Selenium
Selenium is an essential micronutrient that plays a significant role in many physiological processes including immune responses. The immune system needs an adequate intake of this nutrient mostly through its incorporation into selenoproteins to exerts its biological effects [126]. In fact, it has an important antioxidant role to quench ROS, influencing leukocyte and NK cell function and consequently modulating the host antioxidant defense system [85]. In fact, selenoproteins act as redox regulators and cellular antioxidants, potentially counteracting the ROS produced during oxidative stress [82]. Selenium is involved in T-lymphocyte proliferation and the humoral system [85], especially in immunoglobulin production [127]. It helps improve Th cell counts and maintain antibody levels [85] and also increases the production of IFN-γ [95].
Selenium deficiencies have been associated with viral infections such as influenza, determining adaptive and innate immunity responses and leading to a high level of virus-related pathogenicity [128].
Low concentrations of selenium in humans have been linked to the reduced activity of NK cells and the increase in mycobacterial disease. Moreover, selenium deficiency has been shown to allow mutations of coxsackievirus, poliovirus and murine influenza virus, increasing their virulence.
Dietary selenium supplementations were suggested as adjuvant therapies of influenza, supporting the immune response [129].
The beneficial effects of a higher selenium status have been supported for some viral infections, although there are some studies that do not conclusively demonstrate effective improvements in anti-viral immunity. On the contrary, the antioxidant properties of some selenoproteins have been suggested to contribute to boosting anti-viral immunity [129].
Currently, the recommended amounts of adequate selenium intake for adults range between 25 and 100 μg/die [130], with an average of 60 μg/die for men and 53 μg/die for women [131]; the tolerable upper intake level is set at 300–450 μg/die [132]. More research is needed to improve knowledge of selenium metabolism and requirements for optimal health. The relationships between selenium dietary intake and health status, or risk of disease, are complex and require elucidation to inform clinical practice.