Potential Nutraceuticals for COVID-19

Introduction

SARS-CoV-2 has disrupted global health and economic wellbeing since the beginning of 2020. The regional office of World Health Organization (WHO) in China was first alerted to the virus infection in Wuhan on December 31, 2019 and termed the infection as an epidemic on March 11, 2020. Since then, laboratories across the globe have been collaborating to develop vaccines and therapeutic agents for this novel coronavirus.

The SARS-CoV-2 belongs to a group of viruses called the coronaviruses. These are single-stranded, positive sense, RNA viruses, enveloped in a helical capsid with spike shaped trans-membrane proteins. They can further be classified into four subtypes: alpha, beta, delta, and gamma.¹ The coronaviruses are capable of infecting both humans and animals and have the ability to jump the species barrier thereby accelerating the spread of the disease into an epidemic or a pandemic. The severe acute respiratory syndrome coronaviruses (SARS-CoV and SARS-CoV-2) and Middle East respiratory syndrome coronavirus (MERS-CoV) are the three highly pathogenic coronaviruses. The remaining four (HCoV-NL63, HCoV-229E. HCoV-OC43, and HCOV-HKU1) human coronaviruses are less virulent.²

What makes the SARS-CoV-2 threatening is not just the viral infection but the accompanying cytokine storm and other associated comorbidities. Analysis of clinical data from 326 confirmed COVID-19 patients in Shanghai revealed IL-6 and IL-8 to show the most significant changes and the levels of these two interleukins inversely co-related with the lymphocyte count.³ A reduced CD4⁺/CD8⁺ ratio is also a manifestation of the disease. A detailed explanation of the host immune response and the viral evasion methods follows in the later part of this paper.

Initial genomic phylogeny testing revealed that the SARS-CoV-2 virus shared about 79% and 50% gene similarity with SARS-CoV and MERS-CoV, respectively.² Therefore, the initial treatment has included therapeutics known to be effective against SARS-CoV and MERS-CoV for SARS-CoV-2.

In this article we give an overview of the present understanding of the viral lifecycle and host immune responses, which would form the basis for selecting nutraceuticals and natural products that can be potentially explored both as therapeutic and preventive interventions for the SARS-CoV-2 infection.

Virus Replication Cycle and Potential Therapeutic Targets

Understanding the cellular basis of SARS-CoV-2 infection could reveal treatments that prevent the development to a severe disease, and thus reduce mortality. Figure 1 represents a simplified illustration of the infection cycle of SARS-CoV-2.

Figure 1 SARS-CoV-2 replication cycle. (I) The spike protein (S) on SARS-CoV-2 facilitates attachment to the host cell through the ACE2, TMPRSS2. The S protein has two subunits, S1 and S2 (a). The S1 subunit attaches to ACE2 (b) following which TMPRSS2 cleaves ACE2 (c). The S2 subunit facilitates fusion of the viral particle with the host cell membrane thereby leading to viral entry (c). (II and III) Alternatively, the viral entry can also occur via endocytosis. (IV) Release of the viral genome (+ strand) after entry. (V) Translation of the strand leads to the formation of polyproteins (pp1a and pp1ab) which are cleaved by the main protease (MPro) and papain-like protease (PLPro) into the nonstructural proteins (nsps). (VI) The genome is replicated by RdRp. (VII) The transcription of the genome gives the subgenomic transcripts which encode the structural proteins. (VIII) The nucleocapsid is translated in the cytoplasm. (IX) The other structural proteins are translated in the endoplasmic reticulum (ER). (X) The nucleocapsid and the genomic strand form the genomic RNA. (XI) The structural proteins are glycosylated in the golgi bodies. (XII) A budding vesicle forms with the virion particles assembling. (XIII) Exocytosis of the assembled viral particle occurs. (XIV) The newly released viral particles can now infect other host cells.

The spike glycoprotein (S) is a 180–200 kDa transmembrane homotrimer which recognizes and binds to the host angiotensin-converting enzyme 2 (ACE2) receptors.⁴ The S protein can further be subdivided into S1 (the homotrimer head) and the S2 protein (the tail part). Figure 2 gives an enlarged view of the spike protein of the virus. ACE2 is a human receptor largely found in the respiratory and intestinal epithelial cells but it is also present in the kidney, heart, brain, etc. ACE2 has a short C-terminal intracellular domain and a long N-terminal extracellular domain, to which the S2 head of the virus binds.⁵ The spike protein mediates two essential events: binding to ACE2 by the amino-terminal region and fusion of viral and cellular membranes through the carboxyl terminal region.

Figure 2 Enlarged view of spike protein. (A) The binding of the S protein to the ACE2 receptor occurs at the receptor binding domain (RBD site) of the S1 domain, binding to the N-terminal domain of the host receptor, (B) the amino- or N-terminal domain (NTD), carboxy- or C-terminal domain (CTD) and the receptor binding domain (RBD) of the S1 subunit and the S2 subunit that constitute the spike protein (S).

The spike protein bound to ACE2 can be cleaved by two enzymes, the transmembrane serine protease 2 (TMPRSS2) at the S2 site and by furin at the S1/S2 connection site.⁶ TMPRSS2 acts at monobasic cleavage sites, ie it cleaves at a single arginine or lysine residue. Furin, however, cleaves at a polybasic site.⁶ Infection of lung cells requires host proteolytic activation of spike at a polybasic furin cleavage site. Cleavage by the furin protease, therefore, can expand SARS-CoV-2 cell tropism and may have facilitated transmission from bats to humans.² SARS-CoV-2 tropism is, therefore, dependent on expression of cellular proteases, as well as ACE2.

However, TMPRSS2 is not expressed in all the cells expressing ACE2 suggesting the existence of alternative pathways for viral entry such as through use of cathepsin L and cathepsin B.⁷ However, this is a matter of debate since studies have also found that cathepsins are not required for SARS-CoV-2 infection.⁶ Post-cleavage, the virus can enter the host cells either through membrane fusion or through endocytosis.⁵

Two-thirds of the viral genome produces two polypeptides, pp1a and pp1ab which are processed by the viral proteases chymotrypsin like cysteine protease (3CLPro) or main protease (MPro) and one or two papain-like proteases into 16 nonstructural proteins (nsps) that play diverse roles in the subsequent viral replication and infection.⁸ The remaining one-third of the genome encodes for the main viral proteins namely, the spike protein, the nucleocapsid protein, the membrane protein, and the envelope protein. Replication occurs within double membrane vesicles (DMVs) which comprise RNA-dependent RNA polymerase (RdRp) and the nsps. The sub-genomic particles manufactured here then move towards the endoplasmic reticulum golgi complex compartment where they undergo maturation. The virulent particles present in vesicles are further released through the process of exocytosis.¹ Thus, the life cycle of the virus with the host consists of five steps: attachment, penetration, biosynthesis, maturation, and release.

Therapeutics, therefore, can target either the viral proteins or the host proteases. Since the host proteases play multiple roles within the human body, targeting the host proteases may also interfere with other physiological activities in the host body and so caution must be exerted in this approach. Hence, viral proteins seem more attractive and safe targets.

In case of the SARS-CoV-2 virus, the following have been identified as the potential therapeutic targets: the spike glycoprotein (S), the envelope glycoprotein (E), the nucleocapsid protein (NP), the membrane protein (M), the papain like protease (PLPro) or the chymotrypsin like cysteine protease (CLPro), (also known as the main protease MPro) and the RNA dependent RNA polymerase (RdRp). In addition to this, certain nsps could also be targeted. The nsp13 helicase is an important component for replication of the virus which is target for many virus inhibitors.⁹ Nsp10 plays a major role in viral transcription wherein nsp14 3ʹ-5ʹ exoribonuclease and nsp16 2ʹ-O methyltransferase are stimulated by playing a lead role in viral mRNAs cap methylation.¹⁰

The following sections give an overview of the different plant-derived ingredients and more specifically nutraceuticals, that can potentially be developed into therapeutic molecules targeting one or more multiple targets discussed above.

Nutraceuticals and Coronavirus Infection

Nutraceuticals is a portmanteau of “nutrition” and “pharmaceuticals”. The structure of the word itself directs us towards the dual use of these molecules: for preventive health care (source of nutrients that will prevent the occurrence of disease) and as therapeutic molecules (like pharmaceuticals taken to cure a disorder or disease). Table 1 mentions the details of a few nutraceuticals that have been tested against the earlier coronaviruses SARS-CoV and MERS-CoV.

Table 1 Experimentally Tested Nutraceuticals Against Different Coronavirus and Host Proteins

Yi et al¹¹ performed in vitro and animal testing on multiple small molecules and found luteolin to be effective in blocking the S2 protein of the SARS-CoV virus. The SARS-CoV and SARS-CoV-2 S proteins share about 76% amino acid similarity.⁷ Since both the viruses bind to the same host receptor, there is an increased likelihood that molecules that block or interact with SARS-CoV S protein will most likely be effective against SARS-CoV-2 S protein. The authors also tested the effect of quercetin using a pseudotype virus assay, the rationale being that quercetin shared structural similarity to luteolin. Quercetin is an ingredient of antioxidant and antiallergy medicines that had been approved by the US Food and Drug Administration (FDA; the national drug code numbers of the medicines are 65448–3085, 65448–3005).

This strategy by Yi et al¹¹ can be extended to current search by identifying structures similar to the ones that are showing promise in silico and in vitro, but already have a generally recognized as safe (GRAS) status owing to their documented food or medicinal use. This can drastically reduce the time required for regulatory approvals and give more confidence for clinical trials. In fact, a similar strategy was earlier used by Lin et al¹² when they decided to test the efficacy of resveratrol, a stilbene derivative, based on earlier studies that showed antiviral activities on SARS-CoV.

The coronavirus main protease (MPro or 3CLPro) structural backbone and active site conformation are conserved despite sequence variations.¹³ Hence, nutraceuticals such as epigallocatechin gallate (EGCG), gallocatechin gallate (GCG), hesperetin, quercetin, which were previously shown to be effective against SARS-CoV 3CLPro can be tested for their efficacy in SARS-CoV-2 infection. Nguyen et al¹⁴ found that a galloyl moiety present at the 3ʹOH position of a potential inhibitor, belonging to the flavonoid class, is crucial for inhibition of 3CLPro in SARS-CoV. Enzyme kinetics suggested that GCG competitively inhibited 3CLPro with an inhibition constant of 25 µM.

As seen in Table 1, quercetin seems to have multiple targets of action and may prove to be a better candidate molecule for therapeutic development. Administration of quercetin (1000 mg) showed a decrease in the incidence and extent of upper respiratory tract infections (URTIs). Abian et al¹⁷ identified quercetin as a reasonably potent inhibitor of SARS-CoV-2 3CLpro protease, with the inhibition constant being –K_i ~ 7 μM. Quercetin also modulates the cellular unfolded protein response (UPR). As coronaviruses can utilize the UPR to complete different stages of the viral life cycle during infection, this is an important finding.¹⁸ Early clinical data suggests that quercetin has broad antiviral property and acts at various steps of viral life cycle. As an FDA-approved drug ingredient, with potent antiviral action, quercetin offers great promise as a potential drug candidate in the clinical treatment of SARS.

In silico Studies Reveal Potential Therapeutic Nutraceuticals

Given the importance of rapid generation of lead compounds, many authors have used in silico docking and modelling studies to generate a library of potential therapeutic lead molecules targeting different stages of the viral infection life cycle, some of which have been collated in Table 2. It is interesting to observe the structural similarities among these molecules—having a catechol/resorcinol moiety—seems to be a common feature in these nutraceuticals (Figure 3).

Table 2 Potential Nutraceutical Therapeutic Targets as Revealed by in silico Studies

Figure 3 Nutraceuticals shown to be binding to target sites of SARS-CoV-2 are as follows: (A) δ-viniferin binds to RdRP, MPro and human ACE2 receptor, (B) myricetin binds to S protein, (C) silibinin binds to RdRp, (D) curcumin binds to MPro, NP, nsp 10, (E) EGCG binds to MPro, (F) luteolin binds to S protein, (G) carnosol binds to MPro, (H) quercetin binds to S protein.

Khan et al¹⁹ docked 18 known natural antiviral compounds against seven SARS-CoV-2 proteins and compared them with remdesivir and chloroquine. Their modelling studies showed EGCG and curcumin to affect multiple targets of SARS-CoV-2. EGCG showed binding affinity towards main protease, spike protein, post fusion S2 protein, and nsp15 endoribonuclease. Curcumin, however, showed increased binding affinity towards the main protease and nsp15 endoribonuclease. Curcumin binding to NP and nsp10 was observed by Suravajhala et al.¹⁰

Joshi et al¹³ screened a custom-made library of more than 7000 natural molecules such as flavonoids, glucosinolates, terpenoids, alkaloids, and others against the MPro. MPro does not have a sequence homologous structure in humans and hence targeting this protease would be specific for the virus. Since coughing is a major symptom of COVID-19, Joshi et al also screened traditionally known antitussive medicines against various structural proteins of SARS-CoV-2. They followed this up by screening the hits against RdRp and ACE2 in order to find multi-target inhibitors. Multi-target inhibitors have the added advantage that they could be effective in all stages of the virus life cycle. EGCG is shown to reduce TMPRSS2 secretion. Clerodendrum species have been used as an age-old remedy for respiratory ailments. Hence, phytochemicals from 12 different species were assessed for their potential to bind to the coronavirus proteins, wherein stigmasterol was found to be a promising candidate.²⁴

Silibinin not only acts as a direct inhibitor of viral proteins, but is also found to play a role in modifying the inflammatory response. Similarly, EGCG, has additional fat burning action (increasing fat oxidation), which prevents disease progression.

Hu et al,²² observed that flavonoids in general, showed higher affinity towards MPro compared to alkaloids, terpenes and saponins. They suggest that this may be owing to the phenolic hydroxyl group of the flavonoids which can interact more easily with the heteroatoms of the amino acids of MPro. They found rutin to be especially effective in binding to MPro. Buckwheat, apple and tea contain considerable amount of rutin and can be consumed in higher quantities to protect oneself from the infection. The screening study conducted by Fischer et al,²⁵ revealed rhamnetin, found in Moringa oleifera plant, to show greater affinity for MPro. Rhamnetin is already commercially available. Rhamnetin is also structurally similar to quercetin. Myricitrin is a glycosylated analog of myricetin and is found in black grapes. Glycosylated flavonoids have an increased bioavailability compared to the aglycone forms.²⁵ Myricitrin, myricetin, resveratrol, and δ-viniferin, have been found to be potential anti-COVID-19 therapeutics.

Kar et al,²³ evaluated multiple phytochemicals and nutraceuticals for their potential to bind to either S, MPro or RdRp proteins. Post evaluation of in silico binding, these authors have also assessed the adherence of the selected ligands to the Lipinsky’s rule of five. While eugenol was found to adhere to the Lipinsky’s rule of five, mangiferin showed slight deviation. However, these deviations can be corrected through formulation innovations.

Octacosanol, cinametic acid, lauric acid, ascorbyl palmitate, and palmidrol are few of the nutraceuticals found to bind to the envelope protein. These can potentially disrupt the viroporin activity, and can provide relief from edema and inflammation, commonly observed in acute respiratory distress syndrome (ARDS).²⁶

Sulforaphane is an isothiocyanate formed upon hydrolysis of glucosinolates found in cruciferous vegetables like broccoli and cabbage and can modulate levels of TMPRSS2.²⁷ It is also shown to upregulate Nrf2 thus enhancing the anti-inflammatory response of the body. Hence, consumption of cruciferous vegetables may be helpful as a preventive strategy against SARS-CoV-2.

Given that these molecules target different viral proteins at different stages of viral replication, and a majority of them are common ingredients of the food chain (eg curcumin from turmeric, carnosol from rosemary, quercetin in apples, etc), it can be assumed that consumption of such foods may positively impact protection against SARS-CoV-2 infection. However, supraphysiological dosing of nutraceuticals is a double-edged sword, with evidence on both sides—of effectiveness and toxicity. For example, the supraphysiological dosing of vitamins and nutrients (mainly flavonoids and omega-3 fatty acids) for treatment of brain injury is now well established with strong evidence backing their use.²⁸ However, high doses of chlorogenic acid are known to increase plasma homocysteine levels.²⁹ This kind of unexpected spinoff is more pronounced in the case of antioxidants, as they function as antioxidants at physiological dose, while occasionally at higher amounts a pro-oxidant activity is observed. Antioxidants by their very nature are “redox” agents, hence they have both antioxidant and pro-oxidant activities inherent in their structure. It is important to note that the physiological concentration of these nutraceuticals determines their function.

Plant Protease Inhibitors as Potential Therapeutic Molecules Against SARS-CoV-2

Viruses make use of proteases (generally host proteases) to gain entry into the host. Synthetic protease inhibitors have been widely used as treatment tools for a variety of viral diseases such as those caused by HIV, HCV, picornaviruses, SARS, rotavirus and others.³⁰ They are also being investigated as potential treatment options for diabetes, cancer and inflammation. Viral protease inhibitors against HIV, HCV, picornaviruses, SARS have also been studied from bacterial and fungal origins.³¹

TMPRSS2 and furin are host serine proteases that facilitate the entry of SARS-CoV-2 into the host cell. TMPRSS2 is a trypsin like serine protease while furin is a subtilisin type serine protease.³² In addition, PLPro and the MPro which is a chymotrypsin-like cysteine protease, play important roles in viral replication. The epithelial cells of the respiratory tract themselves release proteases/protease inhibitors that help maintain homeostasis required for healthy lung functioning.²⁷ The protease/protease inhibitors balance determines the extent of respiratory viral pathogenesis. A shift towards more proteases can increase viral pathogenesis while the reverse may help protect against it.

Animal studies by Deng et al³³ have shown that coronaviruses can gain quick resistance to 3CLPro inhibitors but this resistance comes at the fitness cost of the virus not being able to replicate fast enough thereby reducing infectivity. TMPRSS2 is a host secreted type 2 transmembrane serine protease and is shown to be sensitive to antiproteases in the midst of an infection.²² Therefore, protease inhibitors or antiproteases against these protease enzymes can be explored as potential therapeutic molecules.

The reviews by Hellinger and Gruber³⁴ as well as Srikanth and Chen³⁵ describe a number of plant protease inhibitors, some of which have been collated in Table 3. This could be a good starting point to start assessing these molecules for their effectiveness against the SARS-CoV-2 virus 3CLPro and papain-like proteases.

Table 3 Examples of Plant Protease Inhibitors (PPIs) as Potential Antiviral Agents

Although the main function of antiproteases is inhibiting or deactivating proteases, newer research is identifying more roles for them in controlling excessive inflammation and microbial infection.²⁷ Hence identifying multifunctional plant protease inhibitors can allow multistep protection in coronavirus infection.

Immunomodulatory Effects and Inflammatory Responses Brought About by the SARS-CoV-2 Virus

The Immune System—Natural Defense Mechanism

During viral infection, the host cells recognize an invading virus through the pathogen-associated molecular patterns (PAMPs). This recognition further activates a series of cell signaling pathways through the pattern recognition receptors (PRRs). The toll-like receptors (TLRs), particularly TLR3 and TLR7 (endosomal receptors) and TLR8, recognize single-stranded RNA while other TLRs recognize the different viral components and set off the cellular signaling pathways. This leads to the production of pro-inflammatory cytokines (IL-6, TNF-α, type-I interferons (IFNs) etc) (Figure 4) and certain chemokines which further triggers the immune responses by the various immune cells (such as the antigen presenting cells, APCs). The release of the cytokines and chemokines is the starting point for the downstream effects such as inflammatory responses, apoptosis or pyroptosis, and the activation of the adaptive immune response. Type-I IFN pathway plays a central role in mediating innate immune responses to viral infections.

Figure 4 Signal cascade following the virus entry into the host cell. Viral entry results in the activation of the toll-like receptors (TLRs) present in the endosome and subsequent activation of the innate immune system through the NFκB transcription factor and interferon-regulatory factors (IRFs) which enter the nucleus and upregulate the expression of cytokines (IL-6, TNFα and type I interferons, etc).

The type-I IFNs activate the JAK-STAT pathway and the phosphorylation of the STAT1and STAT2 causes the transcription of IFN-stimulated genes (ISGs) to further reduce viral replication and thereby reduce the viral load in the system.¹ The release of NF-κB transcription factors occurs downstream of TLRs activation and causes the release of pro-inflammatory cytokines.

The mitogen-activated protein kinases (MAPKs) pathway also gets activated during SARS-CoV infection. SARS-CoV also distinctly enhances the NLRP3 inflammasome pathway leading to a profound expression of pro-inflammatory cytokines and ROS.

The adaptive immune system is activated by the phagocytes that present the antigen to the CD4⁺ T-helper cells which fight against the virus by developing into Th1 cells. The APCs can also activate CD8⁺T cells which further exhibit their cytotoxic effects along with natural killer (NK) cells. The CD4⁺ cells also stimulate the CD8⁺ cells through co-stimulatory signals. The T cells further activate the B cells leading to antibody production (specifically IgM and IgG) (Figure 5).

Figure 5 Activation of cytokines post infection. The release of the virions and the subsequent spread of the infection to other cells leads to the production of several cytokines that activate the adaptive immune system. High viral titer leads to activation of the vascular endothelium and blood coagulation, especially in those suffering from comorbidities.

Immunomodulatory and Inflammatory Responses Triggered by SARS-CoV-2

At the onset of the infection cytokines play an important role in recruiting other phagocytes at the site of infection and eliciting immune response. Increase in viral load and evasion of the innate immune system by the virus causes a cytokine storm viz a marked increase in the levels of pro-inflammatory cytokines through various cellular pathways triggering inflammation, which contributes to morbidity and mortality. It has been reported that the cytokine storm aggravates the disease resulting in ARDS, a characteristic condition exhibited by critically ill patients.

The virus has developed several strategies to evade the human defense system. It avoids the immune system by forming double vesicles that escape detection through PRRs. Furthermore, the viral particles are able to inhibit the STAT family transcription factors and suppress the expression of type-I IFNs.³⁶ The nsps function to alter the structure of the virus to mimic the host cell components and thereby evade detection by the host immune system.¹

Lymphopenia has also been cited as a cause for exacerbation of adverse effects of the disease.³⁷ The low count of T cells results are not only due to the viral load, but also due to the cytokine storm. Previously, it was observed in SARS-CoV infected individuals that there was an imbalance in the CD4⁺ and CD8⁺ ratio with a significant decrease in the CD4⁺ count compared to CD8⁺ cell count.^38,39 Ageing, by itself results in similar disruption of CD4⁺/CD8⁺ ratios, which in turn may be responsible for the increased susceptibility of the aged to SARS- CoV-2 infection. The cytokine storm and the immune dysregulation renders an individual with other comorbidities more susceptible to the disease. Hence, it is essential to build or enhance immunity not only to prevent infection, but also to prevent the acute stage of the disease once infected.

Interventions to Regulate Immune Response in COVID-19 Patients

Several nutraceuticals and natural compounds are known to have antiviral and immunomodulatory properties. Some of these compounds exhibit their functions by counteracting inflammation and ameliorating the ROS. Some of the strategies used in the past to counteract SARS-CoV infection were downregulation of MAPK pathway, reduction of NLRP3 inflammasome effects and inhibition of NF-κB while targeting a decrease in the pro-inflammatory cytokines, especially IL-6.^40,41

Over expression of IL-6 causes enhanced expression of STAT3. Studies suggest that the use of inhibitors of the JAK-STAT pathway can be potential therapeutic strategy. Recent studies have discovered the positive effect of silibinin, a flavolignan obtained from milk thistle and apigenin obtained from dried parsley in inhibiting cytokine storm by preventing STAT3 activation and its nuclear translocation.²¹ Some of the nutraceuticals that can downregulate these different pathways are mentioned in Table 4. Immunity boosters that restore the adaptive immunity are also potential targets. Jacalin (a lectin from jackfruits) was shown to increase the levels of CD4⁺cells and thus restore the CD4⁺ and CD8⁺ cell ratio in an immunocompromised individual. Thus, it could be used to address the issue of lymphopenia and further enhance immunity.⁴²

Table 4 List of Nutraceuticals Exhibiting Immunomodulatory and Anti-inflammatory Effects

COVID-19 and Metabolic Diseases

A large number of people suffering from various metabolic diseases, such as hypertension, obesity, cardiovascular diseases, type 2 diabetes, etc are more susceptible to infection from the SARS-CoV-2 virus.⁶⁰ The immunomodulatory effects due to the infection not only contribute to the severity of the metabolic diseases, but also exacerbate the adverse effects of infection.

An understanding of the onset and subsequent severity of the viral infection along with other comorbidities will help in identifying suitable nutraceuticals that can act on relevant pathways to ameliorate the conditions. Figure 6 succinctly delineates the cross-talk between the immune and metabolic pathways.

Figure 6 The interlinked immune signaling pathways and metabolism. The green box represents adipocytes, orange box represents hepatocytes, and purple box represents pancreatic cells. The green arrows represent the metabolic process and the cellular machinery process, the blue arrows represent the immune response elicited by molecules or factors and the red lines represent inhibition. The PPARs and PGC1α present in the cells influence adipokines and β-oxidation; the blue dotted line represent regulation (either up- or downregulation). The immune signaling involved during the metabolism process includes release of pro-inflammatory cytokines by the adipocytes (IL-6, TNFα, MCP-1, VEGF-A, etc), inhibition of lipolysis via activation of PI3K/Akt pathway, inhibition of insulin function by the adipokines (IL-6 and TNF-α) and activation of AMPK pathway, which upregulates PGC1α further enhancing β-oxidation. Dyslipidemia results in inflammation in the hepatocytes through the JNK and MAPK pathways which release pro-inflammatory cytokines that can inhibit β-oxidation and expression of β-cells in the pancreas. The black box represents a SARS-CoV-2 infected cell where the MCP-1 causes macrophage infiltration (orange circle) at the site of infection resulting in a decrease in HDL levels, loss of PON1 enzyme activity and increase in oxidization of LDL. It also results in activation of inflammatory pathways (JAK/STAT and NF-κB) and hence the cytokine storm.

Noncommunicable Diseases

Two major noncommunicable diseases, obesity and type-2 diabetes, are characterized by the dysregulation of PPARγ and IRS-1 expression occurring through the influence of IL-1 and TNF- α.^61–63 Downregulation of ppargc1a further leads to pulmonary damage of the alveolar macrophages and epithelial cells, a characteristic symptom in the critically-ill COVID-19 patients. Furthermore, in addition to the alveolar epithelial cells, adipose and pancreatic tissues too express ACE2 and are thus prone to SARS-CoV-2 attack.⁶⁴ Diabetic patients are often treated with ACE inhibitors, which results in higher expression of ACE2.⁶⁵ Impaired glucose metabolism in diabetic patients results in dysregulated T cell activation and causes oxidative stress, further compromising the immune system of the infected individual.⁶⁶

Dyslipidemia

A disruption in lipid homeostasis leads to dyslipidemia, characterized by elevated total cholesterol, triglycerides and low-density lipoprotein (LDL) with low levels of high-density lipoprotein (HDL).⁶⁷ IL-6 leads to vascular cell damage by enhancing the permeability of oxidized LDL, resulting in lung injury. During the SARS-CoV-2 infection, the cytokine storm increases LDL oxidation and reduces the levels of HDL-c and PON1, leading to dyslipidemia and exacerbation of other metabolic diseases.⁶⁸

In accordance with these observations, the lipid profile of the epithelial cells in patients suffering from SARS-CoV showed high concentrations of fatty acids and phospholipids and this condition has been considered to be conducive for viral replication.⁶⁴ Furthermore, SARS-CoV-2 entry results in higher levels of angiotensin II which can lead to upregulation of IL-6, CAMs and several other cytokines and chemokines via the NF-κB pathway causing vascular diseases.⁶⁹

Suitable Interventions to Address the Metabolic Diseases

In order to reduce this influence of the metabolic diseases and COVID-19 on one another, the target nodes in focus could be, (a) AMPK pathway (the upregulation of AMPK pathway downregulates inflammation and exhibits hypolipidemic effects), (b) specialized pro-resolving mediators, SPMs (agonists of PPARγ as their activation would target lipid metabolism as well as work as potential SPMs to restore homeostasis in a COVID-19 patient), (c) lipid oxidation (fatty acid oxidation ensures minimization of lipotoxicity as well as reduces damages caused to cells by dyslipidemia).

Nutraceuticals used in the management of metabolic disorders are suitable interventions to alleviate the comorbidities in a COVID-19 patient. These are listed in Tables 5 and 6.

Table 5 Natural Food Sources Exhibiting Anti-obesity, Antidiabetic and Hypolipidemic Effects

Table 6 Natural Products and Their Bioactive Molecules Exhibiting Anti-obesity, Antidiabetic and Hypolipidemic Effects

Probiotics and Prebiotics

Probiotics or the gut microbiota play an important role in curbing inflammatory response and the associated metabolic diseases. Strains of Bifidobacterium and Lactobacillus species are the common probiotics found naturally as well as available commercially. The brain–gut axis facilitates the expression of anti-inflammatory cytokines through inhibition of TLRs.¹¹³ The intestine has a high expression of ACE2 and is thus susceptible to SARS-CoV attack, evident from the GI symptoms exhibited in a few patients.¹¹⁴ Therefore, supplementing a patient with probiotics is a therapeutic approach to control the spread of the infection and can also act as a prophylactic treatment. The presence of microorganisms in the lungs has also been reported, with Bacteroidetes, Firmicutes, and Proteobacteria being the three main phyla characterized.¹¹⁴ Similar to the brain–gut axis, there exists the gut–lung axis—a link between the gut microbiota and the lungs acting through blood vessels, whereby the metabolites from the gut has an influence on the functioning of the lungs, acting via Nod-2 and GM-CSF signaling, and in turn a respiratory ailment could lead to dysbiosis.¹¹³

Several studies reported alleviation of symptoms of the common cold and a reduction in its duration by supplementation with Lactobacillus plantarum HEAL9 (DSM 15312) and Lactobacillus paracasei 8700:2 (DSM 13434) at a concentration of 1 ⨰ 10⁹cfu/day¹¹⁵ and Lactobacillus gasseri PA 16/8, Bifidobacterium longum SP 07/3, Bifidobacterium bifidum MF 20/5 at a concentration of 5 × 10⁷cfu/day.¹¹⁶ When these probiotics were administered along with the influenza vaccine, an increase in the antibody titer was observed.¹¹⁷ An ongoing clinical trial is noteworthy in this regard.^118,119

Prebiotics too have a beneficial role in conferring anti-inflammatory activity by inducing the production of IL-10 and IL-17.¹²⁰ The short chain fatty acids (SCFA) like acetate and butyrate activate FFA receptors, leading to a downregulation of NFκB and MAPK pathways.¹²¹ The levels of certain LPS-induced pro-inflammatory cytokines and chemokines, like TNF-α, CXCL8, CCL20 show downregulation during butyrate administration to the system.¹²⁰ SCFAs also exhibit immunomodulatory functions by regulating the function of T cells.¹²⁰ Oat bran and psyllium husk fiber act as natural prebiotic by undergoing fermentation by the gut microbiota to yield SCFA. These can also bind to PPARs and facilitate homeostasis.¹²²

Bioactive Peptides and Other Natural Molecules and Their Immunomodulatory Actions

Some of the bioactive peptides that have shown beneficial effects in reducing inflammation and functioning as antioxidants are summarized in Table 7.

Table 7 Bioactive Peptides and Their Functional Properties in Ameliorating Metabolic Diseases

Vitamin C

Vitamin C (ascorbic acid) possesses antioxidant and immune-modulatory property. It helps in mitigating the injury caused by oxidative stress during viral infections.¹³⁵ Patients suffering from ARDS show reduced levels of vitamin C (Vit C) and external administration of Vit C reduced the extent of pulmonary inflammation in ARDS.¹³⁶ With inflammation being a major cause of severity of COVID-19, supplementation with Vit C is hypothesized to aid in counteracting the actions of pro-inflammatory cytokines, especially IL-6. Treatment with Vit C (500 mg twice a day)¹³⁷ and its intravenous administration (doses of 6 to 12 g/day and 24 g/day for seven days) (NCT04264533) reduces the levels of IL-6 and other inflammatory cytokines (ferritin and D-dimer).¹³⁸ Vit C as an adjunct treatment could help in reducing the pneumonia-like symptoms and significantly reduce the mortality rates.¹³⁹

Clinical trials are being carried out with a combinatorial treatment of Vit C along with other substances such as quercetin, vitamin D and zinc in different formats (IV or oral dosages) to treat COVID-19.¹³⁷ Co-administering quercetin with Vit C and vitamin Vit B3 in mice with stress-induced H1N1 infection showed a delay in the time-to-death and reduced the mortality, as opposed to administration of single vitamins.¹⁴⁰ A similar co-administration of Vit C along with quercetin, vitamins D and B3 could help to ameliorate the symptoms of severe COVID-19 cases. Vit C is known to increase the efficacy of quercetin¹³⁹ and a combinatorial therapy of quercetin (500 mg)—Vit C (500 mg)—bromelain (50 mg) as a prophylactic treatment on health-care workers was found to be effective over a three-month study.¹⁴¹

Vitamin D

Studies have indicated the role of vitamin D (Vit D) in immune modulation. The review by Grant et al,¹⁴² mentions the different mechanisms by which Vit D may exert its antiviral effectiveness. It has been observed that Vit D downregulates the ACE2 receptors by negatively regulating the renin-angiotensin system (RAS).¹⁴³ A recent study observed an association between COVID-19 severity and the high latitude countries which receive less sunlight leading to Vit D deficiency in people. However, this observation needs to be taken with the understanding that every population would show a mix of people suffering/not suffering from Vit D deficiency and hence generalizations as such may be difficult.¹⁴⁴

Ghavideldarestani et al¹⁴⁵ suggested the role of Vit D in preventing local lung inflammation in COVID-19 patients. Angiotensin II through binding to the receptor AT1 can increase vasoconstriction, oxidative stress and inflammation. This angiotensin II can be cleaved by ACE2 into angiotensin1-7 which counteracts the harmful effects of angiotensin II. However, in patients with COVID-19, the ACE2 is hijacked by the virus for its entry and hence less of it is available for this regular physiological role. Vit D deficiency is shown to increase renin production thereby increasing ACE and angiotensin II production. This can lead to inflammation in the lungs that is observed in severe cases of COVID-19.

Glinsky¹⁶ took a gene-first approach to investigate potential therapeutic targets for COVID-19. They first identified the human genes responsible for the expression and function of ACE2 and FURIN, the attachment molecules of the virus. Next, they studied the expression profile of these genes when infected by the coronavirus. This was followed by searching the literature for molecules that may downregulate the concerned gene expression which led to the discovery of three potential therapeutic molecules—Vit D, quercetin, and estradiol. Vit D was found to alter the expression of 84 of 332 (25%) genes encoding human proteins that serve as target for the SARS-CoV-2. The author suggested a two-ingredient (quercetin-Vit D) or a three-ingredient (quercetin-Vit D-estradiol) formulation as an adjunct therapy for coronavirus patients. In fact, two clinical studies involving Vit D are already registered.

Given the potent antiviral effects of quercetin, especially in the crucial stages of viral entry and its replication, and the ability of ascorbic acid and Vit D to efficiently reduce the oxidative stress induced by inflammation and activate the RAS pathway, respectively, adjunct therapy with these molecules is an approach that warrants further research for its potential benefits.

Lactoferrin

Lactoferrin, experimentally, has been found to inhibit viral entry in murine coronavirus, and human coronaviruses hCOV-NL63 and pseudotype SARS-CoV. Besides preventing viral entry, lactoferrin can also suppress virus replication after the viral entry. Lactoferrin was shown to chelate ferritin thereby downregulating IL-6 and TNF-alpha. There is a clinical trial currently underway to study the effect of lactoferrin as an adjunct therapy for COVID-19.¹⁴⁶

Conclusion

Studies have hitherto revealed many details about the SARS-COV-2 virus infection. Although we have gained insights into the SARS-CoV-2 viral pathogenesis, a definitive treatment course is still distant. In such times, the intelligent way is to maintain the best possible health by taking suitable adjuvants to avoid falling ill.

It is evident that there are many ingredients that can be included in daily diet to possibly gain immunity or protection against coronavirus. Polyphenols, leguminous seeds containing plant protease inhibitors, as well as proteins, such as whey protein or jackfruit seed protein, could be incorporated in the daily diet. Other bioactive polyphenols, such as EGCG, GCG are common constituents of green tea, quercetin is found abundantly in apples while hesperetin is present in citric foods. These foods can also become components of daily diet. Spices such as turmeric, thyme, rosemary, garlic have anti-inflammatory properties and can be used in daily cuisine. In conclusion, the current review highlights the potential benefits of a range of nutraceuticals in the management of COVID-19. Further work aimed at clarifying the mechanisms of actions and potential therapeutic utility requires further investigation.