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Vitamin C is an essential micronutrient for humans, with pleiotropic functions related to its ability to donate electrons. It is a potent antioxidant and a cofactor for a family of biosynthetic and gene regulatory enzymes. Vitamin C contributes to immune defense by supporting various cellular functions of both the innate and adaptive immune system. Vitamin C supports epithelial barrier function against pathogens and promotes the oxidant scavenging activity of the skin, thereby potentially protecting against environmental oxidative stress. Vitamin C accumulates in phagocytic cells, such as neutrophils, and can enhance chemotaxis, phagocytosis, generation of reactive oxygen species, and ultimately microbial killing. It is also needed for apoptosis and clearance of the spent neutrophils from sites of infection by macrophages, thereby decreasing necrosis/NETosis and potential tissue damage. The role of vitamin C in lymphocytes is less clear, but it has been shown to enhance differentiation and proliferation of B- and T-cells, likely due to its gene regulating effects. Vitamin C deficiency results in impaired immunity and higher susceptibility to infections. In turn, infections significantly impact on vitamin C levels due to enhanced inflammation and metabolic requirements. Furthermore, supplementation with vitamin C appears to be able to both prevent and treat respiratory and systemic infections. Prophylactic prevention of infection requires dietary vitamin C intakes that provide at least adequate, if not saturating plasma levels (i.e., 100–200 mg/day), which optimize cell and tissue levels. In contrast, treatment of established infections requires significantly higher (gram) doses of the vitamin to compensate for the increased inflammatory response and metabolic demand.
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nutrients
Review
Vitamin C and Immune Function
Anitra C. Carr 1, * and Silvia Maggini 2
1
Department of Pathology, University of Otago, Christchurch, P.O. Box 4345, Christchurch 8140, New Zealand
2Bayer Consumer Care Ltd., Peter-Merian-Strasse 84, 4002 Basel, Switzerland; silvia.maggini@bayer.com
*Correspondence: anitra.carr@otago.ac.nz; Tel.: +643-364-0649
Received: 21 September 2017; Accepted: 31 October 2017; Published: 3 November 2017
Abstract:
Vitamin C is an essential micronutrient for humans, with pleiotropic functions related to
its ability to donate electrons. It is a potent antioxidant and a cofactor for a family of biosynthetic
and gene regulatory enzymes. Vitamin C contributes to immune defense by supporting various
cellular functions of both the innate and adaptive immune system. Vitamin C supports epithelial
barrier function against pathogens and promotes the oxidant scavenging activity of the skin, thereby
potentially protecting against environmental oxidative stress. Vitamin C accumulates in phagocytic
cells, such as neutrophils, and can enhance chemotaxis, phagocytosis, generation of reactive oxygen
species, and ultimately microbial killing. It is also needed for apoptosis and clearance of the spent
neutrophils from sites of infection by macrophages, thereby decreasing necrosis/NETosis and
potential tissue damage. The role of vitamin C in lymphocytes is less clear, but it has been shown to
enhance differentiation and proliferation of B- and T-cells, likely due to its gene regulating effects.
Vitamin C deficiency results in impaired immunity and higher susceptibility to infections. In turn,
infections significantly impact on vitamin C levels due to enhanced inflammation and metabolic
requirements. Furthermore, supplementation with vitamin C appears to be able to both prevent and
treat respiratory and systemic infections. Prophylactic prevention of infection requires dietary vitamin
C intakes that provide at least adequate, if not saturating plasma levels (i.e., 100–200 mg/day), which
optimize cell and tissue levels. In contrast, treatment of established infections requires significantly
higher (gram) doses of the vitamin to compensate for the increased inflammatory response and
metabolic demand.
Keywords:
ascorbate; ascorbic acid; immunity; immune system; neutrophil function; microbial
killing; lymphocytes; infection; vitamin C
1. Introduction
The immune system is a multifaceted and sophisticated network of specialized organs, tissues,
cells, proteins, and chemicals, which has evolved in order to protect the host from a range of pathogens,
such as bacteria, viruses, fungi, and parasites, as well as cancer cells [
1
]. It can be divided into epithelial
barriers, and cellular and humoral constituents of either innate (non-specific) and acquired (specific)
immunity [
1
]. These constituents interact in multiple and highly complex ways. More than half
a century of research has shown vitamin C to be a crucial player in various aspects of the immune
system, particularly immune cell function [2,3].
Vitamin C is an essential nutrient which cannot be synthesized by humans due to loss of a key
enzyme in the biosynthetic pathway [
4
,
5
]. Severe vitamin C deficiency results in the potentially fatal
disease scurvy [
6
]. Scurvy is characterized by weakening of collagenous structures, resulting in poor
wound healing, and impaired immunity. Individuals with scurvy are highly susceptible to potentially
fatal infections such as pneumonia [
7
]. In turn, infections can significantly impact on vitamin C levels
due to enhanced inflammation and metabolic requirements. Early on, it was noted that scurvy often
Nutrients 2017,9, 1211; doi:10.3390/nu9111211 www.mdpi.com/journal/nutrients
Nutrients 2017,9, 1211 2 of 25
followed infectious epidemics in populations [
7
], and cases of scurvy have been reported following
respiratory infection [8]. This is particularly apparent for individuals who are already malnourished.
Although the amount of vitamin C required to prevent scurvy is relatively low (i.e.,
~10 mg/day) [
9
], the recommended dietary intakes for vitamin C are up to one hundred-fold
higher than that for many other vitamins [
10
]. A diet that supplies 100–200 mg/day of vitamin C
provides adequate to saturating plasma concentrations in healthy individuals and should cover
general requirements for the reduction of chronic disease risk [
11
,
12
]. Due to the low storage capacity
of the body for the water-soluble vitamin, a regular and adequate intake is required to prevent
hypovitaminosis C. Epidemiological studies have indicated that hypovitaminosis C (plasma vitamin C
< 23
µ
mol/L) is relatively common in Western populations, and vitamin C deficiency (<11
µ
mol/L)
is the fourth leading nutrient deficiency in the United States [
13
,
14
]. There are several reasons why
vitamin C dietary recommendations are not met, even in countries where food availability and supply
would be expected to be sufficient. These include poor dietary habits, life-stages and/or lifestyles
either limiting intakes or increasing micronutrient requirements (e.g., smoking and alcohol or drug
abuse), various diseases, exposure to pollutants and smoke (both active and passive), and economic
reasons (poor socioeconomic status and limited access to nutritious food) [
15
,
16
]. Even otherwise
‘healthy’ individuals in industrialized countries can be at risk due to lifestyle-related factors, such
as those on a diet or eating an unbalanced diet, and people facing periods of excessive physical or
psychological stress [15,16].
Vitamin C has a number of activities that could conceivably contribute to its immune-modulating
effects. It is a highly effective antioxidant, due to its ability to readily donate electrons, thus protecting
important biomolecules (proteins, lipids, carbohydrates, and nucleic acids) from damage by oxidants
generated during normal cell metabolism and through exposure to toxins and pollutants (e.g.,
cigarette smoke) [
17
]. Vitamin C is also a cofactor for a family of biosynthetic and gene regulatory
monooxygenase and dioxygenase enzymes [
18
,
19
]. The vitamin has long been known as a cofactor
for the lysyl and prolyl hydroxylases required for stabilization of the tertiary structure of collagen,
and is a cofactor for the two hydroxylases involved in carnitine biosynthesis, a molecule required for
transport of fatty acids into mitochondria for generation of metabolic energy (Figure 1) [19].
Nutrients 2016, 8, 1211 2 of 24
reported following respiratory infection [8]. This is particularly apparent for individuals who are
already malnourished.
Although the amount of vitamin C required to prevent scurvy is relatively low (i.e., ~10 mg/day)
[9], the recommended dietary intakes for vitamin C are up to one hundred-fold higher than that for
many other vitamins [10]. A diet that supplies 100–200 mg/day of vitamin C provides adequate to
saturating plasma concentrations in healthy individuals and should cover general requirements for
the reduction of chronic disease risk [11,12]. Due to the low storage capacity of the body for the water-
soluble vitamin, a regular and adequate intake is required to prevent hypovitaminosis C.
Epidemiological studies have indicated that hypovitaminosis C (plasma vitamin C < 23 μmol/L) is
relatively common in Western populations, and vitamin C deficiency (<11 μmol/L) is the fourth
leading nutrient deficiency in the United States [13,14]. There are several reasons why vitamin C
dietary recommendations are not met, even in countries where food availability and supply would
be expected to be sufficient. These include poor dietary habits, life-stages and/or lifestyles either
limiting intakes or increasing micronutrient requirements (e.g., smoking and alcohol or drug abuse),
various diseases, exposure to pollutants and smoke (both active and passive), and economic reasons
(poor socioeconomic status and limited access to nutritious food) [15,16]. Even otherwise ‘healthy’
individuals in industrialized countries can be at risk due to lifestyle-related factors, such as those on
a diet or eating an unbalanced diet, and people facing periods of excessive physical or psychological
stress [15,16].
Vitamin C has a number of activities that could conceivably contribute to its immune-modulating
effects. It is a highly effective antioxidant, due to its ability to readily donate electrons, thus protecting
important biomolecules (proteins, lipids, carbohydrates, and nucleic acids) from damage by oxidants
generated during normal cell metabolism and through exposure to toxins and pollutants (e.g., cigarette
smoke) [17]. Vitamin C is also a cofactor for a family of biosynthetic and gene regulatory
monooxygenase and dioxygenase enzymes [18,19]. The vitamin has long been known as a cofactor for
the lysyl and prolyl hydroxylases required for stabilization of the tertiary structure of collagen, and is a
cofactor for the two hydroxylases involved in carnitine biosynthesis, a molecule required for transport
of fatty acids into mitochondria for generation of metabolic energy (Figure 1) [19].
Figure 1. The enzyme cofactor activities of vitamin C. Vitamin C is a cofactor of a family of
biosynthetic and gene regulatory monooxygenase and dioxygenase enzymes. These enzymes are
involved in the synthesis of collagen, carnitine, catecholamine hormones, e.g., norepinephrine, and
amidated peptide hormones, e.g., vasopressin. These enzymes also hydroxylate transcription factors,
e.g., hypoxia-inducible factor 1α, and methylated DNA and histones, thus playing a role in gene
transcription and epigenetic regulation. indicates an increase and indicates a decrease.
Figure 1.
The enzyme cofactor activities of vitamin C. Vitamin C is a cofactor of a family of
biosynthetic and gene regulatory monooxygenase and dioxygenase enzymes. These enzymes are
involved in the synthesis of collagen, carnitine, catecholamine hormones, e.g., norepinephrine, and
amidated peptide hormones, e.g., vasopressin. These enzymes also hydroxylate transcription factors,
e.g., hypoxia-inducible factor 1
α
, and methylated DNA and histones, thus playing a role in gene
transcription and epigenetic regulation. indicates an increase and indicates a decrease.
Nutrients 2017,9, 1211 3 of 25
Vitamin C is also a cofactor for the hydroxylase enzymes involved in the synthesis of catecholamine
hormones, e.g., norepinephrine, and amidated peptide hormones e.g., vasopressin, which are central
to the cardiovascular response to severe infection [
20
]. Furthermore, research over the past 15 years
or so has uncovered new roles for vitamin C in the regulation of gene transcription and cell signaling
pathways through regulation of transcription factor activity and epigenetic marks (Figure 1) [
21
,
22
].
For example, the asparagyl and prolyl hydroylases required for the downregulation of the pleiotropic
transcription factor hypoxia-inducible factor-1
α
(HIF-1
α
) utilize vitamin C as a cofactor [
21
]. Recent
research has also indicated an important role for vitamin C in regulation of DNA and histone
methylation by acting as a cofactor for enzymes which hydoxylate these epigenetic marks [22].
Our review explores the various roles of vitamin C in the immune system, including barrier
integrity and leukocyte function, and discusses potential mechanisms of action. We discuss
the relevance of the immune-modulating effects of vitamin C in the context of infections and conditions
leading to vitamin C insufficiency.
2. Barrier Integrity and Wound Healing
The skin has numerous essential functions, the primary of which is to act as a barrier against
external insults, including pathogens. The epidermal layer is highly cellular, comprising primarily
keratinocytes, whilst the dermal layer comprises fibroblasts which secrete collagen fibers, the major
component of the dermis [
23
]. Skin contains millimolar concentrations of vitamin C, with higher levels
found in the epidermis than the dermis [
24
26
]. Vitamin C is actively accumulated into the epidermal
and dermal cells via the two sodium-dependent vitamin C transporter (SVCT) isoforms 1 and 2 [
27
],
suggesting that the vitamin has crucial functions within the skin. Clues to the role of vitamin C in
the skin come from the symptoms of the vitamin C deficiency disease scurvy, which is characterized
by bleeding gums, bruising, and impaired wound healing [
28
,
29
]. These symptoms are thought to
be a result of the role of vitamin C as a co-factor for the prolyl and lysyl hydroxylase enzymes that
stabilize the tertiary structure of collagen (Table 1) [
30
]. Further research has shown that vitamin C can
also increase collagen gene expression in fibroblasts [3135].
Table 1. Role of vitamin C in immune defense.
Immune System Function of Vitamin C Refs.
Epithelial barriers
Enhances collagen synthesis and stabilization [3035]
Protects against ROS-induced damage 1[3640]
Enhances keratinocyte differentiation and lipid synthesis
[4145]
Enhances fibroblast proliferation and migration [46,47]
Shortens time to wound healing in patients [48,49]
Phagocytes (neutrophils,
macrophages)
Acts as an antioxidant/electron donor [5053]
Enhances motility/chemotaxis [5463]
Enhances phagocytosis and ROS generation [6471]
Enhances microbial killing [54,55,57,58,70,72]
Facilitates apoptosis and clearance [71,73,74]
Decreases necrosis/NETosis [73,75]
B- and T-lymphocytes Enhances differentiation and proliferation [62,63,7682]
Enhances antibody levels [78,8385]
Inflammatory mediators Modulates cytokine production [75,77,8694]
Decreases histamine levels [56,61,95101]
1
ROS, reactive oxygen species; NET, neutrophil extracellular trap. Note that many of these studies comprised
marginal or deficient vitamin C status at baseline. Supplementation in situations of adequate vitamin C status may
not have comparable effects.
Vitamin C intervention studies in humans (using both dietary and gram doses of vitamin C) have
shown enhanced vitamin C uptake into skin cells [
26
,
36
] and enhanced oxidant scavenging activity
Nutrients 2017,9, 1211 4 of 25
of the skin [
36
,
37
]. The elevated antioxidant status of the skin following vitamin C supplementation
could potentially protect against oxidative stress induced by environmental pollutants [
38
,
39
].
The antioxidant effects of vitamin C are likely to be enhanced in combination with vitamin E [
40
,
102
].
Cell culture and preclinical studies have indicated that vitamin C can enhance epithelial barrier
functions via a number of different mechanisms. Vitamin C supplementation of keratinocytes in
culture enhances differentiation and barrier function via modulating signaling and biosynthetic
pathways, with resultant elevations in barrier lipid synthesis [
41
45
]. Dysfunctional epithelial
barrier function in the lungs of animals with serious infection can be restored by administration
of vitamin C [
74
]. This was attributed to enhanced expression of tight junction proteins and prevention
of cytoskeletal rearrangements.
Animal studies using the vitamin C-dependent Gulo knockout mouse indicated that deficiency
did not affect the formation of collagen in the skin of unchallenged mice [
103
]; however,
following full thickness excisional wounding there was significantly decreased collagen formation
in vitamin C-deficient mice [
46
]. This finding is in agreement with an earlier study carried out with
scorbutic guinea pigs [
104
]. Thus, vitamin C appears to be particularly essential during wound healing,
also decreasing the expression of pro-inflammatory mediators and enhancing the expression of various
wound healing mediators [
46
]. Fibroblast cell culture experiments have also indicated that vitamin C
can alter gene expression profiles within dermal fibroblasts, promoting fibroblast proliferation and
migration which is essential for tissue remodeling and wound healing [
46
,
47
]. Following surgery,
patients require relatively high intakes of vitamin C in order to normalize their plasma vitamin C status
(e.g., 500 mg/day) [105], and administration of antioxidant micronutrients, including vitamin C, to
patients with disorders in wound healing can shorten the time to wound closure [48,49,106,107].
Leukocytes, particularly neutrophils and monocyte-derived macrophages, are major players in
wound healing [
108
]. During the initial inflammatory stage, neutrophils migrate to the wound site in
order to sterilize it via the release of reactive oxygen species (ROS) and antimicrobial proteins [
109
].
The neutrophils eventually undergo apoptosis and are cleared by macrophages, resulting in resolution
of the inflammatory response. However, in chronic, non-healing wounds, such as those observed
in diabetics, the neutrophils persist and instead undergo necrotic cell death which can perpetuate
the inflammatory response and hinder wound healing [
109
,
110
]. Vitamin C is thought to influence
several important aspects of neutrophil function: migration in response to inflammatory mediators
(chemotaxis), phagocytosis and killing of microbes, and apoptosis and clearance by macrophages
(see below).
3. Vitamin C and Leukocyte Function
Leukocytes, such as neutrophils and monocytes, actively accumulate vitamin C against a concentration
gradient, resulting in values that are 50- to 100-fold higher than plasma concentrations
[111113]
. These cells
accumulate maximal vitamin C concentrations at dietary intakes of ~100 mg/day
[114,115],
although
other body tissues likely require higher intakes for saturation
[116,117]
. Neutrophils accumulate
vitamin C via SVCT2 and typically contain intracellular levels of at least 1 mM [
111
,
118
]. Following
stimulation of their oxidative burst neutrophils can further increase their intracellular concentration of
vitamin C through the non-specific uptake of the oxidized form, dehydroascorbate (DHA), via glucose
transporters (GLUT)
[118,119]
. DHA is then rapidly reduced to ascorbate intracellularly, to give levels
of about 10 mM [
119
]. It is believed that the accumulation of such high vitamin C concentrations
indicates important functions within these cells.
Accumulation of millimolar concentrations of vitamin C into neutrophils, particularly following
activation of their oxidative burst, is thought to protect these cells from oxidative damage [
119
].
Vitamin C is a potent water-soluble antioxidant that can scavenge numerous reactive oxidants and can
also regenerate the important cellular and membrane antioxidants glutathione and vitamin E [
120
].
Upon phagocytosis or activation with soluble stimulants, vitamin C is depleted from neutrophils in
an oxidant-dependent manner [
50
53
]. An alteration in the balance between oxidant generation and
Nutrients 2017,9, 1211 5 of 25
antioxidant defenses can lead to alterations in multiple signaling pathways, with the pro-inflammatory
transcription factor nuclear factor
κ
B (NF
κ
B) playing a central role [
121
]. Oxidants can activate
NF
κ
B, which triggers a signaling cascade leading to continued synthesis of oxidative species and other
inflammatory mediators [
122
,
123
]. Vitamin C has been shown to attenuate both oxidant generation and
NF
κ
B activation in dendritic cells
in vitro
, and NF
κ
B activation in neutrophils isolated from septic Gulo
knockout mice [
75
,
124
]. Thiol-containing proteins can be particularly sensitive to redox alterations
within cells and are often central to the regulation of redox-related cell signaling pathways [
125
].
Vitamin C-dependent modulation of thiol-dependent cell signaling and gene expression pathways has
been reported in T-cells [126,127].
Thus, vitamin C could modulate immune function through modulation of redox-sensitive cell
signaling pathways or by directly protecting important cell structural components. For example,
exposure of neutrophils to oxidants can inhibit motility of the cells, which is thought to be due to
oxidation of membrane lipids and resultant effects on cell membrane fluidity [
63
]. Neutrophils contain
high levels of polyunsaturated fatty acids in their plasma membranes, and thus improvements in
neutrophil motility observed following vitamin C administration (see below) could conceivably be
attributed to oxidant scavenging as well as regeneration of vitamin E [120].
3.1. Neutrophil Chemotaxis
Neutrophil infiltration into infected tissues is an early step in innate immunity. In response
to pathogen- or host-derived inflammatory signals (e.g., N-formylmethionyl-leucyl-phenylalanine
(fMLP), interleukin (IL)-8, leukotriene B4, and complement component C5a), marginated neutrophils
literally swarm to the site of infection [
128
]. Migration of neutrophils in response to chemical stimuli is
termed chemotaxis, while random migration is termed chemokinesis (Figure 2). Neutrophils express
more than 30 different chemokine and chemoattractant receptors in order to sense and rapidly respond
to tissue damage signals [
128
]. Early studies carried out in scorbutic guinea pigs indicated impaired
leukocyte chemotactic response compared with leukocytes isolated from guinea pigs supplemented
with adequate vitamin C in their diet (Table 1) [
54
56
,
64
]. These findings suggest that vitamin C
deficiency may impact on the ability of phagocytes to migrate to sites of infection.
Patients with severe infection exhibit compromised neutrophil chemotactic ability [
129
132
].
This neutrophil ‘paralysis’ is believed to be partly due to enhanced levels of anti-inflammatory and
immune-suppressive mediators (e.g., IL-4 and IL-10) during the compensatory anti-inflammatory
response observed following initial hyper-stimulation of the immune system [
133
]. However, it is also
possible that vitamin C depletion, which is prevalent during severe infection [
20
], may contribute.
Studies in the 1980s and 1990s indicated that patients with recurrent infections had impaired
leukocyte chemotaxis, which could be restored in response to supplementation with gram doses
of vitamin C [
57
60
,
65
67
]. Furthermore, supplementation of neonates with suspected sepsis with
400 mg/day vitamin C dramatically improved neutrophil chemotaxis [134].
Recurrent infections can also result from genetic disorders of neutrophil function, such as
chronic granulomatous disease (CGD), an immunodeficiency disease resulting in defective leukocyte
generation of ROS [
135
], and Chediak-Higashi syndrome (CHS), a rare autosomal recessive disorder
affecting vesicle trafficking [
136
]. Although vitamin C administration would not be expected to
affect the underlying defects of these genetic disorders, it may support the function of redundant
antimicrobial mechanisms in these cells. For example, patients with CGD showed improved
leukocyte chemotaxis following supplementation with gram doses of vitamin C administered either
enterally or parenterally [
137
139
]. This was associated with decreased infections and clinical
improvement [
137
,
138
]. A mouse model of CHS showed improved neutrophil chemotaxis following
vitamin C supplementation [
140
], and neutrophils isolated from two children with CHS showed
improved chemotaxis following supplementation with 200–500 mg/day vitamin C [
141
,
142
], although
this effect has not been observed in all cases [
140
,
143
]. The vitamin C-dependent enhancement of
Nutrients 2017,9, 1211 6 of 25
chemotaxis was thought to be mediated in part via effects on microtubule assembly [
144
,
145
], and
more recent research has indicated that intracellular vitamin C can stabilize microtubules [146].
Nutrients 2016, 8, 1211 6 of 24
[144,145], and more recent research has indicated that intracellular vitamin C can stabilize
microtubules [146].
Figure 2. Role of vitamin C in phagocyte function. Vitamin C has been shown to: (a) enhance
neutrophil migration in response to chemoattractants (chemotaxis), (b) enhance engulfment
(phagocytosis) of microbes, and (c) stimulate reactive oxygen species (ROS) generation and killing of
microbes. (d) Vitamin C supports caspase-dependent apoptosis, enhancing uptake and clearance by
macrophages, and inhibits necrosis, including NETosis, thus supporting resolution of the
inflammatory response and attenuating tissue damage.
Supplementation of healthy volunteers with dietary or gram doses of vitamin C has also been
shown to enhance neutrophil chemotactic ability [61–63,147]. Johnston et al., proposed that the
antihistamine effect of vitamin C correlated with enhanced chemotaxis [61]. In participants who had
inadequate vitamin C status (i.e., <50 μM), supplementation with a dietary source of vitamin C
(providing ~250 mg/day) resulted in a 20% increase in neutrophil chemotaxis [147]. Furthermore,
supplementation of elderly women with 1 g/day vitamin C, in combination with vitamin E, enhanced
neutrophil functions, including chemotaxis [148]. Thus, members of the general population may
benefit from improved immune cell function through enhanced vitamin C intake, particularly if they
have inadequate vitamin C status, which can be more prevalent in the elderly. However, it should be
noted that it is not yet certain to what extent improved ex vivo leukocyte chemotaxis translates into
improved in vivo immune function.
3.2. Phagocytosis and Microbial Killing
Once neutrophils have migrated to the site of infection, they proceed to engulf the invading
pathogens (Figure 2). Various intracellular granules are mobilized and fuse with the phagosome,
emptying their arsenal of antimicrobial peptides and proteins into the phagosome [149]. Components
of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase assemble in the phagosomal
Figure 2.
Role of vitamin C in phagocyte function. Vitamin C has been shown to: (
a
) enhance neutrophil
migration in response to chemoattractants (chemotaxis), (
b
) enhance engulfment (phagocytosis)
of microbes, and (
c
) stimulate reactive oxygen species (ROS) generation and killing of microbes.
(
d
) Vitamin C supports caspase-dependent apoptosis, enhancing uptake and clearance by macrophages,
and inhibits necrosis, including NETosis, thus supporting resolution of the inflammatory response and
attenuating tissue damage.
Supplementation of healthy volunteers with dietary or gram doses of vitamin C has also
been shown to enhance neutrophil chemotactic ability [
61
63
,
147
]. Johnston et al., proposed that
the antihistamine effect of vitamin C correlated with enhanced chemotaxis [
61
]. In participants who
had inadequate vitamin C status (i.e., <50
µ
M), supplementation with a dietary source of vitamin C
(providing ~250 mg/day) resulted in a 20% increase in neutrophil chemotaxis [
147
]. Furthermore,
supplementation of elderly women with 1 g/day vitamin C, in combination with vitamin E, enhanced
neutrophil functions, including chemotaxis [
148
]. Thus, members of the general population may
benefit from improved immune cell function through enhanced vitamin C intake, particularly if they
have inadequate vitamin C status, which can be more prevalent in the elderly. However, it should be
noted that it is not yet certain to what extent improved ex vivo leukocyte chemotaxis translates into
improved in vivo immune function.
3.2. Phagocytosis and Microbial Killing
Once neutrophils have migrated to the site of infection, they proceed to engulf the invading
pathogens (Figure 2). Various intracellular granules are mobilized and fuse with the phagosome,
Nutrients 2017,9, 1211 7 of 25
emptying their arsenal of antimicrobial peptides and proteins into the phagosome [
149
]. Components
of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase assemble in the phagosomal
membrane and generate superoxide, the first in a long line of ROS generated by neutrophils to kill
pathogens. The enzyme superoxide dismutase converts superoxide to hydrogen peroxide, which
can then be utilized to form the oxidant hypochlorous acid via the azurophilic granule enzyme
myeloperoxidase [
149
]. Hypochlorous acid can further react with amines to form secondary oxidants
known as chloramines. These various neutrophil-derived oxidants have different reactivities and
specificities for biological targets, with protein thiol groups being particularly susceptible.
Neutrophils isolated from scorbutic guinea pigs exhibit a severely impaired ability to kill
microbes [
54
,
55
,
70
], and studies have indicated impaired phagocytosis and/or ROS generation in
neutrophils from scorbutic compared with ascorbate replete animals [
68
70
]. Generation of ROS by
neutrophils from volunteers with inadequate vitamin C status can be enhanced by 20% following
supplementation with a dietary source of vitamin C [
147
], and increases in both phagocytosis
and oxidant generation were observed following supplementation of elderly participants with
a combination of vitamins C and E [
148
]. Patients with recurrent infections [
57
,
58
,
66
,
67
,
72
], or the
genetic conditions CGD or CHS [
138
,
139
,
141
,
143
,
150
], have impaired neutrophil bacterial killing
and/or phagocytosis, which can be significantly improved following supplementation with gram
doses of vitamin C, resulting in long lasting clinical improvement. A couple of studies, however,
showed no improvement of ex vivo anti-fungal or anti-bacterial activity in neutrophils isolated from
CGD or CHS patients supplemented with vitamin C [
140
,
151
]. The reason for these differences
is not clear, although it may depend on the baseline vitamin C level of the patients, which is not
assessed in most cases. Furthermore, different microbes have variable susceptibility to the oxidative
and non-oxidative anti-microbial mechanisms of neutrophils. For example, Staphylococcus aureus
is susceptible to oxidative mechanisms, whereas other microorganisms are more susceptible to
non-oxidative mechanisms [
152
]. Therefore, the type of microbe used to assess the ex vivo neutrophil
functions could influence the findings.
Patients with severe infection (sepsis) exhibit a decreased ability to phagocytose microbes and
a diminished ability to generate ROS [
153
]. Decreased neutrophil phagocytosis was associated with
enhanced patient mortality [
154
]. Interestingly, Stephan et al. [
155
] observed impaired neutrophil
killing activity in critically ill patients prior to acquiring nosocomial infections, suggesting that
critical illness itself, without prior infection, can also impair neutrophil function. This resulted in
subsequent susceptibility to hospital-acquired infections. Impaired phagocytic and oxidant-generating
capacity of leukocytes in patients with severe infection has been attributed to the compensatory
anti-inflammatory response, resulting in enhanced levels of immunosuppressive mediators such as
IL-10 [
133
], as well as to the hypoxic conditions of inflammatory sites, which diminishes substrate for
ROS generation [
156
]. Another explanation is the larger numbers of immature neutrophils released
from the bone marrow due to increased demands during severe infection. These immature ‘band’
cells have decreased functionality compared with differentiated neutrophils [
157
]. Thus, conflicting
findings in severe infection could be due to variability in the total numbers of underactive immature
neutrophils compared with activated fully-differentiated neutrophils [
158
,
159
]. Despite displaying
an activated basal state, the mature neutrophils from patients with severe infection do not generate
ROS to the same extent as healthy neutrophils following ex vivo stimulation [
160
]. The effect of
vitamin C supplementation on phagocytosis, oxidant generation, and microbial killing by leukocytes
from septic patients has not yet been explored.
3.3. Neutrophil Apoptosis and Clearance
Following microbial phagocytosis and killing, neutrophils undergo a process of programmed
cell death called apoptosis [
161
]. This process facilitates subsequent phagocytosis and clearance of
the spent neutrophils from sites of inflammation by macrophages, thus supporting resolution of
inflammation and preventing excessive tissue damage (Figure 2). Caspases are key effector enzymes
Nutrients 2017,9, 1211 8 of 25
in the apoptotic process, culminating in phosphatidyl serine exposure, thus marking the cells for
uptake and clearance by macrophages [
162
]. Interestingly, caspases are thiol-dependent enzymes,
making them very sensitive to inactivation by ROS generated by activated neutrophils [
163
,
164
].
Thus, vitamin C may be expected to protect the oxidant-sensitive caspase-dependent apoptotic process
following activation of neutrophils. In support of this premise,
in vitro
studies have shown that loading
human neutrophils with vitamin C can enhance Escherichia coli-mediated apoptosis of the neutrophils
(Table 1) [
71
]. Peritoneal neutrophils isolated from vitamin C-deficient Gulo mice exhibited attenuated
apoptosis [
75
], and instead underwent necrotic cell death [
73
]. These vitamin C-deficient neutrophils
were not phagocytosed by macrophages
in vitro
, and persisted at inflammatory loci
in vivo
[
73
].
Furthermore, administration of vitamin C to septic animals decreased the numbers of neutrophils in
the lungs of these animals [74].
Numerous studies have reported attenuated neutrophil apoptosis in patients with severe infection
compared with control participants [
165
172
]. The delayed apoptosis appears to be related to
disease severity and is thought to be associated with enhanced tissue damage observed in patients
with sepsis [
173
,
174
]. Immature ‘band’ neutrophils released during severe infection were also
found to be resistant to apoptosis and had longer life spans [
157
]. Plasma from septic patients
has been found to suppress apoptosis in healthy neutrophils, suggesting that pro-inflammatory
cytokines were responsible for the increased
in vivo
survival of neutrophils during inflammatory
conditions
[165,174176]
. Interestingly, high-dose vitamin C administration has been shown to
modulate cytokine levels in patients with cancer [
177
] and, although this has not yet been assessed
in patients with severe infection, could conceivably be another mechanism by which vitamin C
may modulate neutrophil function in these patients. To date, only one study has investigated
the effect of vitamin C supplementation on neutrophil apoptosis in septic patients [178]. Intravenous
supplementation of septic abdominal surgery patients with 450 mg/day vitamin C was found to
decrease caspase-3 protein levels and, thus was presumed to have an anti-apoptotic effect on peripheral
blood neutrophils. However, caspase activity and apoptosis of the neutrophils following activation was
not assessed. Furthermore, circulating neutrophils may not reflect the activation status of neutrophils
at inflammatory tissue loci. Clearly, more studies need to be undertaken to tease out the role of
vitamin C in neutrophil apoptosis and clearance from inflammatory loci.
3.4. Neutrophil Necrosis and NETosis
Neutrophils that fail to undergo apoptosis instead undergo necrotic cell death (Figure 2).
The subsequent release of toxic intracellular components, such as proteases, can cause extensive
tissue damage [
179
,
180
]. One recently discovered form of neutrophil death has been termed
NETosis. This results from the release of ‘neutrophil extracellular traps’ (NETs) comprising neutrophil
DNA, histones, and enzymes [
181
]. Although NETs have been proposed to comprise a unique
method of microbial killing [
182
,
183
], they have also been implicated in tissue damage and organ
failure
[184,185]
. NET-associated histones can act as damage-associated molecular pattern proteins,
activating the immune system and causing further damage [
186
]. Patients with sepsis, or who go on
to develop sepsis, have significantly elevated levels of circulating cell-free DNA, which is thought to
indicate NET formation [184,187].
Pre-clinical studies in vitamin C-deficient Gulo knockout mice indicated enhanced NETosis
in the lungs of septic animals and increased circulating cell-free DNA [
75
]. The levels of these
markers were attenuated in vitamin C sufficient animals or in deficient animals that were administered
vitamin C (Table 1). The same investigators showed that
in vitro
supplementation of human
neutrophils with vitamin C attenuated phorbol ester-induced NETosis [
75
]. Administration of
gram doses of vitamin C to septic patients over four days, however, did not appear to decrease
circulating cell-free DNA levels [188], although the duration of treatment may have been too short to
see a sustained effect. It should be noted that cell-free DNA is not specific for neutrophil-derived DNA,
as it may also derive from necrotic tissue; however, the association of neutrophil-specific proteins
Nutrients 2017,9, 1211 9 of 25
or enzymes, such as myeloperoxidase, with the DNA can potentially provide an indication of its
source [184].
The transcription factor HIF-1
α
facilitates neutrophil survival at hypoxic loci through delaying
apoptosis [
189
]. Interestingly, vitamin C is a cofactor for the iron-containing dioxygenase enzymes
that regulate the levels and activity of HIF-1
α
[
190
]. These hydroxylase enzymes downregulate
HIF-1
α
activity by facilitating degradation of constitutively expressed HIF-1
α
and decreasing
binding of transcription coactivators. In vitamin C-deficient Gulo knockout mice, up-regulation
of HIF-1
α
was observed under normoxic conditions, along with attenuated neutrophil apoptosis and
clearance by macrophages [
73
]. HIF-1
α
has also been proposed as a regulator of NET generation by
neutrophils [
191
], hence providing a potential mechanism by which vitamin C could downregulate
NET generation by these cells [75].
3.5. Lymphocyte Function
Like phagocytes, B- and T-lymphocytes accumulate vitamin C to high levels via SVCT [
192
,
193
].
The role of vitamin C within these cells is less clear, although antioxidant protection has been
suggested [
194
].
In vitro
studies have indicated that incubation of vitamin C with lymphocytes
promotes proliferation [
76
,
77
], resulting in enhanced antibody generation [
78
], and also provides
resistance to various cell death stimuli [
195
]. Furthermore, vitamin C appears to have an important
role in developmental differentiation and maturation of immature T-cells (Table 1) [
76
,
79
]. Similar
proliferative and differentiation/maturation effects have been observed with mature and immature
natural killer cells, respectively [196].
Early studies in guinea pigs showed enhanced mitotic activity of isolated peripheral blood
lymphocytes following intraperitoneal vitamin C treatment, and enhanced humoral antibody
levels during immunization [
82
85
]. Although one human intervention study has reported
positive associations between antibody levels (immunoglobulin (Ig)M, (Ig)G, (Ig)A) and vitamin C
supplementation [
85
], another has not [
62
]. Instead, Anderson and coworkers showed that oral
and intravenous supplementation of low gram doses of vitamin C to children with asthma and
healthy volunteers enhanced lymphocyte transformation, an ex vivo measure of mitogen-induced
proliferation and enlargement of T-lymphocytes (Table 1) [
62
,
63
,
81
]. Administration of vitamin C to
elderly people was also shown to enhance ex vivo lymphocyte proliferation [
80
], a finding confirmed
using combinations of vitamin C with vitamins A and/or E [
148
,
197
]. Exposure to toxic chemicals can
affect lymphocyte function, and both natural killer cell activity and lymphocyte blastogenic responses
to T- and B-cell mitogens were restored to normal levels following vitamin C supplementation [
198
].
Although the human studies mentioned above are encouraging, it is apparent that more human
intervention studies are needed to confirm these findings.
Recent research in wild-type and Gulo knockout mice indicated that parenteral administration
of 200 mg/kg vitamin C modulated the immunosuppression of regulatory T-cells (Tregs) observed
in sepsis [
89
]. Vitamin C administration enhanced Treg proliferation and inhibited the negative
immunoregulation of Tregs by inhibiting the expression of specific transcription factors, antigens,
and cytokines [
89
]. The mechanisms involved likely rely on the gene regulatory effects of
vitamin C [
79
,
89
,
199
,
200
]. For example, recent research has implicated vitamin C in epigenetic
regulation through its action as a cofactor for the iron-containing dioxygenases which hydroxylate
methylated DNA and histones [
22
,
201
]. The ten-eleven translocation (TET) enzymes hydroxylate
methylcytosine residues, which may act as epigenetic marks in their own right, and also facilitate
removal of the methylated residues, an important process in epigenetic regulation [
202
]. Preliminary
evidence indicates that vitamin C can regulate T-cell maturation via epigenetic mechanisms involving
the TETs and histone demethylation [
79
,
199
,
200
]. It is likely that the cell signaling and gene regulatory
functions of vitamin C, via regulation of transcription factors and epigenetic marks, play major roles in
its immune-regulating functions.
Nutrients 2017,9, 1211 10 of 25
3.6. Inflammatory Mediators
Cytokines are important cell signaling molecules secreted by a variety of immune cells, both innate
and adaptive, in response to infection and inflammation [
1
]. They comprise a broad range of molecules,
including chemokines, interferons (IFNs), ILs, lymphokines, and TNFs, which modulate both humoral
and cell-based immune responses, and regulate the maturation, growth, and responsiveness of specific
cell populations. Cytokines can elicit pro-inflammatory or anti-inflammatory responses, and vitamin C
appears to modulate systemic and leukocyte-derived cytokines in a complex manner.
Incubation of vitamin C with peripheral blood lymphocytes decreased lipopolysaccharide
(LPS)-induced generation of the pro-inflammatory cytokines TNF-
α
and IFN-
γ
, and increased
anti-inflammatory IL-10 production, while having no effect on IL-1
β
levels [
77
]. Furthermore,
in vitro
addition of vitamin C to peripheral blood monocytes isolated from pneumonia patients decreased
the generation of the pro-inflammatory cytokines TNF-
α
and IL-6 [
86
]. However, another study found
that
in vitro
treatment of peripheral blood monocytes with vitamin C and/or vitamin E enhanced
LPS-stimulated TNF-
α
generation, but did not affect IL-1
β
generation [
87
]. Furthermore, incubation
of vitamin C with virus-infected human and murine fibroblasts enhanced generation of antiviral
IFN [
91
93
]. Supplementation of healthy human volunteers with 1 g/day vitamin C (with and without
vitamin E) was shown to enhance peripheral blood mononuclear cell-derived IL-10, IL-1, and TNF-α
following stimulation with LPS [
87
,
94
]. Thus, the effect of vitamin C on cytokine generation appears
to depend on the cell type and/or the inflammatory stimulant. Recent research has indicated that
vitamin C treatment of microglia, resident myeloid-derived macrophages in the central nervous
system, attenuates activation of the cells and synthesis of the pro-inflammatory cytokines TNF, IL-6,
and IL-1β[90]. This is indicative of an anti-inflammatory phenotype.
Preclinical studies using Gluo knockout mice have highlighted the cytokine-modulating effects of
vitamin C. Vitamin C-deficient Gulo knockout mice infected with influenza virus showed enhanced
synthesis of the pro-inflammatory cytokines TNF-
α
and IL-1
α
/
β
in their lungs, and decreased
production of the anti-viral cytokine IFN-
α
/
β
[
88
]. Administration of vitamin C to Gulo mice with
polymicrobial peritonitis resulted in decreased synthesis of the pro-inflammatory cytokines TNF-
α
and IL-1
β
by isolated neutrophils [
75
]. Another study in septic Gulo mice administered 200 mg/kg
parenteral vitamin C has shown decreased secretion of the inhibitory cytokines TGF-
β
and IL-10 by
Tregs [
89
]. In this study, attenuated IL-4 secretion and augmented IFN-
γ
secretion was also observed,
suggesting immune-modulating effects of vitamin C in sepsis. Overall, vitamin C appears to normalize
cytokine generation, likely through its gene-regulating effects.
Histamine is an immune mediator produced by basophils, eosinophils, and mast cells during
the immune response to pathogens and stress. Histamine stimulates vasodilation and increased
capillary permeability, resulting in the classic allergic symptoms of runny nose and eyes. Studies
using guinea pigs, a vitamin C-requiring animal model, have indicated that vitamin C depletion is
associated with enhanced circulating histamine levels, and that supplementation of the animals with
vitamin C resulted in decreased histamine levels [
56
,
95
98
]. Enhanced histamine generation was found
to increase the utilization of vitamin C in these animals [
96
]. Consistent with the animal studies, human
intervention studies with oral vitamin C (125 mg/day to 2 g/day) and intravenous vitamin C (7.5 g
infusion) have reported decreased histamine levels [
61
,
99
101
], which was more apparent in patients
with allergic compared with infectious diseases [
101
]. Although vitamin C has been proposed to
‘detoxify’ histamine [96,97], the precise mechanisms responsible for the in vivo decrease in histamine
levels following vitamin C administration are currently unknown. Furthermore, effects of vitamin C
supplementation on histamine levels are not observed in all studies [203].
4. Vitamin C Insufficiency Conditions
Numerous environmental and health conditions can have an impact on vitamin C status.
In this section we discuss examples which also have a link with impaired immunity and increased
susceptibility to infection. For example, exposure to air pollution containing oxidants, such as ozone
Nutrients 2017,9, 1211 11 of 25
and nitrogen dioxide, can upset the oxidant-antioxidant balance within the body and cause oxidative
stress [
204
]. Oxidative stress can also occur if antioxidant defenses are impaired, which may be the case
when vitamin C levels are insufficient [
205
]. Air pollution can damage respiratory tract lining fluid
and increase the risk of respiratory disease, particularly in children and the elderly [
204
,
206
] who are
at risk of both impaired immunity and vitamin C insufficiency [
14
,
204
]. Vitamin C is a free-radical
scavenger that can scavenge superoxide and peroxyl radicals, hydrogen peroxide, hypochlorous acid,
and oxidant air pollutants [
207
,
208
]. The antioxidant properties of vitamin C enable it to protect lung
cells exposed to oxidants and oxidant-mediated damage caused by various pollutants, heavy metals,
pesticides, and xenobiotics [204,209].
Tobacco smoke is an underestimated pollutant in many parts of the world. Both smokers and
passive smokers have lower plasma and leukocyte vitamin C levels than non-smokers [
10
,
210
,
211
],
partly due to increased oxidative stress and to both a lower intake and a higher metabolic turnover
of vitamin C compared to non-smokers [
10
,
211
213
]. Mean serum concentrations of vitamin C in
adults who smoke have been found to be one-third lower than those of non-smokers, and it has been
recommended that smokers should consume an additional 35 mg/day of vitamin C to ensure there is
sufficient ascorbic acid to repair oxidant damage [
10
,
14
]. Vitamin C levels are also lower in children
and adolescents exposed to environmental tobacco smoke [
214
]. Research in vitamin C-deficient
guinea pigs exposed to tobacco smoke has indicated that vitamin C can protect against protein
damage and lipid peroxidation [
213
,
215
]. In passive smokers exposed to environmental tobacco smoke,
vitamin C supplementation significantly reduced plasma F
2
-isoprostane concentrations, a measure of
oxidative stress [
216
]. Tobacco use increases susceptibility to bacterial and viral infections [
217
,
218
], in
which vitamin C may play a role. For example, in a population-based study the risk of developing
obstructive airways disease was significantly higher in those with the lowest plasma vitamin C
concentrations (26
µ
mol/L) compared to never smokers, a risk that decreased with increasing vitamin C
concentration [219].
Individuals with diabetes are at greater risk of common infections, including influenza,
pneumonia, and foot infections, which are associated with increased morbidity and mortality [
220
,
221
].
Several immune-related changes are observed in obesity that contribute towards the development
of type 2 diabetes. A major factor is persistent low-grade inflammation of adipose tissue in obese
subjects, which plays a role in the progression to insulin resistance and type 2 diabetes, and which
is not present in the adipose tissue of lean subjects [
222
,
223
]. The adipose tissue is infiltrated by
pro-inflammatory macrophages and T-cells, leading to the accumulation of pro-inflammatory cytokines
such as interleukins and TNF-
α
[
224
,
225
]. A decrease in plasma vitamin C levels has been observed
in studies of type 2 diabetes [
18
,
226
], and a major cause of increased need for vitamin C in type 2
diabetes is thought to be the high level of oxidative stress caused by hyperglycemia [
10
,
227
,
228
].
Inverse correlations have been reported between plasma vitamin C concentrations and the risk of
diabetes, hemoglobin A1c concentrations (an index of glucose tolerance), fasting and postprandial
blood glucose, and oxidative stress [
219
,
229
232
]. Meta-analysis of interventional studies has indicted
that supplementation with vitamin C can improve glycemic control in type 2 diabetes [233].
Elderly people are particularly susceptible to infections due to immunosenescence and decreased
immune cell function [
234
]. For example, common viral infections such as respiratory illnesses, that
are usually self-limiting in healthy young people, can lead to the development of complications such as
pneumonia, resulting in increased morbidity and mortality in elderly people. A lower mean vitamin C
status has been observed in free-living or institutionalized elderly people, indicated by lowered plasma
and leukocyte concentrations [
10
,
235
,
236
], which is of concern because low vitamin C concentrations
(<17
µ
mol/L) in older people (aged 75–82 years) are strongly predictive of all-cause mortality [
237
].
Acute and chronic diseases that are prevalent in this age group may also play an important part in
the reduction of vitamin C reserves [
238
240
]. Institutionalization in particular is an aggravating
factor in this age group, resulting in even lower plasma vitamin C levels than in non-institutionalized
elderly people. It is noteworthy that elderly hospitalized patients with acute respiratory infections
Nutrients 2017,9, 1211 12 of 25
have been shown to fare significantly better with vitamin C supplementation than those not receiving
the vitamin [
241
]. Decreased immunological surveillance in individuals older than 60 years also results
in greater risk of cancer, and patients with cancer, particularly those undergoing cancer treatments,
have compromised immune systems, decreased vitamin C status, and enhanced risk of developing
sepsis [
242
,
243
]. Hospitalized patients, in general, have lower vitamin C status than the general
population [244].
5. Vitamin C and Infection
A major symptom of the vitamin C deficiency disease scurvy is the marked susceptibility to
infections, particularly of the respiratory tract, with pneumonia being one of the most frequent
complications of scurvy and a major cause of death [
7
]. Patients with acute respiratory infections, such
as pulmonary tuberculosis and pneumonia, have decreased plasma vitamin C concentrations relative to
control subjects [
245
]. Administration of vitamin C to patients with acute respiratory infections returns
their plasma vitamin C levels to normal and ameliorates the severity of the respiratory symptoms [
246
].
Cases of acute lung infections have shown rapid clearance of chest X-rays following administration of
intravenous vitamin C [
247
,
248
]. This vitamin C-dependent clearance of neutrophils from infected
lungs could conceivably be due to enhanced apoptosis and subsequent phagocytosis and clearance of
the spent neutrophils by macrophages [
73
]. Pre-clinical studies of animals with sepsis-induced lung
injury have indicated that vitamin C administration can increase alveolar fluid clearance, enhance
bronchoalveolar epithelial barrier function, and attenuate sequestration of neutrophils [
74
], all essential
factors for normal lung function.
Meta-analysis has indicated that vitamin C supplementation with doses of 200 mg or more
daily is effective in ameliorating the severity and duration of the common cold, and the incidence
of the common cold if also exposed to physical stress [
249
]. Supplementation of individuals who
had an inadequate vitamin C status (i.e., <45
µ
mol/L) also decreased the incidence of the common
cold [
203
]. Surprisingly, few studies have assessed vitamin C status during the common cold [
250
].
Significant decreases in both leukocyte vitamin C levels, and urinary excretion of the vitamin, have
been reported to occur during common cold episodes, with levels returning to normal following
the infection [
251
254
]. These changes indicate that vitamin C is utilized during the common cold
infection. Administration of gram doses of vitamin C during the common cold episode ameliorated
the decline in leukocyte vitamin C levels, suggesting that administration of vitamin C may be beneficial
for the recovery process [251].
Beneficial effects of vitamin C on recovery have been noted in pneumonia. In elderly people
hospitalized because of pneumonia, who were determined to have very low vitamin C levels,
administration of vitamin C reduced the respiratory symptom score in the more severe patients [
246
].
In other pneumonia patients, low-dose vitamin C (0.25–0.8 g/day) reduced the hospital stay by
19% compared with no vitamin C supplementation, whereas the higher-dose group (0.5–1.6 g/day)
reduced the duration by 36% [
255
]. There was also a positive effect on the normalization of
chest X-ray, temperature, and erythrocyte sedimentation rate [
255
]. Since prophylactic vitamin C
administration also appears to decrease the risk of developing more serious respiratory infections, such
as pneumonia [
256
], it is likely that the low vitamin C levels observed during respiratory infections are
both a cause and a consequence of the disease.
6. Conclusions
Overall, vitamin C appears to exert a multitude of beneficial effects on cellular functions of
both the innate and adaptive immune system. Although vitamin C is a potent antioxidant protecting
the body against endogenous and exogenous oxidative challenges, it is likely that its action as a cofactor
for numerous biosynthetic and gene regulatory enzymes plays a key role in its immune-modulating
effects. Vitamin C stimulates neutrophil migration to the site of infection, enhances phagocytosis and
oxidant generation, and microbial killing. At the same time, it protects host tissue from excessive
Nutrients 2017,9, 1211 13 of 25
damage by enhancing neutrophil apoptosis and clearance by macrophages, and decreasing neutrophil
necrosis and NETosis. Thus, it is apparent that vitamin C is necessary for the immune system to mount
and sustain an adequate response against pathogens, whilst avoiding excessive damage to the host.
Vitamin C appears to be able to both prevent and treat respiratory and systemic infections by
enhancing various immune cell functions. Prophylactic prevention of infection requires dietary vitamin C
intakes that provide at least adequate, if not saturating plasma levels (i.e., 100–200 mg/day), which
optimize cell and tissue levels. In contrast, treatment of established infections requires significantly
higher (gram) doses of the vitamin to compensate for the increased metabolic demand.
Epidemiological studies indicate that hypovitaminosis C is still relatively common in Western
populations, and vitamin C deficiency is the fourth leading nutrient deficiency in the United
States. Reasons include reduced intake combined with limited body stores. Increased needs occur
due to pollution and smoking, fighting infections, and diseases with oxidative and inflammatory
components, e.g., type 2 diabetes, etc. Ensuring adequate intake of vitamin C through the diet or via
supplementation, especially in groups such as the elderly or in individuals exposed to risk factors for
vitamin C insufficiency, is required for proper immune function and resistance to infections.
Acknowledgments:
Thanks are given to Mark Hampton for critically reviewing the manuscript and Deborah
Nock (Medical WriteAway, Norwich, UK) for medical writing support and editorial assistance on behalf of Bayer
Consumer Care Ltd. A.C.C. is the recipient of a Health Research Council of New Zealand Sir Charles Hercus
Health Research Fellowship.
Author Contributions: A.C.C. and S.M. conceived and wrote the review, and A.C.C. had primary responsibility
for the final content.
Conflicts of Interest:
S.M. is employed by Bayer Consumer Care Ltd., a manufacturer of multivitamins, and
wrote the section on ‘Vitamin C insufficiency conditions’. A.C.C. has received funding, as a Key Opinion Leader,
from Bayer Consumer Care Ltd.
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... Micronutrient deficiencies or inadequate status have been shown to have a deleterious impact on immune function and infection resistance. The roles of vitamins D and C in immunity are especially well-known [66,67]. ...
... Vitamin C has an impact on various elements of immunity, including adaptive and innate immune cell proliferation and function, epithelial barrier function, adaptive and innate immune cell proliferation and function, phagocytosis and microbial death, white blood cell migration to infection sites, and antibody formation [67]. ...
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Purpose of Review This review elaborates the role of malnutrition in PLHIV (people living with HIV) in the context of COVID-19 and emphasis the need of supplementation, dietary intervention, and nutritional counselling in the post-COVID era. One of the most critical challenges among HIV/AIDS patients is malnutrition since it weakens the immune system and increases risk to opportunistic infections. In HIV (human immunodeficiency virus) infection, weight loss is prevalent due to reduced nutritional consumption, malabsorption, abnormal metabolism, and antiretroviral therapy. Sufficient nutrition is required for optimal immune function, as a result, food therapy is now considered an important adjuvant in the treatment of HIV patients. Recent Findings Nutritional intervention, such as the use of dietary supplements, can help to prevent nutrient deficiency, lowering the death risk among malnourished HIV population. Immunocompromised individuals are at very high risk for COVID-19 and malnutrition increases the risk of infection by multiple folds. Interventions, such as nutrition education and counselling are important, to improve the condition of HIV Patients by optimising their nutritional status. Summary A balanced diet should be one of the most important priorities in preventing PLHIV against the potentially deadly consequences of COVID-19. It is to be ensured that HIV-positive persons continue to get enough and appropriate assistance, such as nutrition and psychological counselling, in the context of COVID-19 infection. The use of telemedicine to maintain nutritional intervention can be beneficial. To meet their nutritional needs and minimise future difficulties, PLHIV infected with COVID-19 should get specialised nutritional education and counselling. Graphical abstract
... Cobalamin deficiency causes megaloblastic or pernicious anemia along with neurologic symptoms occurring in the late stage of disease. A study published in 2020 found that low levels of B12 in pregnant women may increase susceptibility to CoV-2 infection [90]. Besides, in a recent review investigating the role of B12 in CoV-2 infection, cobalamin was found useful as an additional treatment of mild to severe COVID-19 symptoms, because of its analgesic function and role in neuromuscular disorders following CoV-2 infection [88]. ...
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The recent outbreak of the novel Coronavirus (SARS-CoV-2 or CoV-2) pandemic in 2019 and the risk of CoV-2 infection during pregnancy led the scientific community to investigate the potential negative effects of Coronavirus infection on pregnancy outcomes and fetal development. In particular, as CoV-2 neurotropism has been demonstrated in adults, recent studies suggested a possible risk of fetal brain damage and fetal brain development impairment, with consequent psychiatric manifestations in offspring of mothers affected by COronaVIrus Disease (COVID) during pregnancy. Through the understanding of CoV-2’s pathogenesis and the pathways responsible for cell damage, along with the available data about neurotropic virus attitudes, different strategies have been suggested to lower the risk of neurologic disease in newborns. In this regard, the role of nutrition in mitigating fetal damages related to oxidative stress and the inflammatory environment during viral infection has been investigated, and arginine, n3PUFA, vitamins B1 and B9, choline, and flavonoids were found to be promising in and out of pregnancy. The aim of this review is to provide an overview of the current knowledge on the mechanism of fetal brain damage and the impact of nutrition in reducing inflammation related to worse neurological outcomes in the context of CoV-2 infections during pregnancy.
... 5 Ascorbic acid is a known anti-oxidant NC which is also capable of boosting immunity by augmenting phagocytosis and chemotaxis. 6 Emodin exhibits anti-proliferative and anti-apoptotic effects on cancer cells and it can regulate PI3K/Akt/NF-kB signaling pathways that are imperative for cell growth and metabolism. 7 Since nutraceuticals have potent anti-inammatory activities 8 we tried to harness these benets for treatment of inammatory conditions characteristic of chronic lymphatic larial infections. ...
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... By inhibiting the generation of proinflammatory cytokines, it modulates nuclear transcription factor-κ B, neutralizes reactive oxygen species, and aids in immunomodulation as a cofactor of different metabolic processes in the immune system. [16][17][18][19][20] Oxidative stress is prominent with infection, particularly lung infection or pressing situations. Vitamin C lowers inflammation and enhances immunoregulatory function in pneumonia or severe disease patients. ...
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