FIGURE 1 Umbrella review process — nutraceuticals
Randa Karzon,1 Grad Cert, Andrew Jackson,2 ND, Iva Lloyd,3 ND, Alexander Hall,4 ND, and Lauren Lee,5 BHSc
ABSTRACT
Background: To identify the results of published review literature regarding nutraceuticals, including probiotics, melatonin, poly-unsaturated fatty acids (PUFAs), quercetin, N-acetyl cysteine (NAC), and propolis as they relate to the prevention and/or treatment of COVID-19 (CV) and/or long COVID (long CV) and to outline key areas to consider for clinical application and for further research.
Methods: This paper is part of a six-part umbrella review which progresses from a living review. This review incorporates systematic reviews and narrative reviews as they relate to nutraceuticals. A live literature search occurred monthly in PubMed and Google Scholar from May 2022 to May 2023. Assessing the Methodological Quality of Systematic Reviews Version 2 (AMSTAR-2) scoring assessed systematic review quality, while the Scale for the Assessment of Narrative Review Articles (SANRA) guidelines evaluated narrative reviews. Only those studies that were relevant to the nutraceuticals outlined above and that addressed COVID-19 prevention and/or treatment of CV and/or long CV were extracted from each review.
Results: Fifteen narrative reviews and 16 systematic reviews were included in this umbrella review. Studies indicate that nutraceuticals may be beneficial in improving the rate of recovery from various COVID-19 symptoms, rate of conversion parameters such as rate or duration of hospital stay and risk of intensive care unit (ICU) admission, and an improvement in various laboratory tests.
Conclusion/Summary: The broad antioxidant, anti-inflammatory, antiviral, and immune modulatory characteristics make the nutraceuticals included in this review reasonable choices for further research. Of the nutraceuticals discussed above, probiotics, melatonin, NAC, and quercetin indicate the greatest potential for benefit in the prevention and treatment of COVID-19 and long CV.
Key Words Anti-viral, anti-oxidant, anti-inflammatory, complementary therapies, immune system, naturopathic medicine, nutritional medicine, respiratory syndrome, SARS-CoV-2 vitamins, minerals
The first cases of atypical pneumonia were reported in December of 2019 and quickly spread to all areas of the globe. Within 1 year of those first cases, COVID-19 was declared a pandemic by the World Health Organization (WHO; see Appendix 1 for a glossary of terms and acronyms used in this manuscript).1 The development of vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, was a prominent focus during the pandemic,2 with the aim of reducing the death rate and the number and severity of new cases of COVID-19.2 Yet 3 years later, there remains a serious public health concern and the need for treatment and prevention is as great as ever, as even fully vaccinated individuals are not completely immune to infection.3 Early on in the pandemic, many varied conventional and natural therapies were suggested as potential agents to reduce the risk of infection and improve the prognosis of the disease.4 In this umbrella review, a team of naturopathic researchers supported by the World Naturopathic Federation (WNF) examine the available literature of common natural health products used by patients that are available over the counter or prescribed by naturopathic practitioners and other traditional, complementary and integrative healthcare (TCIH) practitioners for the prevention and/or treatment of COVID-19 and long COVID (long CV). Nutraceuticals included in this review are melatonin, poly-unsaturated fatty acids (PUFAs), probiotics, quercetin, N-acetyl cysteine (NAC), and propolis.
The nutraceuticals are worth considering for respiratory infections based on their historic use. The understanding of their biochemical properties—defined as nutritionally-based natural health products that are not commonly classified as vitamins or minerals—justify the need to consider their value in the prevention and treatment of viral infections. For instance, NAC is established as a mucolytic nutrient with antioxidant properties and has been found to improve outcomes for acute respiratory distress syndrome and acute lung injury patients.5 Quercetin, a flavonoid found in fruits such as grapes, apples, strawberries, cloudberries, and raspberries and in vegetables such as onions, tomatoes, lettuce, and green beans,6–7 has demonstrated in vitro effects of increasing neutrophil chemotaxis, natural killer (NK) cell lytic activity, and inhibition of influenza A.8 In vitro, in vivo, and clinical studies have highlighted the broad spectrum of antiviral properties of propolis.9 Some common actions of these nutraceuticals include anti-inflammatory, antioxidative, immune-modulating, and antiviral mechanisms.5–10 Melatonin is regarded as a positive regulator of immune responses and a negative regulator of inflammation.11 Lower melatonin levels are common in the elderly and, as there is a correlation between age and severity of COVID-19, there is value in considering the role of melatonin in COVID-19 prevention and treatment.12 PUFAs have demonstrated effectiveness in preventing respiratory tract infections,13 and broad evidence indicates that intestinal immune cells interact with consumed probiotics, and this interaction improves host immune homeostasis and immune function. Furthermore, the meta-analyses investigating probiotic clinical interventions on respiratory tract infections (RTIs) indicates that probiotic use is associated with a lower incidence and duration of mild RTIs, in both children and adults.14
The aim of this paper is to review the available literature on probiotics, melatonin, PUFAs, quercetin, NAC, and propolis with the goal of describing the published peer review literature and identifying key areas to consider for research and clinical application in the treatment of COVID-19.
This umbrella review began as a high-level live review monitoring of all narrative and systematic reviews on nutraceuticals and natural treatments for the prevention and symptomatic management of COVID-19 and its sequellae.15 The umbrella review is part of a six-part review in which this paper focuses on a group of nutraceuticals that are likely of value for the treatment of COVID-19 but do not fit neatly into other categories, such as herbal medicines, vitamins, or minerals. While narrative reviews are not typically included in an umbrella review, this paper included those that met specific criteria in order to provide context to the rationale for considering nutraceuticals in the prevention and/or treatment of COVID-19 and long CV.
Over the course of 1 year (May 2022–May 2023), monthly literature searches were performed to collect the current and emerging data on natural prevention or treatment of COVID-19. Searches followed Cochrane Guidelines for a live systematic review.16 PubMed and Google Scholar were searched using the following terms: “natur*,” “herb*,” “nutraceutical,” “botanical,” “medicinal plant,” “fish oil,” “quercetin,” “melatonin,” “vitamin,” “mineral,” combined with “prevention,” “prophylaxis,” “deficiency,” “treatment,” “management,” and “*COVID*,” “Coronavirus,” “SARS-CoV-2.” Individual names of nutraceuticals, compounds, vitamins, minerals and other health protocols were also searched. These papers were then classified as systematic reviews, narrative reviews, meta-analyses, or other based on their description in the abstract.
Narrative reviews and systematic reviews from the live review were eligible for inclusion in this paper if they were related to the use of the nutraceuticals for the prevention or treatment of COVID-19. Both review types were appraised by at least two blinded reviewers, systematic reviews based on the Assessing the Methodological Quality of Systematic Reviews Version 2 (AMSTAR-2) guidelines17 and narrative reviews according to the Scale for the Assessment of Narrative Review Articles (SANRA) guidelines.18 Systematic reviews were included if the review authors, at least partially, accounted for risk of bias (RoB) in individual studies and used a satisfactory technique for assessing the RoB. Narrative reviews were included if the scientific reasoning score was 1 or 2 and the overall total sum >5. Nutraceuticals that appeared in more than three systemic reviews after AMSTAR-2 assessment were included.
PICO eligibility criteria requirements for study inclusion were as follows:
Excluded were secondary analyses, literature reviews, editorial discussions, best practice guidelines, and book chapters. To ensure that results were not overstated, researchers utilized a citation overlap analysis to track the duplication of papers. Due to the diverse topics covered in this umbrella review, a sub-analysis of overlap was also conducted focusing on each of the nutraceuticals covered. The overlap was categorised as “slight” (0%–5%), “moderate” (6%–10%), “high” (11%–15%), or “very high” (>15%) (see Table S1 in the supplemental material for full details of the overlap analysis).
Data extraction for the systematic and narrative reviews was completed using separate shared, online spreadsheets. Some reviews covered topics that spanned multiple categories of the umbrella review, but only outcomes related to nutraceuticals were extracted for this paper.
Narrative reviews were included to highlight the therapeutic considerations and known mechanism of action of each criterion. The data extracted from the narrative reviews included study number, study identification, citation in American Medical Association (AMA) style, author(s), date, journal, country/world region, review objective, details of any search conducted, area of focus (prevention, treatment, long CV), and therapeutic considerations, including properties (anti-inflammatory, antiviral, etc.), based on the therapeutic properties outlined on https://www.ndhealthfacts.org/wiki/Action_of_Herbs and associations to other vitamins, minerals, or pathways.
The data extracted from the systematic reviews included study number, study identification, citation in AMA style, author(s), date, journal, country/world region, area of focus (prevention, treatment, long CV), review objective, review type (narrative, systematic, or meta-analysis), search date, search databases, study designs included, countries included in the studies, publication date range, tools for assessment of risk of bias and methods of synthesis/analysis. The study results data extracted included the intervention or exposure assessed, the outcomes measured, the number of studies included in the synthesis, the number of participants included in the studies, the age of the participants, the sex of the participants, the results or findings and results of heterogeneity analysis (if applicable). An overview of the umbrella review process is presented in Figure 1.
FIGURE 1 Umbrella review process — nutraceuticals
Given that the role of nutraceuticals in the prevention and treatment of COVID-19 is relatively new, narrative reviews were included as they provide an overview of the biochemical properties, and potential therapeutic considerations and they complement the details of the systematic reviews. Of the 93 narrative reviews included in the live review, 16 were originally identified for the nutraceuticals section and 15 were included (see Table 1).19–33 Of those, 6 were from the region of the Americas (Brazil,19,20 Argentina,21 Canada,22 Mexico,23 USA24), 5 from the European region (Italy,25,26 Denmark,27 Romania,28 Russia29), 2 from the South-East Asia region (India,30 Korea31), and one each from the Western Pacific region (Australia32) and Eastern Mediterranean region (Iran33). The individual nutraceuticals covered by these reviews included melatonin,20,21,25,26,28,29,31,33 quercetin,23,26,27,30,31 PUFAs,22,24,27,32 probiotics,19,24,32 and NAC.28
TABLE 1 Narrative Reviews – Nutraceuticals
The live review included 308 systematic reviews, of which 43 were initially identified as containing reporting about nutraceuticals; 15 were included (see Table 2).34–48 The included systematic reviews were conducted in the Western Pacific region (WPR),34–39 European region (EUR),40,41,42 region of the Americas (AMR),42–45 South-East Asia region (SEA)46,47 and Eastern Mediterranean region (EMR)48 and covered the following nutraceuticals: melatonin,37,38,41,48 probiotics,42,44–46 NAC,35,40,41 propolis,34,36,39,43 quercetin39,47 and PUFAs.46 The overlap analysis of the included systematic reviews found 2.6% overlap, which is classified as a “slight” overlap. Sub-analyses by nutraceutical category found “very high” for the reviews covering flavonoids (20%), PUFAs (24.2%), propolis (25%), and NAC (25%). “Slight” overlap was found for probiotics (1.3%). Melatonin was only covered in one systematic review and as such the overlap was nil.
TABLE 2 Systematic Reviews – Nutraceuticals
Five papers included multiple nutraceuticals and although specific nutraceuticals were listed as being included in the review, those that did not provide adequate details were excluded. Reviews were excluded if they did not refer to COVID-19. Only individual nutraceuticals that had at least three reviews were included.
Eight narrative reviews addressed the clinical and biochemical properties of melatonin as it relates to COVID-19 treatment and/or prevention. The anti-oxidant (n = 3), anti-inflammatory (n = 6), and immune modulating (n = 4) properties were the main focus of the narrative reviews on melatonin.20,21,26,28,29,30,33 Several of the reviews suggested that the actions of melatonin may be valuable for reducing complications from COVID-19 via suppression of the cytokine storm as well as improving long-term outcomes via neuroprotective effects,26,29,31 and sleep regulation.20,21,29 The benefits of melatonin in high-risk, aging populations and in those with long CV was also covered.25,29,33
Four systematic reviews included melatonin as an intervention. These focused mainly on antioxidant and immunomodulatory properties of melatonin clinical outcomes, including changes in laboratory biomarkers indicating a decrease in inflammation,41,48 and three reported on improved rates of recovery.37,38,48 Lan et al. (2022)37 included a meta-analysis of three small studies, only one of which was double-blinded, which showed a statistically significant improvement in clinical recovery rate (odds ratio [OR]: 3.67; 95% confidence interval [CI]: 1.21–11.12; I2 = 0%, p = .02) for patients with COVID-19 treated with melatonin compared with those treated with placebo, but no significant difference in the risk of intensive care unit (ICU) admission (OR: 0.45; 95% CI: 0.16–1.25; I2 = 0%, p = .13), mortality (OR: 0.32; 95% CI: 0.03–3.18; I2 = 0%, p = .33), the rate of CRP (C-reactive Protein; an inflammatory marker) normalization (OR: 1.14; 95% CI: 0.34–3.89; I2 = 0%, p = .83), or values of follow-up CRP (mean difference [MD], −1.03; 95% CI: −3.47 to 1.42; I2 = 0%, p = .41). Wang et al. (2022)38 reported better total effective rate among patients with COVID-19, 18 years of age and older, who were treated with melatonin plus conventional treatments compared with conventional treatments alone (OR = 3.05, 95% CI: 1.47–6.31; p = .003) in a meta-analysis of six small studies. Again, no significant difference was found for the biomarkers CRP (weighted mean difference [WMD] = −0.36, 95% CI:−3.65 to 2.92, p = .83), arterial saturated oxygen (SaO2) (WMD = 1, 95% CI:−1.21 to 3.22, p = .37), or white blood cell (WBC) count (WMD = −1.07, 95% CI:−2.44 to 0.30, p = .13).
The four narrative reviews discussing probiotics focused on their potential for prevention through the beneficial manipulation of the gut microbiome.19,24,30,32 Probiotics were ascribed actions such as antiviral19,30 and anti-inflammatory,19,30,32 mostly in the context of maintaining a functional gut lining and attenuating local and systemic immunity32 and limiting downstream gut–lung mucosal immune responses.24 Through the microbiome–gut–brain axis and the gut–lung axis, probiotics were argued to have the potential to reduce the risk of COVID-19 incidence as well as long CV symptoms.24
The four systematic reviews that included studies investigating probiotics all suggested to some degree that the immune modulatory effects of probiotics, likely due to improvement of mucosal barriers of the gut and lung and associated immune monitoring, may be protective against SARS-CoV-2 infection.40,42,44,46 Three of the studies emphasized the benefits of probiotics for gastrointestinal symptoms due to COVID-19.40,42,44 Neris et al. (2022) indicated that probiotics were associated with a significant 51% reduction in symptoms reported by COVID-19 patients (relative risk [RR] 0.49, 95% CI: 0.40–0.61). There was a significant improvement in cough (RR 0.56, 95% CI: 0.37–0.83), headaches (RR 0.17, 95% CI: 0.05–0.65), and diarrhea (RR 0.33, 95% CI: 0.12–0.96) of patients on probiotic therapy. These findings suggest that probiotic supplementation is effective in improving symptoms of COVID-19.42
The one narrative review exploring the value of NAC highlighted its antioxidant, mucolytic, anti-inflammatory, and immune modulating properties and indicated that NAC can alleviate oxidative stress and dampen the cytokine storm, thus having a protective effect on organ damage.28 It described the NAC as able to reduce inflammation by blocking the nuclear factor kappa B (NF-kappa B) pathway, as a precursor to glutathione, and as a free radical scavenger.28
Three systematic reviews included NAC, of which two reported on studies focused on NAC alone35,40 and one synthesized study involving NAC alongside other nutraceuticals.41 While one systematic review did find that NAC can reduce inflammatory markers interleukin-6 (IL-6), CRP, and procalcitonin (PCT),41 none of the systematic reviews found a relation between NAC treatment and reduced hospital stays, ICU admissions, or mortality.35,40.
Two of the three narrative reviews describing research related to quercetin emphasized its antiviral, antioxidant, and hyperlipidemic properties.26,30 Savant et al. (2021) stated quercetin has broad antiviral properties and is specifically an inhibitor of SARS-CoV-2,30 while Quintal Martinez et al. (2022) highlighted quercetin’s antiplatelet, anticoagulant, fibrinolytic, and nitric oxide (NO) regulating effects.23 The reviews suggest that, due to its antiviral activity and other targets of action, quercetin could play a role in prevention and to help reduce recovery time from COVID-19.
The two systematic reviews on quercetin focused on the treatment benefits of supplementation, and both reported similar findings.39,47 The two reviews were in general agreement and their conclusions were summarized nicely by Cheema et al. (2023), who concluded that “quercetin decreased the risk of intensive care unit admission (OR = 0.31; 95% CI: 0.10–0.99) and the incidence of hospitalisation (OR = 0.25; 95% CI 0.10–0.62) but did not decrease the risk of all-cause mortality and the rate of no recovery and that quercetin may be of benefit in COVID-19 patients but large-scale RCTs [randomized controlled trials] are needed to confirm these findings.”47
No narrative reviews on propolis met the inclusion criteria. However, four systematic reviews were included. Three of the reviews looked mostly at clinical outcomes of COVID-19, including symptom severity, duration of illness, duration of hospitalization, and duration of respiratory support.34,36,39 The rate of recovery results reported by Sobrinho et al. (2022) were not as positive as the other three papers, but the focus of this paper was more on the biochemical actions of propolis, which were stated to be anti-inflammatory, immunoregulatory, and anti-COVID19 effects, including protein kinase-1 (PAK-1) inhibition and binding to angiotensin-converting enzyme 2 (ACE2), one of the main routes of infection for SARS-CoV-2.43 All systematic reviews were in agreement that propolis may be useful but further research into the efficacy for the treatment of COVID-19 is needed to confirm.
The five narrative reviews that included PUFAs highlight their anti-inflammatory,22,24,27,30,32 antioxidant,30 and immune modulating properties.27,32 Paudel et al. (2022)32 and Gareau et al. (2023)24 focused on the ability of PUFAs to modulate immune system functions through alterations in the gut microbiome. Although each of these reviews provides some analysis of PUFAs and their effect on COVID-19, they are relatively inconsistent with regard to the form of PUFA assessed and the outcomes measured. For instance, Bader-Larsen et al. (2021) cautioned against the supplementation of omega-3 fatty acids in cancer patients (despite also suggesting it may be beneficial for COVID-19 symptoms) and barely mentions PUFAs other than to mention that short chain fatty acids (SCFAs) may be helpful for relieving gastrointestinal symptoms associated with SARS-CoV-2 pathology.27
The systematic review by Aldhafiri et al. (2022) was the only review focused on PUFAs included. It indicated that PUFAs together with probiotics may shorten the duration of COVID-19 and may have a direct effect on decreasing inflammation and gastrointestinal symptoms.46.
COVID-19 is a highly communicable disease caused by SARS-CoV-2 infection leading to a wide range of clinical manifestations, from mild forms, such as fever, cough, and myalgia, to moderate forms requiring hospitalization (pneumonia and localized inflammation), to severe/critical forms with fatal outcomes.49 As vaccine efficacy can be jeopardised by the rapid emergence and spread of SARS-CoV-2 variants,50 there remains a need for therapies to reduce the severity and duration of COVID-19 as well as long COVID.51 Three key areas where nutraceuticals may provide benefit relate to prevention, immune modulation, and anti-inflammatory support and supporting aging and high-risk populations.
A focus on prevention reduces the likelihood of contracting COVID-19 and reduces the risk of long CV. Of the nutraceuticals discussed, probiotics may have a unique role to play in reducing the risk of contracting COVID-19 as well as the risk or severity of long CV symptoms. The interaction of the gut microbiome with the immune system and the subsequent effects on inflammation, mental health, and a host of other health outcomes is no longer a novel concept.52 While tailoring the microbiome to precise parameters is not yet (and may never be) a possibility, we are aware of measures that may be taken to alter the gut microbiome to a state of eubiosis or a composition that is beneficial for human hosts.53 It is generally believed that certain beneficial bacteria generate certain SCFAs, such as butyrate, propionate, and acetate, that appear to be key mediators of the beneficial effects elicited by the gut microbiome. Microbial SCFA production is essential for gut integrity by regulating the luminal pH and mucus production, providing fuel for epithelial cells, and procuring beneficial effects on mucosal immune function. SCFAs also directly modulate host metabolic health through a range of tissue-specific mechanisms related to appetite regulation, energy expenditure, glucose homeostasis and immunomodulation, leading the immune system to better recognize infectious agents and to suppress the unnecessary activation of the innate inflammatory response. Therefore, increased microbial SCFA production can be considered a health benefit.54 Given this potential, further studies of probiotics (as well as synbiotics, prebiotics, and postbiotics) are warranted to determine how much benefit they can provide, the expected time frame to show effects, dosing, and the most effective species to prevent and treat COVID-19.
Many of the nutraceuticals in this paper are anti-inflammatory or immune modulating. NAC, for instance, is a precursor molecule to glutathione,55 the most important low-molecular-weight antioxidant synthesized in cells.56 COVID-19 leads to an immune response resulting in what is known as a cytokine storm.57 Long CV may develop via mechanisms involving neuroinflammation owing to unique signaling pathways and blood–brain barrier dysfunction, persistent inflammation related to an altered homeostatic milieu and organs, persistence of proinflammatory cells, altered cytokine production, and altered immune metabolic pathways in the lingering inflammatory response of the SARS-CoV-2 infection COVID-19.58 Anti-inflammatory and immune modulation would seemingly be the ideal properties needed to combat these effects of SARS-CoV-2. Although the nutraceuticals breadth of research in this field is limited, of the nutraceuticals covered in this review, NAC and quercetin seem to have the potential for the greatest benefit, NAC given its particular role as a precursor to one of the most important antioxidants in the body, and quercetin as it seems to be a potent inhibitor of the interaction between spike glycoproteins of SARS-CoV-2 and the ACE2 receptor.59 In addition, active compounds in propolis and honey have anti-inflammatory properties. Galangin, an active compound in propolis, has been shown to inhibit tumor necrosis factor-α and interleukin-8, leading to a decrease in tissue inflammation and clinical symptoms. PUFAs’ primary role is anti-inflammatory, reducing systemic inflammation and infections.
Older adults and those with comorbidities are at greater risk for developing COVID-19 and have worse outcomes. Many of the nutraceuticals studied in this paper may be beneficial indirectly as they are associated with decreasing the risk of comorbidities such as diabetes, cardiovascular disease, gastrointestinal diseases, and others.21,23,31 Melatonin warrants further consideration and research as it is well known that melatonin levels are often significantly lower in the elderly and are associated with decreased immune function.11,12 Melatonin protects against cellular damage induced by reactive oxidative species due to its anti-inflammatory and antioxidant effects, and promotes an adaptive immune activity.
Although the results of many nutraceuticals are promising, there are not nearly enough quality studies available to truly understand the significance of any of the above-mentioned nutraceuticals in their therapeutic role for COVID-19 or long CV treatment and prevention. Further studies are warranted to understand optimal effectiveness, dosing, and delivery methods for any of the nutraceuticals considered above. When interpreting the results, it is important to note that there was great heterogeneity between studies within most of the reviews and much more so between systematic reviews. Many of the studies included in the reviews also had small sample sizes, which limits the statistical power and generalizability of the findings. It is also important to note that there was very high overlap across systematic reviews for some of the nutraceuticals studied. This has implications for this umbrella review as it is important not to overemphasise findings arising from different systematic reviews if the reviews themselves draw upon the same original research articles. However, the high level of overlap is also an important finding for the wider research community investigating natural health products as it suggests significant duplication in research efforts have occurred during the COVID-19 pandemic, and that greater coordination is needed to avoid such duplication in the future.
The need for COVID-19 treatments remains significant, even after the development of effective vaccines. The antioxidant, anti-inflammatory, antiviral and immune modulatory characteristics make the nutraceuticals included in this review reasonable choices for further research. Additional high-quality randomized controlled trials with large sample sizes are required to fully determine their efficacy, with the inclusion of recommendations on dosing and administration. Of the nutraceuticals discussed above, probiotics, melatonin, NAC, and quercetin appear to have the greatest potential for benefit in the prevention and treatment of COVID-19 and long COVID.
1Diamond Health, Queensland, Australia;
2Bastyr University, Kenmore, Washington, USA;
3World Naturopathic Federation, Canada;
4Canadian College of Naturopathic Medicine, Toronto, Ontario, Canada;
5Endeavour College of Natural Health, Sydney, New South Wales, Australia.
The authors are grateful to Maria Karillis, Katrina Reeve, Hannah Richmond, Traceless Shaw, Kristan Gilbert, Beth MacGregor, Tobey-Ann Pinder, Cathrina Geldard, Rebecca Boothe and Gabrielle Covino for their assistance with the critical appraisal of articles included in this review. We also appreciate the editorial oversight and support provided by Dr Iva Lloyd, Dr Kieran Cooley and Associate Professor Amie Steel.
We have read and understood the CAND Journal’s policy on conflicts of interest and declare that we have none.
This research did not receive any funding.
Supplemental material linked to the online version of the paper at https://doi.org/10.54434/candj.165:
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Supplemental material for this article is available online at https://doi.org/10.54434/candj.165
Correspondence to: Randa Karzon, 26 Raff Street, Allora, QLD Australia 4362. E-mail: randakarzon@gmail.com
To cite: Karzon R, Jackson A, Lloyd I, Hall A, Lee L. The role of nutraceuticals in the prevention and/or treatment of COVID-19: An umbrella review. CAND Journal. 2023;30(4):66-80. https://doi.org/10.54434/candj.165
Received: 24 September 2023; Accepted: 16 November 2023; Published: 28 December 2023
© 2023 Canadian Association of Naturopathic Doctors. For permissions, please contact candj@cand.ca.
CAND Journal | Volume 30, No. 4, December 2023