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Immune support by micronutrients is historically based on vitamin C deficiency and supplementation in scurvy in early times. It has since been established that the complex, integrated immune system needs multiple specific micronutrients, including vitamins A, D, C, E, B6, and B12, folate, zinc, iron, copper, and selenium, which play vital, often synergistic roles at every stage of the immune response. Adequate amounts are essential to ensure the proper function of physical barriers and immune cells; however, daily micronutrient intakes necessary to support immune function may be higher than current recommended dietary allowances. Certain populations have inadequate dietary micronutrient intakes, and situations with increased requirements (e.g., infection, stress, and pollution) further decrease stores within the body. Several micronutrients may be deficient, and even marginal deficiency may impair immunity. Although contradictory data exist, available evidence indicates that supplementation with multiple micronutrients with immune-supporting roles may modulate immune function and reduce the risk of infection. Micronutrients with the strongest evidence for immune support are vitamins C and D and zinc. Better design of human clinical studies addressing dosage and combinations of micronutrients in different populations are required to substantiate the benefits of micronutrient supplementation against infection.

The immune system is a complex and sophisticated network of specialized tissues, organs, cells, proteins, and chemicals which has evolved in order to protect the host from a range of dangerous agents such as bacteria, virus, fungi, and parasites. There is a close relationship between nutritional status and immune function. Hence, immunocompetence can be regarded as a measure of adequate nutrition. Inter-individual variations in many immune functions exist within the normal healthy population and are due to age, genetics, gender, ethnic background, socio-economic situation, diet, stress, habitual levels of exercise, alcohol consumption, smoking habits, etc. In addition, seasonal and temperature changes as well as being in crowded spaces (e.g., while commuting in public means of transportation, or working in open space offices) pose an additional burden on the immune system. Among the essential micronutrients required to support a normal immune function, vitamin C, vitamin D and the mineral zinc play a central role. Through their complementary and synergistic effects, they support components of both innate and adaptive immunity which comprise epithelial barriers, cellular defense and antibodies constituting the three main lines of immune defense. Furthermore, vitamin C, D and zinc are actively used by cells of the immune system engaged in fighting infections like upper respiratory tract infections and a state of micronutrient deficiency can arise during severe infections. Here we review the literature on the immune supportive properties of vitamins C, D and zinc and the impact of their supplementation in reducing the incidence or ameliorating symptoms of upper respiratory tract infections, which are among the most common infections in humans. Finally, we discuss their relevance in situations known to challenge the immune system such as exposure to temperature changes, to pollutants, or being in crowded spaces which serve as mixers where pathogens can stay suspended and transfer from host to host thereby increasing the risk of spreading infectious diseases because of close contact and long exposure.

As humans age, the risk and severity of infections vary in line with immune competence according to how the immune system develops, matures, and declines. Several factors influence the immune system and its competence, including nutrition. A bidirectional relationship among nutrition, infection and immunity exists: changes in one component affect the others. For example, distinct immune features present during each life stage may affect the type, prevalence, and severity of infections, while poor nutrition can compromise immune function and increase infection risk. Various micronutrients are essential for immunocompetence, particularly vitamins A, C, D, E, B2, B6, and B12, folic acid, iron, selenium, and zinc. Micronutrient deficiencies are a recognized global public health issue, and poor nutritional status predisposes to certain infections. Immune function may be improved by restoring deficient micronutrients to recommended levels, thereby increasing resistance to infection and supporting faster recovery when infected. Diet alone may be insufficient and tailored micronutrient supplementation based on specific age-related needs necessary. This review looks at immune considerations specific to each life stage, the consequent risk of infection, micronutrient requirements and deficiencies exhibited over the life course, and the available evidence regarding the effects of micronutrient supplementation on immune function and infection.

Adequate intakes of micronutrients are required for the immune system to function efficiently. Micronutrient deficiency suppresses immunity by affecting innate, T cell mediated and adaptive antibody responses, leading to dysregulation of the balanced host response. This situation increases susceptibility to infections, with increased morbidity and mortality. In turn, infections aggravate micronutrient deficiencies by reducing nutrient intake, increasing losses, and interfering with utilization by altering metabolic pathways. Insufficient intake of micronutrients occurs in people with eating disorders, in smokers (active and passive), in individuals with chronic alcohol abuse, in certain diseases, during pregnancy and lactation, and in the elderly. This paper summarises the roles of selected vitamins and trace elements in immune function. Micronutrients contribute to the body's natural defences on three levels by supporting physical barriers (skin/mucosa), cellular immunity and antibody production. Vitamins A, C, E and the trace element zinc assist in enhancing the skin barrier function. The vitamins A, B6, B12, C, D, E and folic acid and the trace elements iron, zinc, copper and selenium work in synergy to support the protective activities of the immune cells. Finally, all these micronutrients, with the exception of vitamin C and iron, are essential for antibody production. Overall, inadequate intake and status of these vitamins and trace elements may lead to suppressed immunity, which predisposes to infections and aggravates malnutrition. Therefore, supplementation with these selected micronutrients can support the body's natural defence system by enhancing all three levels of immunity.

Our bodies are normally well protected against continual attack from pathogens and noxious insult by a complex and integrated immune system. However, daily bombardment from indoor and outdoor air pollutants can compromise immune function and ultimately lead to infection (e.g. acute respiratory tract infections, diarrhoea) and conditions such as sick building syndrome (with mucosal, skin and general symptoms). All of us may be affected by reduced air quality, although certain factors increase the risk of impaired immunity (e.g. young or advancing age, exposure to tobacco smoke, close proximity to areas of high air pollution, office work, commuting). A major exogenous factor modulating immune function is nutrition; even subclinical deficiencies in various nutrients can have adverse effects on the immune system, which may be exacerbated by environmental threats. In particular, the oxidant-antioxidant balance (vital for communication within the immune system) may be affected. Dietary supplementation can help to restore immune function to the normal range, and an antioxidant-containing multivitamin supplement has been shown to ameliorate the symptoms of sick building syndrome, acute respiratory tract infections and diarrhoea. This review looks at the impact of reduced air quality on the oxidant-antioxidant balance and the role of selected micronutrients (vitamins A, D, E, C, B6, B12, folate and the trace elements copper, iron, selenium and zinc) and multivitamin supplementation.

This chapter focuses on the current understanding of vitamin D and immunity, with special attention to the elderly. Vitamin D insufficiency and deficiency are common among the elderly population. Vitamin D plays an immunoregulatory role in innate and adaptive immunity, and low vitamin D status has been implicated in the etiology of immune disorders, such as infectious diseases as well as some autoimmune disorders. As summarized here, the aging of the immune system is well documented and indicates that both innate and adaptive immunity are impaired with increasing age. Nutritional intervention with vitamin D seems to be a valid approach to delay the deterioration of the immune system with increasing age. Based on promising results in targeting some of the impairments of the immune system in older people, supplementation with vitamin D seems to be one strategy in attempting to increase not only the lifespan but also the health span of this increasing elderly proportion of the global population.

Aging is accompanied by physiological, psychological, economic, and social changes. Physiological changes in particular affect the requirements for several micronutrients, and the elderly are at high risk for micronutrient deficiencies. Inadequate intake and status of micronutrients leads to suppressed immunity, predisposing people to infections and aggravating malnutrition. This vicious circle is extremely critical for older adults who have to deal with the age-related impairment of immune function, or immunosenescence. Immunosenescence causes increased susceptibility to infections, cancer, and autoimmune diseases overall, leading to increased morbidity and mortality. This chapter reviews the roles of selected micronutrients and Panax ginseng in relation to immune function support, particularly in the context of aging. Based on their individual properties and synergistic mode of action, a combination of micronutrients and ginseng seems a reasonable association to improve nutritional status and hence the organism’s immune function, the cornerstone of health, in older adults.

Vitamin C concentrations in the plasma and leukocytes rapidly decline during infections and stress. Supplementation of vitamin C was found to improve components of the human immune system such as antimicrobial and natural killer cell activities, lymphocyte proliferation, chemotaxis, and delayed-type hypersensitivity. Vitamin C contributes to maintaining the redox integrity of cells and thereby protects them against reactive oxygen species generated during the respiratory burst and in the inflammatory response. Likewise, zinc undernutrition or deficiency was shown to impair cellular mediators of innate immunity such as phagocytosis, natural killer cell activity, and the generation of oxidative burst. Therefore, both nutrients play important roles in immune function and the modulation of host resistance to infectious agents, reducing the risk, severity, and duration of infectious diseases. This is of special importance in populations in which insufficient intake of these nutrients is prevalent. In the developing world, this is the case in low- and middle-income countries, but also in subpopulations in industrialized countries, e.g. in the elderly. A large number of randomized controlled intervention trials with intakes of up to 1 g of vitamin C and up to 30 mg of zinc are available. These trials document that adequate intakes of vitamin C and zinc ameliorate symptoms and shorten the duration of respiratory tract infections including the common cold. Furthermore, vitamin C and zinc reduce the incidence and improve the outcome of pneumonia, malaria, and diarrhea infections, especially in children in developing countries.

Diet and nutrition are important in the promotion and maintenance of good health throughout the entire life. A common denominator playing a role in the prevention of both chronic diseases and infections and hence important for maintaining health is a good functioning immune system. The competence of the immune system is an excellent marker of proper nutrition status and health and several changes in immune functions have been related to longevity. Preservation of a functionally youthful immune system throughout the years, including ensuring adequate vitamin and mineral status, is the best way to preserve health throughout life and to gain longevity accompanied by good quality of life.