Figure 1. Intranuclear signal transductions can occur in two different pathways: while nuclear factor kappaB (NF-κB) tends to enhance and perpetuate the inflammatory response by triggering the expression of pro-inflammatory cytokines, nuclear factor erythroid 2–related factor 2 (Nrf2) activation through Kelch-like ECH-associated protein-1 (Keap1) oxidation dampens pro-inflammatory signaling by expression of peroxidases and other anti-inflammatory proteins. As E3-ligase, Keap1 also primes inhibitor of NF-κB kinase subunit beta (IKKβ) to degradation via ubiquitination, thereby directly interfering with NF-κB activation. For the sake of clarity, only the reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX)-derived H2O2 is shown as an oxidant signal. Depending on the cellular system and the inflammatory stimulus, NOX-derived H
2O
2 may be supported or replaced by mitochondrial H
2O
2, lipoxygenase products, and S-alkylating electrophiles derived therefrom. NEMO, NF-κB essential modulator; IκB, Inhibitor of NF-κB.
3.1.4. Vitamin E
Vitamin E (alpha-tocopherol) is a fat-soluble vitamin and a potent antioxidant important in protecting cells from oxidative stress, regulating immune function
[65][42], maintaining endothelial cell and heart integrity, and balancing coagulation and gut microbiota
[66,67][43][44]. It has been demonstrated that vitamin E deficiency impairs the normal functions of the immune system in animals and humans, which can be corrected by vitamin E repletion
[65][42]. Furthermore, a low level of vitamin E associated with selenium insufficiency results in specific viral mutations, changing relatively benign viruses into virulent ones
[68][45]. In relation to the long COVID-19 problem, it was reported that a low level of serum alpha-tocopherol improved during the remission phase, as compared to the exacerbation phase, in patients with chronic fatigue syndrome, suggesting that increased oxidative stress may be involved in the pathogenesis and the severity of the symptoms of the syndrome
[69][46].
3.2. Essential Elements
3.2.1. Magnesium
Magnesium is the most abundant divalent cation in living cells and plays essential roles in the regulation of cell growth, division, and differentiation
[70][47]. In the heart, magnesium plays a key role in modulating neuronal excitation, intracardiac conduction, and myocardial contraction by regulating several ion transporters, including potassium and calcium channels. Magnesium also has a role in regulating vascular tone, atherogenesis, and thrombosis, and proliferation and migration of endothelial and vascular smooth muscle cells
[71][48], and it also acts protectively against phosphate-induced kidney injury
[72][49]. It is involved in numerous biological processes (estimated at over 600) and, when present in physiological concentrations, it controls redox homeostasis, reducing the production of oxygen-derived free radicals in various tissues, lowering inflammation. Mechanisms include its “calcium-channel blocking” effects that lead to downstream suppression of NF-κB, IL-1β, IL-6, and TNF-α, as well as C-reactive protein (CRP) production
[73][50]. Latent magnesium deficiency is associated with chronic low-grade inflammation
[74][51], hypertension, metabolic syndrome, type 2 diabetes, and cardiovascular disease
[75][52], as well as with increased levels of free radicals and mitochondrial dysfunction
[76][53], possibly causally related to fatigue and myalgic encephalomyelitis/chronic fatigue syndrome
[77][54], a common manifestation of long COVID-19.
3.2.2. Selenium
Selenium is an essential trace element for mammalian redox biology. Unlike other trace elements that act as cofactors, dietary selenium is converted in the body into aminoacid selenocysteine, which is then incorporated into one of the twenty-five selenoproteins
[84][55] such as glutathione peroxidase, thioredoxin reductases, and methionine sulfoxide reductase, which are important components of the antioxidant defense systems. Reduced expression of selenoproteins as a result of low/sub-optimal selenium status could alter the molecular pathways involved in stress responses and contribute to an aggressive pro-inflammatory environment due to an imbalance between NF-κB and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling (
Figure 1), which may lead to poorer viral disease prognosis
[85][56]. On the contrary, selenium supplementation is associated with lower expression of pro-inflammatory NF-κB signaling
[86][57]. ROS are produced during viral infections, with both positive and negative consequences for the cell
[87][58]. For example, phagocytic cells produce large amounts of ROS to eliminate a wide variety of pathogens without altering the host cell viability, but ROS have also been found to stimulate viral replication
[88][59]. This fact is particularly significant for RNA viruses that exhibit the highest known mutation rates, with up to one mutation per genome per generation cycle
[89][60], and selenium deficiency increases the pathogenicity and severity of infections by benign or mildly virulent strains of Coxsackie and influenza viruses, giving rise to multiple changes in the viral RNA
[90][61]. Thus, dietary insufficiency of this oligo element impacts not only the immune response of the host, but also the virus itself, and dietary selenium deficiency, which causes oxidative stress in the host, can alter a viral genome so that a normally benign or mildly pathogenic virus becomes highly virulent in the deficient host. This has been shown in animal models for the influenza virus
[91][62] and human coxsackie enterovirus
[92][63].
3.2.3. Zinc
Zinc is the second-most abundant trace metal in the human body after iron and an essential component of protein structure and function. It is a vital micronutrient for maintaining cellular physiology
[103][64]. In fact, it is a structural component of ~750 zinc-finger transcription factors
[104][65], allowing gene transcription, and it is a catalytic part of approximately 2000 enzymes on all sides of 6 classes (hydrolase, transferase, oxidoreductase, ligase, lyase, and isomerase)
[9][66]. Zinc acts as a second messenger comparable to calcium
[9,104][65][66]; thus, it is obvious that cellular signals are altered due to changed intracellular zinc concentrations. Therefore, zinc is biologically indispensable for cellular processes, including growth and development, as well as DNA synthesis and RNA transcription
[103][64]. Additionally, zinc contributes to red-ox homeostasis because oxidative stress induces zinc release from metallothioneins as a mechanism to reduce ROS generated by mitochondrial dysfunction or viral infection
[105][67]. Furthermore, zinc deficiency increases IL-6-induced activation of the JAK-STAT3 signaling pathways, which are normalized after zinc supplementation
[106][68]. Zinc is known to be essential, especially for proper T cell and B cell development. During zinc deficiency, the recruitment of naïve Th cells and the percentage of cytotoxic T lymphocytes precursors is diminished, respectively
[107][69]. Zinc inhibits NF-κB signal transduction with the consequent decreased expression of IL-1β and TNFα and decreased CRP levels, lipid peroxidation, and inflammatory cytokines and adhesion molecule expression
[108][70]. More importantly, in the context of COVID-19 pathogenesis and its long-term consequences, zinc mediates the reduction of pro-inflammatory Th17 cells
[109][71], and its deficiency increases endothelial dysfunction
[110][72] and autoimmune susceptibility in general
[96][73]. It has been repeatedly demonstrated that autoimmune diseases are associated with zinc deficiency
[111][74], and an overreacting immune response can be beneficially influenced by the administration of zinc, which seems to be promising to improve the life of patients suffering from autoimmune diseases
[112][75]. In addition, several studies have documented a positive association between zinc deficiency and the risk of depression, and an inverse association between zinc supplementation and depressive symptoms
[102][76].
3.3. Phytochemicals: The Low Hanging Fruit
Viruses probably appeared as parasites of the first bacterial cells over 3.5 billion years ago
[113][77], while plants and homo sapiens appeared on earth 450 million and 300,000 years ago, respectively
[114][78]. Therefore, plants have hundreds of millions of years of greater experience in antiviral defenses than animals, and have most likely developed effective and non-specific defenses, i.e., valid against different viruses. Phytochemicals are naturally occurring plant chemicals that have been used in traditional medicines since ancient times and comprise various bioactive compounds that have now been classified as Alkaloids, Polyphenols, Carotenoids, and Organosulfurs, which are considered a natural weapon against inflammation and oxidation-mediated diseases
[115][79]. Natural substances contained in fruit and vegetables, such as resveratrol, quercetin, sulforaphane, and curcumin, to name but a few, all have a stimulating effect on the intranuclear pathway of transduction of the Nrf2 signal and an inhibitory effect on the NF-κB pathway (
Figure 1)
[116][80], with the result of limiting the effect of the cytokine storm that occurs in patients with severe COVID-19
[117][81] and persistent inflammation and autoimmunity
[118][82] that may occur in long COVID-19. These substances also exert an antiviral effect, both by binding to viruses outside the cell with electrostatic charges, and by preventing the binding process with their receptor, but also by limiting intracellular viral replication and hindering the escape of newly formed viruses from the cell
[68][45]. In fact, the addition of resveratrol to cell cultures infected with the SARS-2 virus prevents its replication and the consequent cell damage
[119][83] in relation to the conformational isomeric interaction between this polyphenol and the enzymes that all viruses use to replicate. The same inhibitory effect has been documented for quercetin
[120][84] sulforaphane and curcumin
[121][85]. The central structure of many synthetic antivirals consists of three rings present in most polyphenols
[122][86].