Adequate Vitamin C is essential to Brain Health and Preventing & Alleviating Chronic Neurological Conditions

Vit C is a nutrient of great importance for proper functioning of nervous system and its main role in the brain is its participation in the antioxidant defense. But apart from this role, it is involved in numerous vital non-oxidant processes like biosynthesis of collagen, carnitine, tyrosine and peptide hormones as well as of myelin. It plays the crucial role in neurotransmission and neuronal maturation and functions. For instance, its ability to alleviate seizure severity as well as reduction of seizure-induced damage have been proved. Two basic barriers limit the entry of Vit C (being a hydrophilic molecule) into the central nervous system: the blood-brain barrier and the blood-cerebrospinal fluid barrier (CSF). Considering the whole body, ascorbic acid uptake is mainly conditioned by two sodium-dependent transporters from the SLC23 family, the sodium-dependent Vit C transporter type 1 (SVCT1) and type 2 (SVCT2). These possess similar structure and amino acid sequence, but have different tissue distribution. SVCT1 is found predominantly in apical brush-border membranes of intestinal and renal tubular cells, whereas SVCT2 occurs in most tissue cells. SVCT2 is especially important for the transport of Vit C in the brain—it mediates the transport of ascorbate from plasma across choroid plexus to the cerebrospinal fluid and across the neuronal cell plasma membrane to neuronal cytosol. Although dehydroascorbic acid (DHA) enters the central nervous system more rapidly than the ascorbate, the latter one readily penetrates CNS after oral administration. DHA is taken up by the omnipresent glucose transporters (GLUT), which have affinity to this form of Vit C. GLUT1 and GLUT3 are mainly responsible for DHA uptake in the CNS . Transport of DHA by GLUT transporter is bidirectional—each molecule of DHA formed inside the cells by oxidation of the ascorbate could be effluxed and lost. This phenomenon is prevented by efficient cellular mechanisms of DHA reduction and recycling in ascorbate. Neurons can take up ascorbic acid using both described ways, whereas astrocytes acquire Vit C utilizing only GLUT transporters.

It is well known that the main function of intracellular ascorbic acid in the brain is the antioxidant defense of the cells. However, vitamin C in the central nervous system (CNS) has also many non-antioxidant functions—it plays a role of an enzymatic co-factor participating in biosynthesis of such substances as collagen, carnitine, tyrosine and peptide hormones. It has also been indicated that myelin formation in Schwann cells could be stimulated by ascorbic acid [7,29].

The brain is an organ particularly exposed to oxidative stress and free radicals’ activity, which is associated with high levels of unsaturated fatty acids and high cell metabolism rate [16]. Ascorbic acid, being an antioxidant, acts directly by scavenging reactive oxygen and nitrogen species produced during normal cell metabolism [30,31]. In vivo studies demonstrated that the ascorbate had the ability to inactivate superoxide radicals—the major byproduct of fast metabolism of mitochondrial neurons [32]. Moreover, the ascorbate is a key factor in the recycling of other antioxidants, e.g., alpha-tocopherol (Vitamin E). Alpha-tocopherol, found in all biological membranes, is involved in preventing lipid peroxidation by removing peroxyl radicals. During this process α-tocopherol is oxidized to the α-tocopheroxyl radical, which can result in a very harmful effect. The ascorbate could reduce the tocopheroxyl radical back to tocopherol and then its oxidized form is recycled by enzymatic systems with using NADH or NADPH [33]. Regarding these facts, vitamin C is considered to be an important neuroprotective agent.

One non-antioxidant function of vitamin C is its participation in CNS signal transduction through neurotransmitters [16]. Vit C is suggested to influence this process via modulating of binding of neurotransmitters to receptors as well as regulating their release [34,35,36,37]. In addition, ascorbic acid acts as a co-factor in the synthesis of neurotransmitters, particularly of catecholamines—dopamine and norepinephrine [26,38]. Seitz et al. [39] suggested that the modulating effect of the ascorbate could be divided into short- and long-term ones. The short-term effect refers to ascorbate role as a substrate for dopamine-β-hydroxylase. Vit C supplies electrons for this enzyme catalyzing the formation of norepinephrine from dopamine. Moreover, it may exert neuroprotective influence against ROS and quinones generated by dopamine metabolism [16]. On the other hand, the long-term effect could be connected with increased expression of the tyrosine hydroxylase gene, probably via a mechanism that entails the increase of intracellular cAMP [39]. It has been stated that the function of ascorbic acid as a neuromodulator of neural transmission may be also associated with amino acidic residues reduction [40] or scavenging of ROS generated in response to neurotransmitter receptor activation [34,41]. Moreover, some have studies showed that ascorbic acid modulates the activity of some receptors such as glutamate as well as γ-aminobutyric acid (GABA) ones [22,40,42,43,44]. Vit C has been shown to prevent excitotoxic damage caused by excessive extracellular glutamate leading to hyperpolarization of the N-methyl-d-aspartate (NMDA) receptor and therefore to neuronal damage [45]. Vit C inhibits the binding of glutamate to the NMDA receptor, thus demonstrating a direct effect in preventing excessive nerve stimulation exerted by the glutamate [26]. The effect of ascorbic acid on GABA receptors can be explained by a decrease in the energy barrier for GABA activation induced by this agent. Ascorbic acid could bind to or modify one or more sites capable of allosterically modulating single-channel properties. In addition, it is possible that ascorbic acid acts through supporting the conversion from the last GABA-bound closed state to the open state. Alternatively, ascorbic acid could induce the transition of channels towards additional open states in which the receptor adopts lower energy conformations with higher open probabilities [40,44].

There have also been reports concerning the effect of Vit C on cognitive processes such as learning, memory and locomotion, although the exact mechanism of this impact is still being investigated [26]. However, animal studies have shown a clear association between the ascorbate and the cholinergic and dopaminergic systems, they also suggested that the ascorbate can act as a dopamine receptor antagonist. This was also confirmed by Tolbert et al. [46], who showed that the ascorbate inhibits the binding of specific dopamine D1 and D2 receptor agonists.

Another non-antioxidant function of Vit C includes modulation of neuronal metabolism by changing the preference for lactate over glucose as an energy substrate to sustain synaptic activity. During ascorbic acid metabolic switch, this vitamin is released from glial cells and is taken up by neurons where it restraints glucose transport and its utilization. This allows lactate uptake and its usage as the primary energy source in neurons [47]. It was observed that intracellular ascorbic acid inhibited neuronal glucose usage via a mechanism involving GLUT3 [48].

Vit C is involved in collagen synthesis, which also occurs in the brain [26]. There is no doubt that collagen is needed for blood vessels and neural sheath formation. It is well recognized that vitamin C takes part in the final step of the formation of mature triple helix collagen. In this stage, ascorbic acid acts as an electron donor in the hydroxylation of procollagen propyl and lysyl residues [16]. The role of Vit C in collagen synthesis in the brain was confirmed by Sotiriou et al. [49]. According to these authors in mice deficient in SVCT2 ascorbate transporter, the concentration of ascorbate in the brain was below detection level. The animals died due to capillary hemorrhage in the penetrating vessels of the brain. Ascorbate-dependent collagen synthesis is also linked to the formation of the myelin sheath that surrounds many nerve fibers [26]. In vitro studies showed that ascorbate, added to a mixed culture of rat Schwann cells and dorsal root ganglion neurons, promoted myelin formation and differentiation of Schwann cells during formation of the basal lamina of the myelin sheath [7,29].

Vit C is important for proper nervous system function and its abnormal concentration in nervous tissue is thought to be accompanied with neurological disorders. The fact that Vit C can neutralize superoxide radicals, which are generated in large amount during neurodegenerative processes, seems to support its role in neurodegeneration. Moreover, plasma and cellular Vit C levels decline steadily with age and neurodegenerative diseases are often associated with aging. An association of Vit C release with motor activity in central nervous system regions, glutamate-uptake-dependent release of Vit C, its possible role in modulation of N-methyl-d-aspartate receptor activity as well as ability to prevent peroxynitrite anion formation constitute further evidence pointing to the role of Vit C in neurodegenerative processes.

The role of Vit C in AD disease was studied in APP/PSEN1 mice carrying human AD mutations in the amyloid precursor protein (APP) and presenilin (PSEN1) genes (transgenic mouse model of Alzheimer’s disease) with partial ablation of vitamin C transport in the brain [9,62,63].

Warner et al. [9] demonstrated that decreased brain Vit C level in the 6-month-old SVCT2+/− APP/PSEN1 mice (obtained by crossing APP/PSEN1 bigenic mice with SVCT2+/− heterozygous knockout mice, which have the lower number of the sodium-dependent Vit C transporter) was associated with enhanced oxidative stress in brain, increased mortality, a shorter latency to seizure onset after kainic acid administration (10 mg/kg i.p.), and more ictal events following treatment with pentylenetetrazol (50 mg/kg i.p.). Furthermore, the authors reported that Vit C deficiency alone in SVCT2+/− mice increased the severity of kainic acid- and pentylenetetrazol-induced seizures [62]. According to another study even moderate intracellular Vit C deficiency displayed an important role in accelerating amyloid aggregation and brain oxidative stress formation, particularly during early stages of disease development. In 6-month-old SVCT2+/− APP/PSEN1 mice increased brain cortex oxidative stress (enhanced malondialdehyde, protein carbonyls, F2-isoprostanes) and decreased level of total glutathione as compared to wild-type controls were observed. Moreover, SVCT2+/− mice had elevated levels of both soluble and insoluble Aβ1-42 and a higher Aβ1-42/Aβ1-40 ratio. In 14-month old mice there were more amyloid-β plaque deposits in both hippocampus and cortex of SVCT2+/−APP/PSEN1+ mice as compared to APP/PSEN+ mice with normal brain Vit C level, whereas oxidative stress levels were similar between groups [62]. Ward et al. [63], in turn, showed that severe Vit C deficiency in Gulo−/− mice (lacking l-gulono-1,4-lactone oxidase (Gulo) responsible for the last step in Vit C synthesis) resulted in decreased blood glucose levels, oxidative damage to lipids and proteins in the cortex, and reduction in dopamine and serotonin metabolites in both the cortex and striatum. Moreover, Gulo−/− mice displayed a significant decrease in voluntary locomotor activity, reduced physical strength and elevated sucrose preference. All the above-mentioned behaviors were restored to control levels after treatment with Vit C (250 mg/kg, i.p.). The role of Vit C in protecting the brain against oxidative stress damage seems to be also proved by the recent study performed by Sarkar et al. [64]. The researchers share a view that cerebral ischemia-reperfusion-induced oxidative stress may initiate the pathogenic cascade leading eventually to neuronal loss, especially in hippocampus, with amyloid accumulation, tau protein pathology and irreversible Alzheimer’s dementia. Being the prime source of ROS generation, neuronal mitochondria are the most susceptible to damage caused by oxidative stress. The study proved it that l-ascorbic acid loaded polylactide nanocapsules exerted a protective effect on brain mitochondria against cerebral ischemia-reperfusion-induced oxidative injury [64]. Kennard and Harrison, in turn, evaluated the effects of a single intravenous dose of Vit C on spatial memory (using the modified Y-maze test) in APP/PSEN1 mice. The study was performed on APP/PSEN1 and wild-type (WT) mice of three age spans (3, 9 or 20 months). It was shown that APP/PSEN1 mice displayed no behavioral impairment as compared to WT controls, but memory impairment along with aging was observed in both groups. Vit C treatment (125 mg/kg, i.v.) improved performance in 9-month old APP/PSEN1 and WT mice, but improvements in short-term spatial memory did not result from changes in the neuropathological features of AD or monoamine signaling, as acute Vit C administration did not alter monoamine levels in the nucleus accumbens [65]. Cognitive-enhancing effects of acute intraperitoneal (i.p.) Vit C treatment in APP/PSEN1 mice (12- and 24-month-old) were investigated by Harrison et al. Vit C treatment (125 mg/kg i.p.) improved Y-maze alternation rates and swim accuracy in the water maze in both APP/PSEN1 and wild-type mice; but like in the previous study had no significant effect on the age-associated increase in Aβ deposits and oxidative stress, and did not also affect acetylcholinesterase (AChE) activity either, which was significantly reduced in APP/PSEN1 mice [66]. Murakami et al. [67] in turn reported that 6-month-treatment with Vit C resulted in reduced Aβ oligomer formation without affecting plaque formation, a significant decrease in brain oxidative damage and Aβ42/Aβ40 ratio as well as behavioral decline in an AD mouse model. Furthermore, this restored the declined synaptophysin and reduced the phosphorylation of tau protein at Ser396.

Besides the presented roles, Vit C has also been suggested to prevent neurodegenerative changes and cognitive decline by protecting blood–brain barrier (BBB) integrity [68]. A randomized control trial involving 276 elderly participants demonstrated that 16-week-co-supplementation of vitamin E and C with β-carotene significantly improved cognitive function (particularly with higher doses of β-carotene). Furthermore, the authors suggested that such a treatment markedly reduced plasma Aβ levels and elevated plasma estradiol levels [80]. Vit C and E co-supplementation for more than 3 years was also shown to be associated with a reduced prevalence and incidence of AD [81]. Moreover, an adequate Vit C plasma level seems to be associated with less progression in carotid intima-media thickness (C-IMT)—the greater C-IMT is suggested to be a risk factor in predicting cognitive decline in the general population, in the elderly population and in patients with Alzheimer’s disease.

Kook et al., in the study performed on KO-Tg mice (generating by crossing 5 familial Alzheimer’s disease mutation (5XFAD) mice with mice lacking Gulo), found that oral Vit C supplementation (3.3 g/L of drinking water) reduced amyloid plaque burden in the cortex and hippocampus by ameliorating BBB disruption (via preventing tight junction structural changes) and morphological changes in the mitochondria [69]. This seems to be confirmed by other studies that proved that Vit C might affect levels of proteins responsible for the tightness of BBB, like tight junction-specific integral membrane proteins (occludin and claudin-5) as well as matrix metalloproteinase 9 (MMP-9). Allahtavakoli et al. demonstrated that in a rat stroke model Vit C administration (500 mg/kg; 5 h after stroke) significantly reduced BBB permeability by reducing serum levels of matrix metalloproteinase 9 [70]. Song et al. reported that Vit C (100 mg/kg i.p.) protected cerebral ischemia-induced BBB disruption by preserving the expression of claudin 5 [71], whereas Lin et al. observed that Vit C (500 mg/kg i.p.) prevented compression-induced BBB disruption and sensory deficit by upregulating the expression of both occludin and claudin-5 [72].

In the available literature, there were only few studies investigating the role of Vit C in AD disease in human and the existing ones have yielded equivocal results.

Some studies have shown significantly lower plasma/serum Vit C level in AD patients as compared to healthy individuals, whereas others have found no difference [73,74]. However, meta-analysis performed by Lopes da Silva et al. proved significantly lower plasma levels of Vit C in AD patients [75]. It seems that the above discrepancies may result from the fact that not plasma but rather intracellular Vit C may be associated with AD.

Generally, studies involving human participants are limited to assessing the effect of Vit C supplementation administrated with other antioxidants on AD course.

Arlt et al. [76] found that 1-month and 1-year co-supplementation of Vit C (1000 mg/day) with vitamin E (400 IU/day) increased their concentrations not only in plasma but also in cerebrospinal fluid (which reflects the Vit C status of the brain), while cerebrospinal fluid lipid oxidation was significantly reduced only after 1 year. However, vitamins’ supplementation did not have a significant effect on the course of AD [76]. These findings were aslo confirmed by the randomized clinical trial of Galasko et al. [77], which showed that treatment of AD patients for 16 weeks with vitamin E (800 IU/day) plus Vit C (500 mg/day) plus α-lipoic acid (900 mg/day) did not influence cerebrospinal fluid levels of Aβ42, tau and p181tau (widely accepted biomarkers related to amyloid or tau pathology), but decreased F2-isoprostane level (a validated biomarker of oxidative stress). Moreover, is should be emphasized that the above treatment increased risk of faster cognitive decline. This seems to be consistent with results of the recent study which revealed it that Vit C was a potent antioxidant within the AD brain, but it was not able to ameliorate other factors linked to AD pathogenesis as it was proved to be a poor metal chelator and did not inhibit Aβ42 fibrillation [78]. In the study considering an association between nutrient patterns and three brain AD-biomarkers, namely Aβ load, glucose metabolism and gray matter volumes (a marker of brain atrophy) in AD-vulnerable regions, it was found that the higher intake of carotenoids, vitamin A, vitamin C and dietary fibers was positively associated only with glucose metabolism [79].

On the other hand(1), a randomized control trial involving 276 elderly participants demonstrated that 16-week-co-supplementation of vitamin E and C with β-carotene significantly improved cognitive function (particularly with higher doses of β-carotene). Furthermore, the authors suggested that such a treatment markedly reduced plasma Aβ levels and elevated plasma estradiol levels [80]. Vit C and E co-supplementation for more than 3 years was also shown to be associated with a reduced prevalence and incidence of AD [81]. Moreover, an adequate Vit C plasma level seems to be associated with less progression in carotid intima-media thickness (C-IMT)—the greater C-IMT is suggested to be a risk factor in predicting cognitive decline in the general population, in the elderly population and in patients with Alzheimer’s disease. Polidori et al. showed significant decrease (with a linear slope) in Vit C level among old individuals with no or very mild cognitive impairment from the first to the fourth C-IMT quartile [82].

The study (2) observations substantiate the previous in vitro findings that ascorbate specifically prevents oxidative degradation of microsomal membranes. The results indicate that vitamin C may exert a powerful protection against degenerative diseases associated with oxidative damage and play a critical role in wellness and health maintenance.

(2) Vitamin C prevents oxidative damage, Free Radic Res 1996 Aug;25(2):173-9. M K Ghosh et al, https://pubmed.ncbi.nlm.nih.gov/8885335/ & Vitamin C for Chronic Conditions, B Windham (Ed) www.myflcv.com/VitCrp.html