Table 3.

Identified nutrient and food supplements along with their sources and mechanisms improving the immunity against Covid-19.

Nutrients	Proposed Mechanism	Sources	Ref
Micronutrients	The elements such as Zn, Se, Cu, Fe, and Mg, enhance both cell-mediated and humoral immunity. Also, they are associated with the production of Th1, Th17, Th9 having antioxidant properties along with the production of IgG	Zn: red meat, shellfish Mg: dark chocolate, black beans, avocados and whole grains Se: red meat, shellfish, chicken, garlic, grains	Gasmi et al. (2020)
Antioxidants	They possess redox mechanisms and allow in scavenging free radicals thus assisting in repairment of tissue and regulation the innate immune system	Green tea, black tea, chilli pepper, oregano, red onion, fennel leaf, grapes, apples, green vegetables, peppers, N-acetylcysteine	Nasi et al. (2020)
Probiotics	Consist of live microorganism (Lactobacillus, Bifidobacterium and Saccharomyces) that are intended to promote gut immunity and reduce the acute upper tract respiratory infections	Yoghurt, fermented oats, kefir, kombucha, sauerkraut, tempeh, sourdough bread, and some cheese	Baud et al. (2020)
Vitamin D	Enhance the innate immunity, through the production of antimicrobial peptides, supports in the production of monocytes, macrophages and dendritic cell, decrease the pro-inflammatory cytokines and enhance anti-inflammatory cytokines	Salmon, tuna, spinach, hazelnuts	Ilie et al. (2020)
Vitamin C and E	Both these vitamins possess antioxidant as well as anti-inflammatory properties that reduce oxidative stress. They have potential as an antiviral agent and supports host defence against the infection by affecting the growth and development of T lymphocytes and natural killer (NK) cells, thus preventing cellular damage	Vit. C: lemon, grapefruits, oranges, tangerines, sweet red pepper, broccoli, papaya, eggplant Vit. E: nuts, sunflower seeds, avocados, spinach and hazelnuts	Carr, 2020
Vitamin A	Enhance both innate (through natural killer cells, macrophages, neutrophils) and adaptive immunity (by differentiation, proliferation, functioning and movement) as well as assist in the maintenance of mucosal epithelium integrity and possess anti-inflammatory properties.	Carrots, spinach, kale, apricots, sweet potato, squash and cantaloupe	Jovic et al. (2020)
Water content-rich	Water intake keeps the mucus membrane moist and produces lymph which circulates the white blood cells and immune cell throughout the body lower the chances of diseases such as cold and flu.	Cucumber, watermelon, celery, coconut water,	Kau et al. (2011)
Amino acids and proteins	They are used to boost the immune system and also possess antioxidant properties.	Soybean, seafood, meat, fish, eggs, dairy products, tofu, cereal and pulses	Ferrara et al. (2020)