary intake (13) as do individuals with chronic hyperglycemia due to diabetes, sepsis, or stress (14). The elderly are at high risk of insufficient intake for a number of reasons including limited mobility, low income, institutionalization, reduced appetite, and poorer cognitive function. Additionally, adult males consistently exhibit lower plasma vitamin C concen-trations throughout their lives than do their female counterparts. The use of vitamin C as an additive to food and feed to enhance product quality and stability is often erroneously considered an additional vitamin source. However, the antioxidant properties of ascorbic acid as additive are used for its technological functions, such as retardation of oxidative rancidity for fats and oils or protection from enzymatic browning in processed fruits and vegetables, and do not carry out the vitamin’s health maintaining functions.

Natural and synthetic sources of vitamin C appear to be equally bioavailable and provide similar antioxidant protection after ingestion (15). However, the stability of vitamin C in foods is precarious and readily influ-enced by oxygen, heat, pH, and metallic ions, resulting in the oxidation of the vitamin C. The lability of vitamin C in foods is an important consideration since many populations worldwide consume products that are transported, stored, and processed prior to purchase.

Antioxidant and reductive functions

Most of the functions of vitamin C in the human body are related to its role as a reducing agent (electron donor); hence, ascorbic acid is the active, stable form of vitamin C in tissues. When used as a cofactor or antioxidant, ascorbic acid is oxidized to the more unstable dehydroascorbic acid, which is readily recycled back to ascorbic acid by several enzyme systems (16). As an effective reducing (electron donating) agent, vitamin C serves as a powerful antioxidant, scavenging reactive oxygen and nitrogen species in the body. Reactive species are generated by normal cell processes as well as environmental stressors. When their production overwhelms the cellular antioxidant defenses (e.g., enzymes such as superoxide dismutase and catalase) they can cause oxidative damage to lipids, cell proteins, and nucleic acids in DNA. Such oxidative stress is thought to be involved in the development of diseases including atherosclerosis, diabetes, neurode-generative diseases, chronic inflammatory diseases and cancer and in the ageing process (17). Thus, the antioxidant properties of vitamin C can potentially reduce cellular damage.

The ability of vitamin C to prevent oxidative DNA damage is of particular interest because some of these DNA lesions are thought to lead to mutations that can cause the cell to become malignant, and thus cause cancer. Several human studies have investigated potential effects of vitamin C supplementation on preven-ting oxidative damage to the DNA in blood cells by measuring biomarkers such as DNA base lesions and strand breaks (18). Most of the studies showed either a vitamin C-mediated reduction in oxidative DNA damage or no effect. A possible reason for these inconclusive findings might be that most studies measured plasma vitamin C concentrations. However, it is known that blood cells saturate at lower vitamin C concen-trations than plasma (19). Intracellular saturation can be easily achieved from a normal diet. It is therefore likely that an additional increase of plasma vitamin C by supplementation would only produce moderate beneficial effects, which would be difficult to detect, leading to non-significant results. Consistent with this notion, most of the studies that have shown a protective effect were carried out with smokers or patients with pathological conditions (e.g., cancer, diabetes and cataracts) associated with oxidative stress and low plasma vitamin C levels. Is it thus possible that the initial level of vitamin C in the cell predetermines whe-ther supplementation trials may have a positive response or no response (18).

Although the direct antioxidant protection afforded by vitamin C is limited to water-soluble environments, vitamin C does play an antioxidant role in lipids through its regeneration of fat-soluble vitamin E. Vitamin C readily donates an electron to the vitamin E radical to regenerate the active form of vitamin E, alpha-toco-pherol. The antioxidant function of alpha-tocopherol limits lipid peroxidation in the membranes of cells and mitochondria, thereby maintaining cell integrity (20).

In addition, vitamin C functions as an enzyme cofactor in a number of reactions, where it specifically maintains metal ions within these enzymes in a reduced state, required for enzyme activity. Although alternative electron donors can function in these roles, vitamin C is the most effective cofactor for these enzymes (21). Three of these enzymes are involved in the biosynthesis of collagen, the main component of connective tissue. In these enzymes, vitamin C ensures that iron ions remain in the reduced ferrous state.

Vitamin C also serves as an iron reducing agent in two enzymes involved in the production of carnitine. Carnitine is required for the transport of fatty acids from the intracellular fluid into the mitochondria during the breakdown of lipids for the generation of metabolic energy (22). During the biosynthesis of the amino acid tyrosine, vitamin C keeps the iron and copper of two enzymes in a reduced state. In addition to this, the production of certain neurotransmitters (the conversion of dopamine to norepinephrine) and hormones (including calcitonin, oxytocin and vasopressin) require vitamin C to keep copper in its cuprous form for enzyme activity (23). Furthermore, it has been shown that vitamin C keeps the cofactor of the enzyme ‘nitric oxide synthase’ in a reduced, active state which may prevent atherosclerosis-promoting processes (24).

In the intestinal tract, vitamin C enhances iron bioavailability by keeping non-heme iron in the ferrous state. It also promotes enzyme activity further contributing to the absorption potential of dietary iron (25). In tis-sues, vitamin C up-regulates the synthesis of the iron storing protein ferritin, thereby increasing intracellular iron storage and preventing iron-induced oxidative damage within cells (26). These data provide strong evi-dence that vitamin C has a potent regulatory influence on iron metabolism.

Moreover, new in vitro research indicates that vitamin C is able to affect redox-sensitive signaling pathways. Reactive oxygen species are now recognized as important signaling molecules that may influence cell proli-feration, cell death and the expression of genes, besides being involved in the activation of several signaling cascades such as inflammatory signaling pathways (27). The relevance of these effects in vivo is currently unknown and subject to future investigations.

Other functions

Due to its chemical structure, vitamin C (ascorbic acid) is an electron donor and therefore a reducing agent. It thus has two different biochemical roles: as an antioxidant and as a cofactor for enzymes involved, for example, in collagen and carnitine synthesis. This enzymatic property has an impact on cell metabolism that could influence gene expression. However, these mechanisms are probably not directly involved in the control of gene expression and cell signaling. New research indicates that ascorbic acid may be involved in cell differentiation: vitamin C seems to promote a differentiation process in embryonic stem cells (28). This effect is specific to vitamin C and is not shared with other antioxidants, so antioxidant mechanisms are probably not involved in this process. The action of vitamin C on cell division is seen as a promising area for further research. It is known that the intracellular vitamin C concentration varies according to specific body tissues in adults– some tissues have high concentrations, others have much lower ones (29). If the same is true in embryos, the division rate could be affected by the local vitamin concentration. As the progression and inhibition of cell division are crucial during embryogenesis and cell differentiation, it could be hypothe-sized that the local vitamin C concentration influences these processes. This difference in vitamin C concen-tration is probably linked to the concentration of the transporter SVCT2 (30), which may act as both a transporter and a receptor. The local vitamin C concentration could modulate the expression of a battery of genes and influence cell division (31). An alternative mechanism may be the direct action of vitamin C on the expression of genes involved in development.

It has been suggested that cAMP-dependent pathways could function as signaling pathways. Increases in concentration of cyclic adenosine monophosphate (cAMP) are thought to lead to the activation of the enzyme protein kinase A, which can modulate transcription factors, regulating gene expression. Vitamin C has been suggested to be a global regulator of the intracellular cAMP pool (32). Therefore, the vitamin could modulate the expression of a battery of genes (e.g., those involved in DNA repair) expressed under the control of cAMP-dependent pathways.

Studies on disease prevention

The chemical properties and potential functions of vitamin C have led to the hypothesis that increased intake of vitamin C may prevent chronic diseases such as cardiovascular disease and cancer as well as age-related cognitive decline and respiratory tract infections. Despite this strong rationale, results from intervention studies investigating the efficacy of a vitamin C-rich diet or supplements are contradictory. The lack of consensus in the literature may be due to erroneous or inappropriate analysis or classification of data (33). Study results need to be evaluated whilst paying careful attention to potential methodological biases. For example, the assessment of study participants’ vitamin C status using measuring blood concentrations is more precise than calculations based on dietary intake questionnaires. In addition, it needs to be considered that measuring plasma vitamin C concentrations does not necessarily reflect the intracellular status which is relevant for potential vitamin effects in the cell. As blood cells, for example, saturate at lower vitamin C concentrations than plasma (19) and intracellular saturation can be easily obtained from a normal diet, it is likely that additional increase of plasma vitamin C by supplementation would only show small effects, lea-ding to non-significant results. Thus, it is crucial to consider the study participants’ initial status of vitamin C. The absence of such critical information precludes the drawing of appropriate conclusions. Most of the studies that have shown a preventative effect against disease were carried out on subjects with oxidative stress and low plasma vitamin C levels, and not on well-nourished individuals (18).

Clinical trials show mixed results regarding the role of supplemental vitamin C in reducing cardiovascular disease risk. A large-scale randomized controlled trial in more than 14, 000 men did not show a beneficial effect of supplemental vitamin C (500 mg/day for 8 years) for myocardial infarction, total stroke, or cardio-vascular mortality (34). In smaller clinical trials, vitamin C supplementation (500 to 1000 mg/day for up to 8 weeks) was associated with reduced systolic and diastolic blood pressure, reduced systemic arterial stiff-ness, and reduced elevated C-reactive protein concentrations in blood, a biomarker for inflammation (35-37). Moreover, increased intake of vitamin C enhanced endothelium-dependent dilation of blood vessels from 40% to 180%, which provides theoretical mechanisms for the reported ability of supplemental vitamin C to reduce cardiovascular disease risk (38,39).

High intakes of vitamin C have been associated with a decreased risk of certain cancers, particularly cancers of the pharynx, oral cavity, oesophagus, lung, and stomach (40). Although the anticarcinogenic effects of vitamin C are not well defined, it is thought that the antioxidant properties of vitamin C protect against molecular damage (e.g., oxidative DNA lesions) that is associated with cancer development and/or that vitamin C may modulate signal transduction and gene expression (41). Randomized controlled trials have not demonstrated a benefit for supplemental vitamin C in cancer prevention (42-44). Meta-analyses indicate that individuals with high intakes of vitamin C are at reduced risk of oesophageal cancer (45), lung cancer (46) and breast cancer (47). However, these analyses examined only relationships between diet and cancer risk and cannot distinguish if the relationship is specific to dietary vitamin C or related to other com-ponents in vitamin C-rich fruits and vegetables.

There is a large body of evidence supporting the fact that maintaining adequate vitamin C levels can pre-vent age-related cognitive decline and Alzheimer’s disease, but avoiding vitamin C deficiency is likely to be more beneficial than taking supplements on top of a normal, healthy diet. The optimal level of vitamin C intake for brain function is unknown, but vitamin C plays a critical role in brain development and protection across the life-span (48). It is becoming increasingly clear that oxidative stress generated by reactive oxygen species is a critical component in the development of dementias of the Alzheimer’s type and other neurodegenerative disorders, which has been shown in human studies (49,50). Several studies on nutrition and cognition in people showing healthy and abnormal aging processes have been reviewed in detail numerous times (51–53). Such reviews generally conclude that there is some supporting evidence for the advantages of using dietary fruit and vegetables and vitamin C and E supplements to slow cognitive deterioration, but that support is not universal (33).

Vitamin C is thought to reduce the duration and severity of common cold symptoms by enhancing immune responses and counteracting histamine, a mediator for the common symptoms of colds and allergy. Meta-analyses of randomized controlled trials have shown modest beneficial effects of vitamin C supplementation for reducing common cold duration (8% to 14%) and severity – as indicated by days confined to home and off work or school (54,55). The most pronounced benefit of vitamin C supplementation for reducing cold incidence and severity has been demonstrated in populations experiencing extreme physical stress such as athletes (56,57).

Individuals who supplement vitamin C regularly have been shown to maintain higher plasma concentrations of the vitamin (58), and the long-term safety of vitamin C supplementation seems evident as several large investigations have indicated a reduced risk of mortality in populations who use vitamin C supplements (59,60) and in populations with elevated plasma vitamin C concentrations (61). However, the link between vitamin C and diseases, such as cardiovascular disease, various cancers forms, age-related cognitive decline or the common cold has not been clearly established. More carefully planned trials are warranted to determine if vitamin C can play a protective or even a therapeutic role in these conditions.