Table 2.

Important dietary sources of nutrients that support the immune system. The reference daily intakes (RDIs) are for healthy individuals. The RDI for infants, children and lactating women is different which should be consulted with the physician. Adopted from Calder (2020).

Nutrient	Dietary Source	Main Roles on The Immune System	Reference Daily Intake (RDIs) in Adults and Children≥ 4 Years According to FDA	Tolerable Upper Intake Level (UL) Per Day
Vitamin A	Milk and cheese, eggs, liver, oily fish, fortified cereals, dark orange or green vegetables (e.g., carrots, sweet potatoes, pumpkin, squash, kale, spinach, broccoli), orange fruits (e.g., apricots, peaches, papaya, mango, cantaloupe melon), tomato juice	Vitamin A has both promoting and regulatory roles on the innate immune system and adaptive immunity; therefore, it can enhance the organisms' immune function and provide enhanced defense against multiple infectious diseases. Its deficiency is associated with the impaired barrier, function, altered immune responses, and increased susceptibility to a range of infections (Z. Huang et al., 2018) (Z. Huang et al., 2018)	900 mcg	3000 mcg Institute of Medicine et al. (2002)
Vitamin B6	Fish, poultry, meat, eggs, whole grain cereals, fortified cereals, many vegetables (especially green leafy) and fruits, soya beans, tofu, yeast extract	Cell-mediated immunity and, to a lesser extent, humoral immunity are being affected by Vitamin B6 deficiency in both animal and human studies. Its intake or supplementation improves some immunological parameters in vitamin B6-deficient animals and humans(Van Doremalen et al., 2020b)	1.7 mg	100 mg (Haugen et al., 2018; Stover and Field, 2015)
Vitamin B12	Fish, meat, some shellfish, milk and cheese, eggs, fortified breakfast cereals, yeast extract	The phagocytic and bacterial killing capacity of neutrophils is decreased by Vitamin B12 deficiency. Vitamins B6, B12, and folate all support the activity of natural killer cells and CD8+ cytotoxic T lymphocytes, effects which would be important in antiviral defense. Patients with vitamin B12 deficiency had low blood numbers of CD8+ T lymphocytes and low natural killer cell activity (Calder, 2020)	2.4 mcg	4 mcg (NDA, 2015)
Folate	Broccoli, brussels sprouts, green leafy vegetables (spinach, kale, cabbage), peas, chickpeas, fortified cereals	Folic acid, the synthetic form of folate, plays an important role in cell division and cell production in blood-forming organs and bone marrow. Many studies recommend that adequate dietary levels of folic acid and B12 can act as preventative measures for inflammation, immune dysfunction, and disease progression (Alpert, 2017; Mikkelsen and Apostolopoulos, 2019; van Doremalen et al., 2020b)	400 mcg	1000 mcg (Sawaengsri et al., 2016; Watson et al., 2018; World Health Organization & FAO, 2004)
Vitamin C	Oranges and orange juice, red and green peppers, strawberries, blackcurrants, kiwi, broccoli, brussels sprouts, potatoes	Vitamin C contributes to immune defense by supporting various cellular functions of both the innate and adaptive immune systems. Vitamin C supports epithelial barrier function against pathogens. It also has roles in several aspects of immunity, including leukocyte migration to infection sites, phagocytosis and bacterial killing, natural killer cell activity, T lymphocyte function (especially CD8+ cytotoxic T lymphocytes), and antibody production. People deficient in vitamin C are susceptible to severe respiratory infections such as pneumonia (Calder, 2020; Carr and Maggini, 2017).	90 mg	2000 mg (Hemilä and Chalker, 2020; Institute of Medicine et al., 2002; Levine et al., 1998)
Vitamin D	Oily fish, liver, eggs, fortified foods (spreads and some breakfast cereals)	Vitamin D is able to modulate innate and adaptive immune responses. Vitamin D deficiency is associated with increased autoimmunity and increased susceptibility to infection. Vitamin D supplementation can reduce the risk of respiratory tract infections and reduce influenza risk during winter. Besides, avoidance of severe vitamin D deficiency improves immune health and decreases susceptibility to autoimmune diseases. (Bouillon, 2017; Calder, 2020; Martens et al., 2020)	20 mcg	100 mcg (Bouillon, 2017; Martens et al., 2020; Pludowski et al., 2018)
Vitamin E	Many vegetable oils, nuts and seeds, wheat germ (in cereals)	Immunomodulatory effects of vitamin E have been observed in animal and human models under normal and disease conditions. With advances in understating the development, function, and regulation of dendritic cells (DCs), macrophages, natural killer (NK) cells, T cells, and B cells, recent studies have focused on vitamin E's effects on specific immune cells (Lee and Han, 2018). Vitamin E supplementation of the diet of laboratory animals enhance antibody production, lymphocyte proliferation, T helper 1-type cytokine production, natural killer cell activity and macrophage phagocytosis. Moreover, vitamin E promotes interaction between dendritic cells and CD4+ T lymphocytes (Calder, 2020)	15 mg	1000 mg (Institute of Medicine et al., 2002; Jansen et al., 2016)
Zinc	Shellfish, meat, cheese, some grains and seeds, cereals, seeded or whole-grain bread	Zinc deficiency has a remarkable effect on bone marrow, decreasing the number of immune precursor cells, reduced naive B lymphocytes' output causes thymic atrophy, and reduces naive T lymphocytes' output. Therefore, zinc is vital in maintaining T and B lymphocyte numbers. Zinc deficiency impairs many aspects of innate immunity, including phagocytosis, respiratory burst, and natural killer cell activity. Zinc also supports the release of neutrophil extracellular traps that capture microbes.115 There are also marked effects of zinc deficiency on acquired immunity. Circulating CD4+ T lymphocyte numbers and function (e.g., IL-2 and IFN-γ production) are decreased, and there is a disturbance in favor of T helper 2 cells. Likewise, B lymphocyte numbers and antibody production are decreased in zinc deficiency (Gammoh and Rink, 2017; Hojyo and Fukada, 2016)	11 mg	50 mg (R. S. Gibson and Anderson, 2009; World Health Organization & FAO, 2004)
Selenium	Fish, shellfish, meat, eggs, some nuts, especially brazil nuts	Selenium deficiency in laboratory animals adversely affects several innate and acquired immunity components, including T and B lymphocyte function, including antibody production, and increases infection susceptibility. Lower selenium concentrations in humans have been linked with diminished natural killer cell activity and increased mycobacterial disease (Avery and Hoffmann, 2018; Cheng and Sandeep Prabhu, 2019; Hatfield et al., 2006)	55 mcg	400 mcg (Cheng and Sandeep Prabhu, 2019; World Health Organization & FAO, 2004)
Iron	Meat, liver, beans, nuts, dried fruit (eg, apricots), whole grains (e.g., brown rice), fortified cereals, most dark green leafy vegetables (spinach, kale)	Iron exhibits important effects on immune cell function and differentiation, and fourth almost every immune activation, in turn, impacts iron metabolism and spatiotemporal iron distribution (Haschka et al., 2020). The role of iron in immunity is in immune cell proliferation and maturation, specifically lymphocytes, associated with generating responses to infection(Alpert, 2017; Haschka et al., 2020)	18 mg	45 mg (Schümann et al., 2002; World Health Organization & FAO, 2004)
Copper	Shellfish, nuts, liver, some vegetables	Copper supports neutrophil, monocyte, and macrophage function and natural killer cell activity. It promotes T lymphocyte responses such as proliferation and IL-2 production. Copper deficiency in animals impairs a range of immune functions and increases susceptibility to bacterial and parasitic challenges (Calder, 2020). Cu can kill several infectious viruses such as bronchitis virus, poliovirus, human immunodeficiency virus type 1(HIV-1), other enveloped or nonenveloped, single- or double-stranded DNA and RNA viruses (Raha et al., 2020)	0.9 mg	10 mg Institute of Medicine et al. (2002)
Essential Amino Acids	Meat, poultry, fish, eggs, milk and cheese, soya, nuts and seeds, pulses	Amino acid metabolism plays a key role in this metabolic rewiring, and it supports various immune cell functions beyond increased protein synthesis. Immune cells critically depend on such pathways to acquire energy and biomass and to reprogram their metabolism upon activation to support growth, proliferation, and effector functions (Cruzat et al., 2018; Kelly and Pearce, 2020; Raha et al., 2020)	–
Essential Fatty Acids	Many seeds, nuts, and vegetable oils	Clinical signs of essential fatty acid deficiency include a dry scaly rash, decreased growth in infants and children, increased susceptibility to infection, and poor wound healing	Acceptable Macronutrient Distribution Ranges (AMDR) - Total fat is 20–35 E% - linoleic acid: 5 to 10 E% - α-linolenic acid) 0.6 to 1.2 E% - (NDA, 2010)
Long Chain Omega-3 Fatty Acids	Oily fish	This family of polyunsaturated fatty acids exerts major alterations on the activation of cells from both the innate and the adaptive immune system, although the mechanisms for such regulation are diverse	The FDA recommends that the intake for consumers not exceed 3 g/day of combined EPA and DHA, with no more than 2 g/day from dietary supplementation (51). However, according to EFSA Panel on Dietetic Product (52),long-term consumption of EPA and DHA supplements at combined doses of up to about 5 g/day and supplemental intakes of EPA alone up to 1.8 g/day, appears to be safe. Additionally, available data are insufficient to establish a UL for the omega-3 long-chain polyunsaturated fatty acids (LC–PUFA), individually or combined, for any population group
Got microbiota	A plant-based diet, fiber, and fermented food including yogurt, kefir, sauerkraut, tempeh	Indigenous commensal bacteria within the gastrointestinal tract are believed to play a role in host immune defense by creating a barrier against colonization by pathogens. Improving gut microbiota profile by personalized nutrition and supplementation are known to improve immunity can be one of the prophylactic ways by which infectious diseases can be minimized in older people and immune-compromised patients (Childs et al., 2019).	–