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Table 1 Summary of dietary recommendations of selected studies for strengthening the immune system against COVID-19 

From: Effective food hygiene principles and dietary intakes to reinforce the immune system for prevention of COVID-19: a systematic review

Author/Year Study design Nutrients or Foods or Lifestyle with immune-supporting roles Recommendations Score
Adams KK., et al./ 2020 Commentary Vitamins C, D. zinc, silver, elderberry Physicians and patients should not rely on dietary supplements to prevent or treat COVID-19  
Ali N./2020 Narrative Review Vitamin D Vitamin D supplementation has been reported to protect against respiratory tract infections 9
BourBour F., et al./2020 Systematic Review Protein, Omega-3 fatty acids, Vitamins A, D, E, B1, B6, B12, C iron, zinc, selenium Following a balanced diet may play a vital role in prevention of COVID-19
Supplementation according to the RDA is recommended for most of healthy people who do not have sufficient intake of nutrients
Moderate
Bousquet J., et al./ 2020 Narrative Review Cabbage, fermented milk products, Resveratrol These foods may reduce angiotensin-converting enzyme activity or are anti-oxidants 9
Briguglio M., et al./ 2020 Opinion Protein, vitamins A, D, E, B, Iron and micronutrients Mal-nutritional status is associated with immune dysfunction and Malnourished individuals may be more susceptible to SARS-CoV-2 infection  
Butler MJ., et al./ 2020 Narrative Review Healthy diet including foods high in fiber, whole grains, unsaturated fats and antioxidants
Unhealthy diet including foods high in saturated fats, refined carbohydrates and sugar
Individuals recommended for healthy diet to boost immune function and refrain from unhealthy foods 8
Calder PC., et al./ 2020 Narrative Review Vitamins A, B6, B12, C, D, E, folate
Trace elements, including zinc, iron, selenium, magnesium, and copper
Omega-3 fatty acids (EPA + DHA)
Suboptimal status of these nutrients negatively affects immune function and can decrease resistance to infections
Supplementation with micronutrients and omega-3 fatty acids based on RDA, is a safe, effective, and low-cost way to support optimal immune function
9
Author/Year Study design Nutrients or Foods or Lifestyle with immune-supporting roles Findings Score
Carter SJ., et al./ 2020 Perspective Vitamin D
Physical activity
Obesity
Obesity may increase the risk of symptom complications following a positive COVID-19 diagnosis
Calcitriol make protective effects from lipopolysaccharide- induced lung injury by modulating the expression of angiotensin-converting enzymes I and II
Physical activity may effectively improve vitamin D status. Supplementation with vitamin D (D2 or D3) has been shown to protect from acute respiratory infections, among individuals exhibiting vitamin D deficiency
 
Cena H., et al./ 2020 Mini Review Healthy diet, Mediterranean diet Healthy diet prevents undesired hyper-inflammation and might be useful for patients with mild signs of infection
Mediterranean diet which is rich in polyphenols has immune-protective and anti-inflammatory properties
8
Chakhtoura M., et al./ 2020 Commentary Vitamin D Vitamin D metabolites reduce the expression of cytokines due to the viral infection. Modulation of macrophage chemotactic protein1, interleukin 8, type 1 interferon, TNF-α and reducing oxygen reactive species  
Ciavarella C., et al./ 2020 Narrative Review Sea food and fish oil, pomegranate, herbs and spices (curcuma, thyme, oregano, hot pepper, rosemary, sage, lemongrass) These nutritional PPAR- γ agonists make an inhibitory effect on pro-inflammatory cytokines 12
D’Avolio A., et al./ 2020 Retrospective study- Brief report Vitamin D status The 25(OH)D level is significantly lower in SARS-CoV-2 PCR-positive patients than in PCR-negative patients  
Fan Y., et al./ 2020 Narrative Review Liquorice, garlic, ginger, turmeric, pomegranate, black pepper Functional food plants with immunomodulatory and antiviral properties 12
Gombart AF., et al./ 2020 Narrative Review Vitamins A, D, C, E, B6, and B12
Folate, zinc, iron, copper, selenium
Supplementation of these micronutrients may modulate immune function and reduce the risk of infection
Vitamins C and D and zinc are nutrients with the strongest evidence for immune supporting
10
Grant WB., et al./ 2020 Narrative Review Vitamin D Vitamin D supplementation is required for many individuals to reach 25(OH)D concentrations above 30 ng/mL, especially in winter 12
Author/Year Study design Nutrients or Foods or Lifestyle with immune-supporting roles Findings Score
Handu D., et al./ 2020 Narrative Review Nutrient-dense eating pattern and energy needs Dieticians should advise consuming a nutrient-dense eating pattern to meet protein and energy needs, with oral supplementation when necessary, to prevent and treat malnutrition in adults with comorbidities and not infected with COVID-19 10
Iddir M., et al./ 2020 Narrative Review Protein, and micronutrients including vitamins A, D, C, E, Bs, zinc, selenium and iron omega-3 versus lower saturated, trans fat, and omega-6 fatty acids, low refined sugars, high fiber content such as whole grains, phytochemicals Strengthening the immune system during the COVID-19 crisis can be obtained through healthy diet 10
Jayawardena R., et al./ 2020 Systematic Review Vitamins A, D, E, C Trace elements, Zinc, Selenium, Copper, Magnesium, Nutraceuticals and probiotics supplements Treating malnutrition and weight reduction in obese healthy subjects. Nutrition principles based on adequate nutrients could be useful in possible prevention of COVID-19.Selective micronutrient supplementations may be beneficial especially for vulnerable populations Moderate
Kara M., et al./ 2020 Narrative Review Vitamin D Call attention for the possible association between severe vitamin D deficiency and mortality pertaining to COVID-19 10
Kieliszek M., et al./ 2020 Narrative Review Selenium Selenite inhibits the entrance of viruses into the healthy cells and abolish their infectivity 9
Mailhot G., et al./ 2020 Narrative Review Vitamin D Vitamin D supplementation decreases the rates of viral respiratory tract infections  
Mehta S./ 2020 Rapid Report Healthy diet Adequate calorie intake and an optimal diet including variety of fresh fruit and vegetables, unsaturated fats, complex carbohydrate and sufficient protein and vitamin intakes  
Messina G., et al./ 2020 Narrative Review Omega-3 fatty acids (flaxseed oil), Vitamin C (Oranges, lemons, mangoes) Polyphenols (epigallo-catechin 3 gallate in green tea), flavonoids (Red wine, oranges, red fruits and vegetables) Modification of the dietary regimen to improve the levels of adiponectin could be very useful to prevent the infection 10
Razzaque, M S., et al./ 2020 Commentary Zinc Adequate zinc intake is important to protect from microorganisms, including viral infections  
Roy A., et al./ 2020 Narrative Review Zinc, Curcumin The cytokine suppression by curcumin can be related to clinical improvement in conditions associated with cytokine storm in viral infections 9
Ribeiro, Kdds., et al./ 2020 Commentary Healthy diet, vitamins A, D, and E, minerals zinc and selenium, fiber, and essential fatty acids Adequate dietary intake may be necessary to protect against inflammation of SARS-Cov-2 infection; Some nutrients including vitamins A, D, and E, minerals zinc and selenium, fiber, and essential fatty acids, have been reported to promote the immune system  
Zabetakis I., et al./ 2020 Narrative Review Mediterranean diet, unhealthy diet Fruit and vegetables, fiber, Fish and fish oils, vitamins C, D, E, Zinc, copper Following healthy diet which provides adequate nutrients will support immune function
People who are deficient in some micronutrients may warrant supplementation or modify their dietary patterns
10