**3. Selenium biofortification**

Selenium is an essential inorganic element for humans and animals, and one of the organic forms of Se, methylselenocysteine, appears to be an effective food source of Se [16]. Se is incorporated into a range of selenoproteins involved in several important metabolic activities such as synthesis of thyroid hormones and antioxidative activity [46, 47]. Selenium is an important inorganic component for the antioxidant metabolism of enzymes, making up part of the active site of enzymes from the group of glutathione peroxidases (GSH-Px) which plays an important role in detoxification of free radicals. The GSH-Px catalyze the reduction of hydroperoxide radicals (H2O2 for instance) by the oxidation of glutathione (GSH), a nonenzymatic component of the antioxidative metabolism [48].

It is noted that the accumulation of Se in foods is closely related to the content of this nutrient in the soil. However, consumption of foods poor in Se or low ingestion of foods containing Se is associated with the emergence of numerous diseases such as cancer, type II diabetes, heart disease, pulmonary dysfunction, seizures in children, impaired development and cerebral functions, as well as pregnancy and conception [49–51].

Currently, Se deficiency affects about 1 billion people worldwide due to soils lacking this mineral nutrient in some countries [52]. This edaphic characteristic was registered in countries such as Sweden, Finland, USA and China [16, 53, 54]. Because there is a close relationship between plant mineral nutrition and human health, food fortification with Se, via a strategy known as biofortification, is an effective way to add Se to human food and prevent the emergence of diseases related to deficient Se intake. This strategy proved to be effective due to the fact that Se presents chemical similarity to S, and both have the same carrier membranes and biochemical pathway of assimilation [28, 33].

Because vegetables are considered an important source of bioactive compounds that contain polyunsaturated fatty acids, phytochemicals such as flavonoids and glucosinolates, many of which can inhibit cell proliferation, induce apoptosis and act synergistically when combined in foods [55], some vegetable groups are more suitable for biofortification because they are natural accumulators of Se such as brassicas [16]. This group of plants has significant levels of glucosinolates, a substance of great nutraceutical interest. In this context, recent studies confirm the biofortifying effect of Se in edible plants in the group of brassicas. For example, in a major study [21] showed the positive impact of Se biofortification in broccoli. This study found that the broccoli extract showed high levels of phenolic compounds, significant antioxidant effect and anti-proliferative activity of tumor cells on the effect of Se in the synthesis of glucosinolates and phenolic compounds, and anticancer activity, see [16, 56].

Although Se biofortification is a strategy undertaken in edible and domesticated plants, it can occur naturally, as in the Brazilian Amazon, where important food sources of Se are the shrub species of the family Lecythidaceae, *Bertholletia excelsa* Humb. and Bonpl and *Lecythis usitata* Miers, which produce nuts presenting high Se concentrations [57, 58]. For these species, biofortification is a natural phenomenon, because the average levels of Se in their nuts range from 0.03 to 512 μg g−1 [59, 60]. Thus, consumption of only one nut per day appears to be sufficient to meet the daily Se requirement of an adult, because according to the Scientific Committee on Foods of the European Commission, the human daily intake requirement of Se is around 55 μg dia−1 [27]. However, the production of nuts is seasonal and the season of low production affects the nutrition of people near regions of natural occurrence of these two species [61, 62].

Among domesticated plants of food interest, the biofortification strategy can be carried out by providing Se to the soil, nutrient solution or leaf. However, considering large-scale plantations, the crop cycle and the economic value of the Se sources can make biofortification expensive, with the need for less costly and more practical strategies. One example is pelletizing the seeds of Se accumulator species, like radish, because of its short life cycle. Another strategy is to add Se sources to fertilizers used in agriculture. This strategy was adopted by Whelan and Barrow [63] in cultivation of the grass *Trifolium subterraneum* L in a soil classified as podzol laterite. These authors used a fertilizer containing 1% Se, composed of the sources Na2SeO4 and BaSeO4 in a 1:1 ratio, which is characterized by slow release of Se in the soil. This strategy proved to be interesting due to the fact that there is a synchrony between plant growth and Se liberation, a fact which favors Se absorption and culture biofortification based on soil-plant interaction.
