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Immunity and nutrition

Rohini Diniz

Dietary deficiencies of selected vitamins and minerals can adversely affect aspects of both innate and adaptive immunity thereby making one vulnerable to infection and disease. Besides vitamin A, vitamins D, B-6 and C; minerals like zinc, selenium, iron and copper and some probiotics have been shown to have a role in the normal functioning of the immune system.

Vitamin D is not just a vitamin that we eat, but also a hormone that our body produces. The metabolically active form of vitamin D, 1,25-dihydroxyvitamin D3or calcitriol, in addition to its role in bone metabolism is now recognised to be a potent modulator of the immune system. Studies have shown that 1,25-dihydroxyvitamin D3modulates both innate and adaptive immune responses. Though more research is needed epidemiological as well as clinical studies have shown that higher serum vitamin D status reduces the risk of certain types of autoimmune disease. Good dietary sources of vitamin D include milk and milk products, egg yolk, fish and fish liver oils and vitamin D fortified foods. A balanced diet coupled with daily exposure to the sun for half an hour around mid-day (11 a.m. to 3 p.m.) is sufficient for the body to build up vitamin D stores.

Vitamin B-6 is widely distributed in foods but rich sources include poultry, fish, liver, whole grain cereals and pulses. The metabolically active form of vitamin B-6, pryridoxal phosphate is a coenzyme primarily involved in the metabolism of amino acids. Amino acids are needed for the synthesis of immune factors like cytokines and antibodies. Human and animal studies have shown that vitamin B6 deficiency affects the proliferation, differentiation and maturation of the lymphocyte as well as the production of cytokines and antibodies.

Vitamin C or ascorbic acid is a water-soluble vitamin that is found in large amounts in fresh fruits and vegetables like amla, guava, cashew fruit, green leafy vegetables; yellow orange fruits and vegetables like citrus fruits like orange, sweet lime, torange, lime, grapefruit, kiwi fruit, strawberries, and in tomatoes, green chillies and capsicum.

Vitamin C is highly concentrated in immune cells and being a powerful antioxidant, it protects the immune cells from oxidative damage. It has also been shown to modulate the functions of phagocytes, proliferation of T- lymphocytes, production of cytokines and gene expression of monocyte adhesion molecules. Research studies have shown that for most people, vitamin C supplements do not reduce the risk of catching colds but reduces the duration and severity of a cold.

Zinc is another nutrient that is important for the normal development and function of the cells that mediate both innate and adaptive immunity. Zinc is needed for the biological activity of thymulin, a thymus specific hormone that promotes T cell functions such as signal transduction pathways that control gene expression of various immune-regulatory cytokines. Zinc is also a cofactor of several enzymes involved in antioxidant responses that contribute to reduced oxidative damage in immune cells. Marginal as well as severe zinc deficiency impairs both the innate and adaptive immune functions leading to increased susceptibility to infections. Major dietary sources of zinc are meat, dairy products, seafood and cereals.

Foods such as meat, fish, nuts and seeds, wheat germ and oats contain the mineral selenium. Selenium is essential for optimal immune response because it is a cofactor for a group of enzymes known as selenoproteins that function as antioxidants thereby protecting the cells from oxidative damage. Selenium deficiency impairs aspects of innate as well as adaptive immunity by adversely affecting antibody production and cell mediated immunity.

The role of iron in immunity is like a double-edge sword. On one hand sufficient iron stores is critical to several immune functions that are required to kill pathogens. On the other hand, it is used by most infectious agents for replication and survival. Iron deficiency impairs immune functions and anaemia increases the risk of developing infections.

Copper is an important component of a number of essential enzymes known as cuproenzymes. Though the exact mechanism of its action is still not clear, copper plays an important role in the development and maintenance of the functions of immune systems.

Probiotics are live microorganisms that confer health benefits on the host by improving the intestinal microbial flora when administered in adequate amounts. Fermented dairy products such as curd, yoghurt, kefir, koumiss, some soft cheeses and foods such as sauerkraut, miso, tempeh, kombucha, kimchi, sour mustard-based pickles and indigenous foods such as fermented rice gruel, pazhankanji, pakhala bhaat or panta bhaat; fermented ragi porridge and traditional beverages such gajar ki kanji are rich in probiotics. These foods contain live culture of microorganisms either as a result of fermentation or added intentionally that beneficially affect the host by improving the intestinal microbial balance. During the process of transient colonisation some probiotic strains have been shown to boost both the innate and acquired immune response.

(Writer is a consultant nutritionist with 21 years of experience, practicing at Panaji and can be contacted on rohinidiniz@gmail.com)

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