**2. Clinical applications**

Selenium deficiency results in impaired immunity [27], and supplementation is immune stimulant. Deficiency increases the virulence of some viruses such as in Keshan disease [28, 29] by altering their genome to a more virulent one. Deficiency may also be associated with low mood [30], increased cognitive decline in older people, and susceptibility to epilepsy [31], exacerbates hypothyroidism in iodine deficiency [32], protective against cardiovascular disease and cancer [33, 34], beneficial in a variety of inflammatory conditions by reducing oxidative stress, and in male fertility [35] through testosterone synthesis and spermatozoa function.

The conventional cancer therapies currently available are surgery, radiotherapy, and chemo‐ therapy. Chemotherapy is typically the main regimen for most cancers. However, many tumours develop resistance to these harsh drugs, known as chemo resistance, which can lead to other complications [36]. A number of natural products and compounds have been shown to act as effective chemo sensitizers. However, taking antioxidants during cancer treatment could interfere with the way chemotherapy work and diminish their benefits to the patient [37]. It was conclude that the use of supplemental antioxidants during chemotherapy and radiation therapy should be discouraged because of the possibility of tumour protection and reduced survival [37].This is because radiation and some chemotherapy agents work by generating free radicals, which then kill rapidly dividing cancer cells. Since antioxidants scavenge free radicals, they might interfere with the therapeutic effects of these treatments. However, oxidation supports the proliferation of malignant cells and may itself interfere with treatment [38]. People who hold this view maintain that antioxidants may counter the harmful effects of oxidation in the malignant process and thereby increase the effects of drugs or radiation therapy to the benefit of the patient. Moreover, they note that some evidence suggests that antioxidant supplements offer patients protection from the toxic effects of therapy [39].

The balance between oxidants and antioxidants can be a key issue in the development of cancer [40] and reactive oxygen species (ROS) are involved in aging, chronic illness, and cancer. Oxidants also cause free radical damage, thus the body generates large amounts of antioxi‐ dants to prevent harm and maintain health. However, Watson [41] reported that antioxidants can promote cancer growth of late stage metastatic cancers. The process is explained in terms of redox signalling, redox being a term used for reduction and oxidation. Healthy cells and also malignant ones benefit from intact Keap1/Nrf2 signalling, making a dysregulated hydroperoxide signaling a plausible explanation for the increased cancer risk in selenium deficiency [42]. Therefore, a research agenda to investigate the synergistic operation of such substances in cancer treatment is required.
