The Effects of Selenium on Bone Health: From Element to Therapeutics

Osteoporosis, characterized by low bone mass and a disruption of bone microarchitecture, is traditionally treated using drugs or lifestyle modifications. Recently, several preclinical and clinical studies have investigated the effects of selenium on bone health, although the results are controversial. Selenium, an important trace element, is required for selenoprotein synthesis and acts crucially for proper growth and skeletal development. However, the intake of an optimum amount of selenium is critical, as both selenium deficiency and toxicity are hazardous for health. In this review, we have systematically analyzed the existing literature in this field to determine whether dietary or serum selenium concentrations are associated with bone health. In addition, the mode of administration of selenium as a supplement for treating bone disease is important. We have also highlighted the importance of using green-synthesized selenium nanoparticles as therapeutics for bone disease. Novel nanobiotechnology will be a bridgehead for clinical applications of trace elements and natural products.


Introduction
Bones play multiple roles in the body, including providing support to the body and an environment for generating blood cells, enabling movement, protecting vital organs, and storing minerals [1][2][3]. Thus, it is not surprising that the deterioration of bone health results in morbidity, disability, and occasionally worse outcomes [4,5]. Osteoporosis, which is characterized by low bone mass and a disruption of bone microarchitecture, is the most common bone disease in humans [6][7][8][9][10]. It is prevalent in menopausal women and elderly men and is a major risk factor for fractures [11,12]. With an increase in longevity and societal aging in many parts of the world, osteoporosis is becoming a global epidemic [13,14].
Several primary and secondary preventive strategies for osteoporosis have been recommended, including drugs, diet, and other lifestyle modifications [15,16]. Among dietary elements, most previous studies have focused on calcium and vitamin D supplementation [17][18][19][20][21][22][23][24], which have traditionally been provided as anti-resorptive treatments for every adult at risk; however, fractures can be prevented in their absence, and the effects of these agents on bone density appear to be negligible [25][26][27].
Selenium is a trace mineral and an indispensable component of various enzymes and proteins [28][29][30]. Selenoprotein is synthesized from selenocysteine, an essential

Selenium Deficiency and Toxicity
Selenium is a microelement that rarely occurs in its elemental form in nature; instead, organic and inorganic mineral forms of selenium, such as selenide, selenate, and selenite, have been detected in soil [118][119][120]. Previous studies have shown that humans can absorb organic compounds better than inorganic compounds [121][122][123]. Inorganic forms of selenium, such as selenite, are used as dietary supplements [124]. Selenate is the most common inorganic form of selenium in soil; it dissolves in water and reaches rivers and oceans [125]. Organic forms of selenium occur mostly in plant and animal food as selenomethionine and selenocysteine [120,126]. Many whole grains, dairy products, meat, and seafood are good sources of selenium, although geographic variation in selenium content in soil may determine the level of selenium in food [127].
The margin between selenium deficiency and toxicity is narrow. A small amount of selenium is necessary for cell homeostasis because of its antioxidant activities, while supranutritional levels of selenium act as pro-oxidants in vivo [128]. Selenium poisoning, termed selenosis, can develop if selenium intake exceeds the tolerable limit of 400 µg/day [129]. Selenosis is characterized by garlicky breath, gastrointestinal disorders, hair loss, nail discoloration, fatigue, irritability, and neurotoxic effects [130]. Recent studies have suggested that excess selenium intake may increase the risk of type 2 diabetes [131]. Hydrogen selenide, a corrosive gas, is the most toxic selenium compound [132]. Interestingly, consumption of lower doses of selenium, such as 300 µg/day, may also exert negative effects on the body. Long-term consumption of 300 µg/day in the form of selenium-enriched yeast increased mortality in the cohort of the Danish Prevention of Cancer by Intervention with Selenium (PRECISE) trial. Hence, high daily selenium supplementation or intake should be avoided [133].
According to the World Health Organization (WHO) standards, the recommended daily intake of selenium for adults is 55 µg/day [51]. The optimal selenium concentration in plasma has been suggested to be 90-120 g/L, which is sufficient to saturate selenoproteins in blood [134]. People living in low selenium regions and those with compromised bowel absorption and undergoing chronic hemodialysis are at high risk of developing selenium deficiency [135]. Keshan disease, caused by a combination of dietary deficiency of selenium and Coxsackie virus infection, was first noted in people living in Northeast China, where soil contains low levels of selenium [136][137][138]. The affected patients suffer from congestive cardiomyopathy and pulmonary edema, often culminating in death [139,140]. Selenium deficiency has been suggested to be related to immune reactions against viruses [141,142]. People with Kashin-Beck disease (KBD) are distributed from Northeastern to Southwestern China. KBD is probably caused by the ingestion of fungal-contaminated grains [143] and is characterized by pain, limited motion in many joints of the body, and growth retardation in children, and selenium deficiency is considered a predisposing factor for the KBD [144]. Plasma selenium levels in children with KBD were lower than 27 ng/mL in the Tibet Autonomous Region, whereas appropriate selenium concentrations for optimal GPx activity are 80-100 ng/mL [33,[145][146][147].

Roles of Selenoproteins in Bone Cells
Compared to boron, iron, zinc, and copper, selenium is a trace element present in bones that affects bone metabolism [1]. Bone has the second-highest proportion of body selenium (16%), followed by skeletal muscles (27.5%) [148]. Selenium deficiency is associated with low bone mass in male rats [149] and osteoarthropathy in KBD [150,151], as it adversely affects the biosynthesis of several antioxidant selenoproteins, which impairs bone metabolism [152].

Bone Remodeling
Healthy skeletal remodeling is maintained by an equilibrium between the activities of mesenchymal stem cell-derived osteoblasts and hematopoietic cell-derived osteoclasts on the bone surface [153]. The molecular triad of the receptor activator of the NF-κB ligand (RANKL) expressed by osteoblast lineage cells, osteoprotegerin (OPG) produced by osteoblastic or stromal cells, and the receptor activator of NF-κB (RANK) expressed by osteoclasts forms an important signaling link between osteoblasts and osteoclasts [154][155][156]. The RANK/RANKL pathway has been identified as the major signaling mechanism responsible for osteoclast differentiation. OPG is a non-signaling decoy receptor for RANKL that plays a critical regulatory role in osteoclast-mediated bone resorption by suppressing osteoclastogenesis and regulating mature osteoclasts [157,158]. Reactive oxygen species (ROSs) act as critical intracellular signaling mediators for RANKL-induced osteoclastic differentiation. Large amounts of ROSs are produced during bone remodeling, which are harmful for normal bone physiology, as they suppress osteoblastic differentiation and promote osteoclastic differentiation independent of NF-κB activation [159,160].

Selenoproteins in Bone Metabolism
The majority of selenoprotein-related genes and at least nine selenoprotein biosynthesisrelated genes have been identified in osteoblasts or osteoclasts [161][162][163][164]. Figure 1 shows the mechanism underlying the metabolic effect of selenium on osteoblasts and osteoclasts.
TrxR1 is a selenoprotein that is upregulated by 1,25(OH)2D3 and is expressed early in the osteoblast differentiation signaling cascade, highlighting a part of the mechanism by which selenium promotes bone formation [45]. 1,25(OH)2D3 did not increase TrxR activity in human fetal osteoblast (hFOB) cells cultured in selenium-deficient media. The co-treatment of 1,25(OH)2D3 and selenium increased TrxR1 protein and activity, suggesting that 1,25(OH)2D3-mediated osteoblastic differentiation and/or maintenance of the differentiation program would be impaired if selenium was deficient in bone [165]. GPx1 is a critical antioxidant enzyme in osteoclasts that can inhibit osteoclastogenesis when activated [45]. Bone marrow matrix cells cultured at low selenium concentrations poorly expressed GPx and TrxR, and showed signs of chromosomal damage, which were restored by selenium supplementation; in addition, basal selenoprotein activity was recovered [162].
Additionally, evidence indicates that supplementing with supranutritional selenium can modulate specific pathological changes via various mechanisms, including GPx and TrxR suppressing NF-κB activation and its inflammatory reactions. [166][167][168][169]. Genetic studies have shown that a single nucleotide polymorphism (SNP) at codon 198 of GPx1 is associated with low bone mineral density and increase in the levels of bone turnover markers [170].
Adequate selenium consumption appears to be important for osteoclast and osteoblast cell proliferation and differentiation, mostly via ROS regulation. Studies have shown that extracellular signal-regulated kinases, ERK1/2, mediate the inhibitory effect of hydrogen peroxide on osteoblast differentiation [171][172][173]. Selenium treatment has been shown to protect bone marrow stromal cells from the hydrogen-peroxide-induced suppression of osteoblastic differentiation by inhibiting oxidative stress and ERK activation [174][175][176]. In contrast, ROS levels increased when selenoprotein expression decreased, which was related to pathologically exacerbated signaling and enhanced osteoclast activity [163,177].

Figure 1.
Mechanism of selenium-related bone metabolism. Selenium supplement encourages the activity of GPx and TrxR, and GPx activity is more activated in those with a selenium-deficient status. GPx and TrxR suppress NFκB activation at supranutritional selenium levels and regulate osteoclastogenesis and osteoblastogenesis. Co-treatment of 1α,25(OH)2D3 and selenium synergistically elevates TrxR1 protein and activity. GPxs: glutathione peroxidases; TrxRs: thioredoxin reductases; RANK: receptor activator of NFκB; RANKL: receptor activator of NFκB ligand.
Adequate selenium consumption appears to be important for osteoclast and osteoblast cell proliferation and differentiation, mostly via ROS regulation. Studies have shown that extracellular signal-regulated kinases, ERK1/2, mediate the inhibitory effect of hydrogen peroxide on osteoblast differentiation [171][172][173]. Selenium treatment has been shown to protect bone marrow stromal cells from the hydrogen-peroxide-induced suppression of osteoblastic differentiation by inhibiting oxidative stress and ERK activation [174][175][176]. In contrast, ROS levels increased when selenoprotein expression decreased, which was related to pathologically exacerbated signaling and enhanced osteoclast activity [163,177].
Recently, Kim et al. demonstrated that SELENOW contributes to osteoclastogenesis, but is downregulated via RANKL/RANK/tumor-necrosis-factor-receptor-associated factor 6/p38 signaling. They suggested that this RANK-dependent inhibition of SELENOW blocks overactive osteoclasts to prevent osteoporosis [178]. SELENOP is a major selenium transporter in plasma that is produced and released by hepatocytes and resorbed by bone cells via apolipoprotein receptor 2(ApoER2) [179][180][181]. In SELENOP-mediated tissue-specific selenium absorption, transcription of mRNA and LDL-receptor-related protein 8 (LRP8) protein expression has been proceeding. A crucial component of the intracellular Figure 1. Mechanism of selenium-related bone metabolism. Selenium supplement encourages the activity of GPx and TrxR, and GPx activity is more activated in those with a selenium-deficient status. GPx and TrxR suppress NFκB activation at supranutritional selenium levels and regulate osteoclastogenesis and osteoblastogenesis. Co-treatment of 1α,25(OH)2D3 and selenium synergistically elevates TrxR1 protein and activity. GPxs: glutathione peroxidases; TrxRs: thioredoxin reductases; RANK: receptor activator of NFκB; RANKL: receptor activator of NFκB ligand.
Recently, Kim et al. demonstrated that SELENOW contributes to osteoclastogenesis, but is downregulated via RANKL/RANK/tumor-necrosis-factor-receptor-associated factor 6/p38 signaling. They suggested that this RANK-dependent inhibition of SELENOW blocks overactive osteoclasts to prevent osteoporosis [178]. SELENOP is a major selenium transporter in plasma that is produced and released by hepatocytes and resorbed by bone cells via apolipoprotein receptor 2(ApoER2) [179][180][181]. In SELENOP-mediated tissuespecific selenium absorption, transcription of mRNA and LDL-receptor-related protein 8 (LRP8) protein expression has been proceeding. A crucial component of the intracellular selenium transportation system, the LRP8/ApoER2 complex is increased in chronic selenium deficiency, suggesting this process as important feedback to avoid chronically low SELENOP levels [46,161]. Interestingly, bones, brain, and testis are resistant to limited selenium supply, ensuring that the most essential selenium-dependent processes continue in specific tissues even under selenium deficiency due to increases in the expression of ApoER 2 [180][181][182].

Selenium and Bone: Animal Model Studies
Few studies have investigated the effects of selenium on bone metabolism in animal models [183][184][185]. Table 2 presents the results of animal studies regarding intervention  with selenium supplementation. Decades ago, Sasaki et al. discovered that a low-selenium diet of 3-11 months reduced bone volume, bone mineral density, and femur ash weight in rats [186]. Turan et al. [187] fed young rabbits of both sexes with a selenium-and vitamin-E-adequate diet (control group), a selenium-and vitamin-E-deficient diet, or excessive selenium for 12 weeks. They evaluated three-point bending, representing the risk of bone fracture [188]. In this study, plasma red blood cell GPx activity increased or decreased relative to plasma selenium levels in the group fed a selenium-and vitamin-E-deficient diet and the group fed a selenium-excess diet. Although selenium-deficient diets did not affect body weight or growth patterns, the biomechanical strength of the bones decreased significantly in the groups fed the selenium-and vitamin-E-deficient diet or the selenium-excess diet. Notably, bone histopathology observed in the selenium-and vitamin-E-deficient group was indicative of osteomalacia [187]. Osteomalacia is a disorder of decreased bone mineralization with a histological feature that is different from osteoporosis, which shows normal bone mineralization but an ongoing loss of bone matrix that leads to decreased bone mass [189]. Later, Turan et al. [190] developed a heparin-induced osteoporosis model in adult female New Zealand white rabbits. They were divided into three experimental and control groups, each on the basis of supplementation with deionized water, a combination of vitamin C and E, and sodium selenite plus a combination of vitamin C and E. The long bone tissue of rabbits from the group fed selenium plus combined vitamin supplementation showed almost the same structure as that of normal rabbits. [190]. Similarly, Cao et al. administered a four-month selenium-deficient or -adequate diet to adult male C57BL/6J mice and reported that bone microarchitecture was impaired in the selenium-deficient diet group, accompanied by increases in the levels of bone absorptive markers and decreases in GPx activity in the blood [177]. Yao et al. examined the effects of supplemented selenium combined with iodine, which was designed for a diet for the KBD endemic area, on the histology of bone and growth plate cartilage in 96 Wistar rats of both sexes [191]. Supplementation with selenium and iodine decreased necrosis of the chondrocytes in the growth plate and promoted trabecular bone formation in rats. Additionally, they observed increases in bone volume/tissue volume ratio, trabecular thickness, and trabecular number and decreases in trabecular separation [191].
The effects of a selenium diet on bone metabolism appear to be clear in animal studies conducted over generations [149,192,193]. Moreno-Reyes et al. provided a seleniumdepleted diet to growing male rats for two generations [149] and observed growth retardation and low bone mineral density in the second-generation rats fed a selenium-depleted diet compared with the first-generation rats. The selenium-depleted diet was associated with reductions in the levels of pituitary growth hormone and insulin-like growth factor I [149].
Ren et al. designed an animal study where a selenium-and/or iodine-deficient diet was supplied for two generations and reported that the combination of selenium and iodine deficiency impaired bone and cartilage growth [192]. Additionally, Min et al. showed that selenium deficiency in rats fed a low-selenium diet over two generations can lead to epiphyseal plate thickness shortening, pathological changes, and decreased antioxidant activity [193]. Combination of vitamins E and C with selenium prevented structural alterations in the long bones in an osteoporosis model.
Tibial length, the thickness of the growth plate cartilage ↓ in Seand iodine-deficient rats. PTHrP expression ↑. ColX expression ↓ in Se-deficient rats in both generations, independently of iodine deficiency.
In two generations of rats, gene expressions of COL II, GPx1, and GPx4 ↓ in Se-sufficient rats. Epiphyseal plate lesion ↓, cartilage type II collagen production ↓, and GPx1 activity ↓.

Clinical Studies on the Effects of Selenium on Bone Health
As observed in preclinical studies, selenium deficiency may potentially cause bone damage. Despite efforts to prove the association between selenium and bone health in humans, clinical studies have yielded controversial outcomes. An extensive literature search was done on correlations between bone health, selenium blood level, and selenium dietary intakes and on the effectiveness of selenium supplementation in humans. For the present section, we reviewed a total of 21 studies: 6 case-control studies, 10 cross-sectional studies, 2 cohort studies, 1 longitudinal study, and 2 randomized, double-blind, placebocontrolled studies. Tables 3 and 4 compare the results of previous observational, clinical studies, and they were grouped according to the ethnicity of the population and are listed in order. Most of these observational studies assessed selenium status by measuring blood selenium levels alone or paired with selenoprotein P levels, or by using a questionnaire on selenium intake. The outcomes of bone health were considered by BMD or fracture history.

Association of Selenium Intake Levels with Bone Health: Observational Studies
Several case-control studies have been performed to determine the differences in selenium levels between patients with osteoporosis and healthy individuals. In a study which is the first of its kind, the differences in the concentrations of elements, including magnesium, zinc, copper, manganese, and selenium, were determined in 77 postmenopausal women with osteoporosis and 61 healthy controls. However, the plasma concentrations of selenium and other elements did not vary significantly between the patients and controls [194]. Liu et al. investigated the serum concentrations of macro and trace elements such as zinc, iron, copper, and selenium in 290 postmenopausal women in the Chinese urban area. Comparison of bone mineral density of healthy individuals with those with osteoporosis and osteopenia did not reveal any significant difference in the levels of these elements [198]. Wang et al. grouped 91 elderly Chinese men with osteoporosis and showed that plasma levels of Mn, Zn, Cu, Se, and Pb were not related to bone mineral density [199]. Some studies have documented that the levels of triglycerides and low-density lipoprotein cholesterol correlated negatively with selenium content in postmenopausal women [209,210]. There were no differences in the levels of trace elements such as selenium and lipids in 107 women who were divided into postmenopausal osteopenia, osteoporosis, and health disorder groups, and there was no direct correlation between selenium and bone mineral density [47].
Other population-based cohort studies have used large numbers of participants. A prospective study [48] that characterized the association between selenium status and bone mass among 1144 healthy euthyroid postmenopausal women demonstrated that plasma selenium levels or SELENOP concentrations correlated with hip and lumbar spine bone mineral density and bone turnover markers. Higher selenium status was related to low bone turnover markers and high bone mineral density, but was not related to a risk of vertebral fracture [48]. Beukhof et al. determined selenium content by measuring both plasma selenium and SELENOP levels in a cohort study of 387 healthy elderly men in the Netherlands. Selenium and SELENOP levels were positively associated with total and femoral trochanter bone mineral density in subjects without selenium deficiency [195]. Al-E-Ahmad et al. showed an association between selenium level and lumbar spine and femoral neck bone mineral density among elderly individuals living in Iran. As selenium possesses antioxidant activity, they investigated the correlation between SNPs in ROSrelated genes [211][212][213] and selenium levels and revealed that selenium reduced ROS activity. In addition, selenium levels correlated positively with bone mineral density [197].
Recent studies have shown a positive correlation between selenium levels and bone mineral density. In a study on 1167 healthy Korean adults, Park et al. found differences in hair selenium levels between the low and normal bone mineral density groups. Participants with low bone mineral density had significantly lower hair selenium levels adjusted for osteoporosis-related risk factors compared to a healthy control [200]. In the populationbased Hortega study [196] involving 1365 participants, a non-linear dose-response relationship of selenium was observed with reductions in bone mineral density, showing an inverse association below 105 µg/L, which became increasingly positive above 105 µg/L. While a U-shaped dose-response curve of selenium was observed with the incidence of osteoporosis-related bone fractures in a prospective analysis, the positive association above 105 µg/L was markedly stronger than that observed in the cross-sectional analysis [196]. In a study that did not directly evaluate selenium levels, Mendelian randomization analysis was performed on summary data from large Genome-wide Association Study (GWAS) datasets, and serum selenium levels were found to correlate positively with the heel bone mineral density. A nominally significant relationship was observed between serum selenium levels and forearm bone mineral density. However, selenium significantly affected only bone mineral density at specific skeletal sites such as heel [214].
Other studies have investigated selenium intake instead of measuring selenium levels in the blood. Wolf et al. measured bone mineral density in 11,068 postmenopausal women aged 50-79 years enrolled in the Women's Health Initiative Observational Study and Clini-cal Trial at three clinics in the USA, where the level of selenium was estimated using a selfreported food-frequency questionnaire. However, dietary selenium intake and total bone mineral density were not associated [49]. Two more studies showed a lack of correlation between selenium intake and bone mineral density [202,205]. In contrast, several studies have shown a positive correlation with bone health. Zhang et al. constructed a case-control study and showed that selenium intake was inversely associated with a reduced risk of osteoporotic hip fracture in 2564 elderly smokers (OR = 0.27, 95% CI = 0.12-0.58) [201]. An earlier case-control study also showed that a higher dietary intake of selenium was associated with a lower risk of hip fracture (OR = 0.43, 95% CI = 0.26-0.70) [206]. Two Spanish studies of postmenopausal and non-menopausal women showed that a selenium diet was associated with bone mineral density at specific sites, including the calcaneus or phalanges [203,204].
In a cross-sectional study of 6267 Chinese people, the prevalence of osteoporosis was high (OR = 0.72, 95% CI = 0.55-0.94) in both men and women with lower dietary selenium intake levels. Although several confounding factors were considered in this study, unfortunately, detailed information on hormonal status is lacking [207]. Zhang et al. used data from 17,150 participants from the China Health and Nutrition Survey, in which the selfreported history of fractures showed a non-linear association with selenium intake [208]. Recently, Wu et al. analyzed the data of 2983 adults in the National Health and Nutrition Examination Survey (NHANES) 2013-2014 [50] and observed that a higher selenium level, measured in plasma and diet, correlated with a lower bone mineral density and a decreased FRAX score, which represents the 10-year fracture risk [50].
Although there are studies showing non-significant results between selenium and bone health [47,49,194,198,199,202,205], a positive correlation was observed between selenium and bone health, showing a positive relationship between selenium and bone mineral density [48,50,[195][196][197]200,203,204,207]and a reduction in the risk of osteoporotic fracture [50,196,201,206,208]. The limitations of these observational studies include the fact that not all, but some, were conducted with sample sizes too small to achieve statistically valid results. The majority of studies found no evidence of a selenium deficiency, as demonstrated by the Recommended Daily Allowance (RDA).

Randomized Clinical Trial of Selenium Intake: Intervention Studies
Based on the positive results obtained in the above observational studies, two randomized clinical trials were performed. The characteristics of interventional studies are shown in Table 5. The effect of selenium level on bone health was tested for the first time in a randomized controlled trial by Walsh et al., where they completed a 6-month randomized, double-blind, placebo-controlled trial involving postmenopausal women aged >55 years with osteopenia or osteoporosis in the UK. An equal number of participants were randomly assigned to a placebo or to groups administered 50 µg or 200 µg of oral sodium selenite per day. Among the 120 participants recruited, 115 (96%) completed the follow-up. The primary endpoint was a biochemical marker of bone resorption (urinary N-telopeptide, uNTx) at 26 weeks, the levels of which did not differ significantly between the treatment groups. Secondary assessments of other markers of bone turnover, i.e., bone mineral density, muscular performance, and levels of glutathione peroxidase, highly sensitive C-reactive protein, and interleukin-6, revealed that there were no significant changes after selenium supplementation [215]. The authors of that study mentioned that the dietary selenium intake at baseline was higher than that expected in the UK population. Second, the study population included only postmenopausal women, suggesting that further studies investigating elderly people with selenium deficiency are required [215]. Selenium supplementation reduced P1NP at 6 months, but there were no significant effects on other BTM or after 5 years.
A population with relatively low selenium level was investigated in a single-center, randomized, double-blinded, placebo-controlled, multi-arm, parallel clinical trial (PRE-CISE) in Denmark [216]. The 491 volunteers, aged 60-74 years, were randomly and equally assigned to treatment groups administered 100, 200, or 300 µg selenium per day via selenium-enriched yeast for 5 years (from randomization in 1998-1999) and were followed up for further 10 years. The plasma selenium level was within the optimal level during the follow-up period. The levels of bone turnover markers, measured at 6 months, showed a significant linear decrease in the level of procollagen type 1 N-terminal propeptide (P1NP), although this effect was not apparent at 5 years; no significant effect of selenium supplementation on any other bone formation markers was observed [216]. As P1NP is a marker of osteoblast function [217], the decrease in PINP level indicated a reduction in bone formation. Therefore, the effect of selenium on bone turnover and bone health remains to be determined on the basis of the results of these randomized clinical trials.

Novel Approach of Selenium Supplement
The contradictions in the results of clinical studies can be because of the differences in the levels of dietary selenium and selenium supplements [218]. Therefore, current studies have focused on developing various selenium-based therapeutic strategies instead of traditional selenium intake trials. Here, we have focused on various novel therapeutic strategies involving selenoproteins.

Selenium Nanoparticles
The amount of bioavailable selenium in the diet largely differs with the food and soil fodder to which humans and animals are exposed [219,220]. Moreover, in the environment, selenium is present in various oxidation states (2, 0, 4+, and 6+) that are cytotoxic. Owing to its high toxicity (Se 4+ ), selenite is reduced to elemental selenium (Se 0 ) by biogeochemical cycles [221][222][223]. Therefore, a high dose of selenium is harmful for humans and has a narrow safety-to-toxicity margin [224,225]. Selenium nanoparticles are potential candidates with low cytotoxicity [226,227] that can be used to circumvent the various problems associated with inorganic and organic selenium compounds, such as ROS generation and low redox activity [228][229][230][231].

Synthesis of Selenium Nanoparticles
Selenium nanoparticles can be synthesized chemically [232], physically [233], or even biologically using microorganisms or plant extracts, a process known as green synthesis [234,235]. Gao et al. [236] showed that hollow, chemically synthesized, spherical selenium nanoparticles possess antioxidant properties that can reduce selenium toxicity. Selenium nanoparticles are less toxic than inorganic and organic selenium. The lower hepatotoxicity and nephrotoxicity of these nanoparticles have been demonstrated in in vivo studies [226,237,238]. However, these chemically synthesized nanoparticles are not suitable for biomedical use, limiting their application. As harsh chemicals are used in chemical synthesis, even small amounts of these compounds in humans can cause systemic toxicity. Therefore, green synthesis is gaining popularity in the biomedical field [239][240][241].
Furthermore, oral supplementation with nanoparticles is considered to be the most appropriate and cost-effective method. However, absorption barriers in the digestive tract may render the absorption of nanoparticles difficult. For example, to metabolize nanoparticles, the mucus covering the intestinal mucosa should be overcome [242]. Thus, numerous compounds have been used to stabilize selenium nanoparticles, including amino acids, bovine serum albumin, polysaccharides such as chitosan [243][244][245], gum arabic [243,246], and polymers such as poly(lactic-co-glycolic acid) [247].
Experiments have been performed with green selenium nanoparticles. Polysaccharideprotein complex-coated selenium nanoparticles (PTR-SeNPs) synthesized using the mushroom polysaccharide isolated from Pleurotus tuberregium and selenium nanoparticles (Cs4-SeNPs) synthesized using the mushroom polysaccharide isolated from Cordyceps sinensis inhibited osteoclastogenesis while promoting osteoblastogenesis, thereby stimulating bone formation via preosteoblast differentiation [257,258]. The PTR-SeNP system has been rationally designed and identified as a potent bone-formation therapeutic that can antagonize osteoporosis. The nanosystem is incorporated by osteoblasts and significantly enhances bone formation in vitro and in vivo [259]. Fatima et al. [260] found that modest concentrations of selenium nanoparticles can improve human mesenchymal stem cell survival and osteogenic potential by reducing oxidative stress. Selenium nanoparticles may promote osteogenesis by activating the JNK/FOXO3 pathway. Another study also demonstrated increases in the osteogenic maturation of MC3T3-E1 cells and the downregulation of ROSs by hydrogen peroxide [261].
Many clinical studies have investigated the effects of cancer chemotherapy on osteoporosis. Anastrozole is used to treat breast cancer; however, bone toxicity limits its clinical use. In vitro studies on human osteoblasts have shown that selenium nanoparticles reduce cell death. At the examined doses, selenium nanoparticles significantly prevented decreases in bone density and increases in the levels of biochemical markers of bone resorption, eventually preventing anastrozole-induced osteoporosis [262]. Therefore, selenium nanoparticle co-supplementation might prevent bone loss by enhancing osteogenesis and may be an effective candidate for treating osteoporosis.

Implanted Selenium Therapeutics
Numerous metal ions, including selenium, possess osteoinductive properties. Hence, the effects of coating or implanting selenium and selenium nanoparticles on bone structures and tumors [263], or other orthopedic medical devices [264], have been investigated. The integration of selenium into biomaterials is an effective therapy for promoting bone remodeling [265,266].
Previous studies have shown that the local use of selenium correlates with bone formation and reduces the risk of osteoporosis due to its antioxidant property [175,267]. The effect of calcium phosphate ceramics on bone healing [268] and the strong affinity between selenium and calcium phosphate have been shown [266]. Li et al. [269] conducted a study with selenium-modified calcium phosphate cement (Se-CPC), CPC with 3% selenium nanoparticles, and scaffolds. These were implanted into the femoral epiphysis of an ovariectomized rat with osteoporosis and incubated for 12 weeks. The results of a reverse transcription-quantitative polymerase chain reaction revealed that the Se-CPC group had higher OPG and lower RANKL levels. This stimulated bone regeneration and bone formation by activating the OPG/RANKL pathway and inhibiting local oxidative stress [269]. Additional experiments should be performed in the future to expand the availability of localized selenium nanoparticles.

Conclusions and Future Prospects
The effects of selenium on human health are numerous and complex, and there are significant information gaps that have to be filled. In the present review, we highlighted the importance of selenium in maintaining bone health, especially in osteoporosis, which is one of the major diseases affecting bone health.
Osteoporosis contributes significantly to a social management burden due to the high mortality, morbidity, and treatment costs associated with the disease [270]. By 2020, the number of people suffering from osteoporosis worldwide exceeded 200 million, which will increase rapidly in successive years. The number of annual hip fractures will surpass 3 million, and the cost of treatment is estimated to be 25 billion USD by 2025 [13,271].
New drugs are available for the treatment and prevention of osteoporosis. As mentioned earlier, the daily intake of calcium-and vitamin-D-rich food is part of the traditional lifestyle modification therapy [272]. Only subtle adverse events have occurred with antiosteoporotic drugs including romosozumab, calcium supplements, bisphosphonates, and selective estrogen receptor modulators, which are currently safely prescribed and widely used [273][274][275]. In addition to anti-osteoporotic drugs, natural products, such as trace elemental particles that are safe for human health, can be used in conjunctive therapy. Selenium may be used as an adjuvant for the treatment of osteoporosis, as some studies have demonstrated that selenium exerts cardiovascular protective and anti-osteoporotic effects [276][277][278][279][280].
Most observational studies have reported that the level of selenium correlates positively with bone health, as it increases bone mineral density or reduces the risk of osteoporotic fracture, while some studies have shown a lack of association between selenium status and bone health. However, there is a discrepancy between the results of randomized clinical trials and those of observational studies. The baseline selenium level or the formula of selenium supplement might be a key factor influencing the effect of selenium on bone health. Further research on the different functions and effects of selenium biomarkers and on more effective and safe methods of selenium supplementation are required. Therefore, the novel approach of synthesizing selenium supplements, such as selenium nanoparticles, is being investigated.
Nanobiotechnology has been a hot topic of research ever since its emergence in the biomedical domain. Green nanoparticles are produced using natural microbes, which can be optimal for human use. However, reports on green-synthesized selenium nanoparticles are limited. Further studies are required to understand the relationship between selenium and selenium nanoparticles, as well as the biological pathways underlying the differences in the therapeutic efficacies of selenium and selenium nanoparticles. In the future, the use of selenium nanoparticles should be expanded to treat bone health and other medical conditions. Finally, the effect of selenium on bone health is still an open area of investigation, and more studies on the role of selenium nanoparticles are required.

Conflicts of Interest:
The authors declare no conflict of interest.