**1. Introduction**

Many nutritional recommendations for human well-being and disease prevention have highlighted dietary styles based on the growing consumption of fresh fruits and vegetables and the reduction of simple carbohydrates, sodium, and saturated and transfats consumption [1].

In order to maintain a good health, people require several mineral nutrients that must be included in the diet. The essentiality of minerals can be demonstrated by the fact that vitamins cannot be absorbed solely or work in the absence of specific minerals, which are necessary in many physicochemical processes [2].

Deficiencies of specific mineral elements affect, in both underdeveloped areas and industrialized countries, up to two-thirds of the world's population [3–5] and the insufficient intake can cause severe damage to people's health [6]. For instance, in Europe and Central Asia, malnutrition problems related to diets with low micronutrient contents are increasing the number of women and children with anemia. In fact, iron and iodine deficiency disorders are the most common forms of malnutrition [7]. Besides, a recent study conducted in South Italy showed that the population has low intake of calcium and potassium [8].

Food, mainly plant-based, is the source of all important minerals, therefore it is important to keep on a regular basis a good and balanced diet that can provide the adequate

**Citation:** Buturi, C.V.; Mauro, R.P.; Fogliano, V.; Leonardi, C.; Giuffrida, F. Mineral Biofortification of Vegetables as a Tool to Improve Human Diet. *Foods* **2021**, *10*, 223. https://doi.org/ 10.3390/foods10020223

Academic Editor: Antonello Santini

Received: 22 December 2020 Accepted: 19 January 2021 Published: 21 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

proportion of minerals [9]. The enrichment of food with health related compounds and mineral elements could, however, be considered a strategy to fight undernourishment or to face with specific nutritional need [10].

In the case of not processed food, such as vegetables, the only option to enhance the nutrient content of products in preharvest using improved genotypes or adopting specific agronomical techniques [11].

The increasing interest in the enrichment of fresh consumed vegetables with mineral elements has encouraged intensive research activity focusing on the elaboration of suitable application protocols. This review describes developments in agronomic biofortification of vegetables with reference to some mineral elements often lacking or not adequately present in human diets, i.e., calcium (Ca), magnesium (Mg), iodine (I), zinc (Zn), selenium (Se), iron (Fe), copper (Cu), and silicon (Si). After synthetically considering the role in human nutrition and in plant physiology, this review aims to discuss the most successful agronomic strategies to increase the amount of the considered minerals in the edible portion of vegetables.

#### **2. The Role of Vegetables for Human Health and How Biofortification Can Have an Impact**

Plant foods make up a substantial part of the human diet and they provide most of calories, nutrients, and bioactive compounds necessary to keep a healthy status and prevent diseases. Vegetables are one of the pillars of a plant-based good diet, providing in particular dietary fiber, phytochemicals (such as vitamins, antioxidants), and minerals [12,13]. Minerals are considered essential nutrients: they are not synthesized by humans and must be obtained from the diet. Humankind evolved thanks to the dietary assumption of a significant number of vegetables and their insufficient intake is one of the reasons for many noncommunicable diseases, which are spread in Westernized societies. As an example, potassium, calcium, selenium, and iodine obtained through a vegetable-rich diet, can contribute to maintaining good blood pressure, bone strength, hormonal production, heart, and mental health [14,15]. In a recent study carried out in the UK, a data analysis from more than 40,000 people showed that changes in fruit and vegetable consumption may not only benefit physical health in the long-run, but also mental well-being in short term [16,17], besides the general population these benefits were also observed in cancer survivors [18]. On the other hand, vegetables play an important role in the economy, fighting poverty, hunger, and undernutrition, since they can be locally cultivated and consumed in a high diversity of shapes, sizes, colors, and tastes [12,19,20].

Nonoptimal intake of micronutrients and undernutrition, the so called hidden hunger, can be particularly severe for people following restricted diet for religious, ethical, or medical reasons [4,5,21]. Health authorities have established dietary reference intakes (DRI) based on recommended daily allowances (RDA) and tolerable upper levels (UL). As general principle, strategies that address vitamin or mineral deficiency must aim to achieve the DRI for each component without exceeding the UL [22].

However, the actual contribution of phytochemicals and minerals to human diet is not only related to their concentration in a certain plant tissue. The micronutrients must be released from the food matrix during the passage in the gastrointestinal track, absorbed into the blood and transported to their target tissues [23]. In fact, only the fraction released from the plant tissue become eventually available for absorption. This fraction is indicated as bioaccessible and to increase the bioaccessibility of plant phytochemicals and minerals is a promising target of agronomical strategies to improve the nutritional quality of vegetables [24].

Vegetable consumption should increase in the coming years for sustainability and health reasons. To deal with the rise of global population, more sustainable food sources will be needed [25]. According to Schreinemachers et al. [15], the most important vegetables in the current global economy are tomatoes, cucurbits (pumpkins, squashes, cucumbers, and gherkins), alliums (onions, shallots, and garlic), chilies, spinach, potatoes, carrots, and brassicas, therefore, it makes sense to focus the biofortification efforts on these species.

#### **3. Biofortification of Vegetables**

The approaches to address micronutrient malnutrition are different; medical supplementation and product fortification are the most commonly adopted. Fortification is the process of food enrichment with nutrients, adopting different methods during processing. However, in some contexts, fortification is challenged due to poor investments, infrastructure, and delivery system [26]. Under these conditions, an alternative strategy is to adopt new genotypes, characterized by improved compositional profiles, or to tailor specific agronomic techniques aimed to enhance the content of specific health effective compounds in widespread crops [26]. While this can be considered an option for products which are transformed before they are used (e.g., staple foods), for fresh consumed products, such as vegetables, biofortification is the only choice to improve the content of health-related compounds in the edible portion.

Among the different strategies to obtain biofortified vegetables there are agronomic and genetic approaches, the latter can be done either through conventional breeding or transgenic methods [27,28]. The objectives are to increase in the edible portion the minerals content or other specific health related compounds. Transgenic programs involve biotechnology studies that allow to genetically modify a species, to obtain a plant with targeted characteristics (i.e., higher content of specific nutrients). Even though this approach could be cost effective in the long run, it is the least employed methodology today because the phase of research and development is still very slow and expensive. In any case, in developed countries the higher prices involved in the production of biofortified vegetables is countered by the achievement of a premium product with a superior nutritional quality, that can satisfy the new consumers' demand willing to pay for a healthier way of eating [29]. In addition, some countries have restrictive laws, that forbid genetically modified organisms (GMOs). Along the same lines, there is the option to cross different genotypes, with the aim to introduce in new cultivars desirable traits naturally occurring in plants. This genetic approach (traditional breeding) has been performed for decades and can allow to create new varieties with a higher content of certain nutrients. In this case, the limitation is to find the desired characteristics in the available genetic resources [30]. On the other hand, breeding programs, even when effective, may eliminate their effect due to the high renewal rate of cultivars coming from the large number of new introductions made by the vegetable seed industry [31].

Biofortification programs carried out through the agronomic approach are the best option, since they involve simple techniques to accumulate or to stimulate the production of specific compounds at plant level. A substantial part of the biofortification research that has been carried out in the last decades focused the attention on specific compounds such as vitamins and amino acids, rather than minerals [4,13,32–34]. A variety of biofortified products with vitamins or their precursor include banana, mango, sweet potato, wheat, and cauliflower [35]. In the same line, biofortification with amino acids proved to be effective in producing high lysine-rice, using the double strategy of maximizing its biosynthesis and minimizing its catabolism [36]. Besides, evidence shows that sulfur fertilization on wheat, barley, and potato can increase the sulfur-containing amino acids (SAAs) methionine and cysteine content in its edible part. In the same way, the application of nitrogen plus potassium has potential in increasing carotenoid content in carrots [37].

However, besides the increase of the content of some specific compounds (e.g., antioxidants) with controlled doses of stressors [38], agronomic biofortification consists in increasing or optimizing the application of mineral elements to the crop in order to increase the corresponding content in the edible organs. In this case the focus is on setting up the form of the mineral, the concentration, and the application form; indeed, certain mineral forms or quantities can cause indirect effects, damaging or compromising a crop [5,27].

#### **4. Agronomic Mineral Biofortification**

The production of vegetables is carried out in very diversified agronomic contexts as regards crop cycles, soil conditions, or growth environments. Agronomic approaches to increase the concentration of minerals in edible organs generally rely on the supply of mineral fertilizers and/or improvement of the mobilization and solubilization of mineral elements in the rhizosphere [27]. Vegetable crops are generally grown in agro-systems characterized by a high degree of intensification of the production processes and in which the supply of nutrients is increasingly based on the use of fertigation, soilless cultivation, and foliar fertilization. These alternatives offer different opportunities to implement targeted biofortification programs. In the case of the application of mineral elements by fertigation on soil cultivated crops, some interference may derive from element availability for the plant (phytoavailability), therefore selecting mineral forms and concentrations may have a relevant importance [27,32]. One alternative strategy to overcome the low mineral phytoavailability into the soil is the cultivation through hydroponic systems (soilless cultivation). The possibility of optimizing limited water resources and cultivating in the absence of suitable agricultural soils, has led to a considerable spread of soilless cultivation systems. It has been observed, for example, that hydroponic cultures can be among the best options to increase the nutrient content of plant tissues [39,40]. In the case of minerals not readily translocated to the edible tissues, such as for crops grown on soil and/or for minerals with scarce mobility, another alternative is the use of foliar fertilization [41].
