Influence of Dietary Compounds on Arsenic Metabolism and Toxicity. Part II—Human Studies

Exposure to various forms of arsenic (As), the source of which may be environmental as well as occupational exposure, is associated with many adverse health effects. Therefore, methods to reduce the adverse effects of As on the human body are being sought. Research in this area focuses, among other topics, on the dietary compounds that are involved in the metabolism of this element. Therefore, the aim of this review was to analyze the influence of methionine, betaine, choline, folic acid, vitamin B2, B6, B12 and zinc on the efficiency of inorganic As (iAs) metabolism and the reduction in the severity of the whole spectrum of disorders related to As exposure. In this review, which included 62 original papers (human studies) we present the current knowledge in the area. In human studies, these compounds (methionine, choline, folic acid, vitamin B2, B6, B12 and zinc) may increase iAs metabolism and reduce toxicity, whereas their deficiency may impair iAs metabolism and increase As toxicity. Taking into account the results of studies conducted in populations exposed to As, it is reasonable to carry out prophylactic activities. In particular nutritional education seems to be important and should be focused on informing people that an adequate intake of those dietary compounds potentially has a modulating effect on iAs metabolism, thus, reducing its adverse effects on the body.


Introduction
Exposure to arsenic (As) affects people living in many regions of the world. This problem occurs not only in less affluent areas (such as Chile, Argentina, Bangladesh, and Mexico), but also in several hotspot regions of Europe (Poland, Hungary, Serbia, Romania, Czech Republic, Croatia, Finland, Greece, and Italy) [1]. The source of exposure to different chemical forms of As can be both environmental and occupational. Environmental pollution can be of natural origin (rocks, soils, and volcanoes, among others) and can be caused by human activities (e.g., mining, burning fossil fuels, residues of agricultural chemicals in the soil and groundwater) [2,3]. Environmental sources also include water and food contamination. The As content in food products varies and depends on the type of product and its origin. Foods that contribute most to general population exposure to inorganic arsenic (iAs) include rice and rice-based products, grains and grain-based processed products, and drinking water [4]. The second source of As exposure is occupational exposure. As is used in many industries (agriculture; horticulture; mining; wood preservation; and production of, e.g., ammunition, glass, semiconductors, dyes) [2,3].
Both environmental and occupational exposures include exposure to inorganic as well as organic forms. Inorganic forms (such as As trioxide, As pentoxide, arsenous acid, and arsenic acid) are more toxic than organic forms (monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), arsenobetaine). Among iAs, As on the third and fifth oxidation state is the most toxic, with forms of the third oxidation state being more toxic [5][6][7].

Folic Acid and Zinc as Modulators of iAs Metabolism and Toxicity-In Vitro Studies
The protective effect of folic acid (two studies) and zinc (two studies) has been analyzed in four studies on cell lines that were exposed to iAs. The results are summarized in Table 1.

Folic Acid-iAs Metabolism
The association between folic acid and iAs metabolism was analyzed only in one in vitro study. In a study performed in Chang human hepatocytes, folate deficiency decreased the levels of methylated arsenicals, and supplementation with folic acid showing no effects on iAs metabolism [19].
In the in vitro study performed on human embryonic kidney 293 cells, folic acid showed a protective effect against adverse effect of iAs by increasing the viability of cells and decrease the oxidative stress levels through decreasing the levels of reactive oxygen species and p66Shc expression [20].
Furthermore, a study with a human monocyte cell line, carried out in the presence of iAs, demonstrated that zinc deficiency increased oxidative stress (increased reactive oxygen species production) and inflammatory response (inter alia, through increased production of inflammatory markers) [21].
In the human keratinocyte cell lines exposed to iAs, zinc reduced DNA damage by affecting the production of poly(ADP-ribose) polymerase-1 (increased content zinc in this protein and its activity; decreased As binding with this protein) [22].

Folic Acid and Zinc-Summary
In vitro studies have shown that folic acid deficiency decreased iAs metabolism and supplementation had no influence. Folic acid deficiency (1 h) could exacerbate the adverse effect of exposure to iAs in a short period of time, and the addition of folic acid (also only for 1 h) did not improve iAs metabolism; in both cases, a long exposure time to iAs (24 h) was used.
However, these compounds reduced the adverse changes induced by iAs in three cell lines, mainly through decreased apoptosis, oxidative stress and DNA damage. Although the studies were carried out on different cell lines and different doses of folic acid (10, 100 µM) zinc (2 µM) and iAs (2, 20 µM; 5 mM) with the same exposure times iAs (24 h), favorable results were obtained.
In two in vitro studies with iAs exposure (for 4 or 24 h), folic acid and zinc deficiency intensified oxidative stress, apoptosis and inflammation. In these cases, both short folic acid deficiency (1 h) and long zinc deficiency (4 weeks) had an adverse effect.

Relationship between Dietary Intake of Selected Compounds and iAs Metabolism and Toxicity
Twenty-two studies in subjects exposed to iAs (mainly in drinking water) analyzed the relationships between the intake of a whole range of nutrients and iAs metabolism as well as toxicity (Table 2).

Nutrient Intake-iAs Metabolism
The main results of studies that analyzed the relationships between intakes of methionine, betaine, choline, vitamins B 2 , B 6 , B 12 , folate, zinc and biomarkers of iAs metabolism in children and then in adults are presented below.
The relationship between nutrient intake and iAs metabolism was reported in four studies conducted with children.
In the case of vitamin B 2 , no significant relationship was observed between its intake and the urinary excretion of As metabolites (%MMA, %DMA, and %iAs) [23]. A negative association between vitamin B 6 intake and %MMA in urine was observed only in one study involving children, and there were no significant relationships between vitamin B 6 intake and the percentage of DMA and iAs in the urine [23]. In contrast, in the study by Kurzius-Spencer et al. [24], no significant association was observed between vitamin B 6 intake and the urinary excretion of iAs metabolites. Three studies showed no relationship between vitamin B 12 intake and iAs metabolism in children [23][24][25]. Folate intake was negatively associated with urinary %MMA [25,26] and, additionally, in one of these studies, a positive association with urinary %DMA was observed [26]. In these studies, no association was observed between intake of folic acid and %iAs, tAs in the urine [25,26] and urinary excretion of iAs metabolites [24].
The relationship between nutrient intake and iAs metabolism was also analyzed in the adult population (10 studies).
In the study by Heck et al. [27], methionine intake (in groups of men and women) was positively associated with urinary %MMA, whereas no such relationship was observed in another study involving women [28]. In the study by Heck et al. [28], methionine intake also was positively associated with urinary %DMA [28], but in a subsequent study by these authors, this relationship was not significant [27]. Additionally, these two studies showed a negative association between methionine intake and %iAs in the urine [27,28]. Moreover, in the study by Heck et al. [29], a higher methionine intake was associated with an increased excretion of tAs in the urine. Another analyzed ingredient was betaine; there was no relationship between betaine intake and iAs metabolism [27,28].
Regarding choline, two studies showed intake of this compound was not associated with urinary %MMA, but a positive association was observed with the DMA/MMA ratio [27,28]. In contrast, one study demonstrated associations with the other iAs metabolites in urine (positive association of choline intake with %DMA and negative association with %iAs) [28].
The relationships between vitamin B 2 intake and iAs metabolism have been analyzed in several studies. One study showed a positive association between vitamin B 2 intake and urinary %MMA [27], while another study showed a negative association with this form [30]. Additionally, in several studies, there were no relationships [28,[31][32][33]. In the study by Spratlen et al. [30], vitamin B 2 intake was positively related with urinary %DMA, while no such relationship was shown in other studies [27,28,[31][32][33]. In contrast, a negative association with %iAs was shown in the study by Spratlen et al. [30], while no such relationship was observed in other studies [27,28,[31][32][33]. There was no significant relationship between vitamin B 2 intake and urinary excretion of tAs [34]. One study revealed a positive association between vitamin B 2 intake and first methylation step (MMA/iAs), and negative with the second methylation step (DMA/MMA) [27].
The relationship between vitamin B 6 intake and iAs metabolism was analyzed in seven studies. In the study by Spratlen et al. [30], vitamin B 6 intake was negatively associated with urinary %MMA and positively with %DMA. In the study by Kurzius-Spencer et al. [24], these relationships were not significant. Both studies showed a negative association with urinary %iAs [24,30], while the study by Argos et al. [34] showed a positive association with tAs in the urine. On the other hand, in several studies, there was no relationship between vitamin B 6 intake and urinary %iAs metabolite excretion [27,28,31,33].
Vitamin B 12 intake correlated positively with urinary %MMA in the study by Heck et al. [27]; other studies did not show such a relationship [24,28,30,32]. The study by Lopez-Carillo et al. [28] also showed a positive association with urinary %DMA, which was not observed in the studies of other authors [27,30,32]. Only in two studies, the intake of this vitamin was negatively associated with %iAs in the urine [27,28]. This relationship was not confirmed in other studies [24,30,32]. Positive associations with ratios of various forms of As in the urine were also observed in studies by Heck et al. [27] and Lopez-Carillo et al. [28]. However, the intake of this vitamin did not affect the urinary excretion of tAs [34].
None of the studies observed a relationship between folic acid intake and urinary %MMA and %DMA [27,28,[30][31][32][33]. In contrast, the study by Howe et al. [35] showed that intake of sum of vitamins B 2 , B 6 , B 12 and folate had a negative association with the proportion of monomethyl arsenic species. In turn, folate intake was negatively associated with urinary %iAs in one study [28], which was not shown in other studies [27,[30][31][32][33]. Folate intake was positively associated with ratio DMA/iAs [28]. No relationship between folate intake and tAs in the urine was reported in the study by Argos et al. [34].
Associations between zinc intake and iAs metabolism were demonstrated in two studies. They included one negative association with urinary %MMA and a positive association with urinary %DMA [28,33]. Moreover, the study by Lopez Carillo et al. [28] demonstrated a negative association with urinary %iAs and a positive association with ratios DMA/MMA and DMA/iAs.

Nutrient Intake-Toxicity of As
Several studies have analyzed the relationship between nutrient intake and the severity of adverse changes in the body and the risk of developing diseases associated with exposure to As.
In one study involving adults and children, an adverse effect of high vitamin B 6 intake was observed; it was associated with an increased risk of diabetes and metabolic syndrome [31]. On the other hand, in another study, a low intake of vitamin B 2 , B 6 , B 12 and folic acid in subjects exposed to iAs with drinking water was associated with worse cardiovascular outcomes (through increased pulse pressure and marginally systolic hypertension) [36]. A protective effect of nutrients was demonstrated in one study, in which a reduction in the severity of oxidative stress (reduced urinary 15-F 2t -isoprostane) was observed in adults who had a higher intake of B vitamins [35].
However, no consistent effect was observed between folic acid intake and bladder cancer risk in adults [37]. Three studies demonstrated an increased risk of skin lesions in individuals who had low intakes of choline, vitamin B 2 , folate and zinc [38][39][40]. In turn, a reduced risk of As-related skin lesions was observed in adults who consumed higher amounts of vitamin B 2 , B 6 and folic acid [41].
Low folic acid intake was not associated with cognitive performance in children, but higher intake was positively associated with cognitive abilities. In this study, moreover, several relationships were observed between tAs in the urine and cognitive performance depending on the level of folic acid intake, but it was an inconsistent effect [42]. In another study by the same author, there was no relationship between urinary tAs and achievement (broad math and reading scores) among children and B vitamin intake [43].
In turn, negative associations between vitamin B 12 intake and tAs concentrations in toenails were observed in the study by Gruber et al. [44].

Nutrient Intake-Summary
The results of human studies on the relationship between nutrient intake and iAs metabolism are inconclusive. They indicate that some nutrient intake may contribute to iAs elimination from the body. In several studies, the intake of such nutrients as methionine, choline, vitamin B 2 , vitamin B 6 , B 12 , folate, zinc was observed to be correlated with iAs metabolism. However, some of the correlations between nutrient intakes (betaine, choline, vitamin B 2 , vitamin B 6 , vitamin B 12 , and folate) and the urinary content of various forms of iAs suggest that these nutrients may or may not impair iAs metabolism.
The authors also observed a relationship between a high intake of B vitamins and folic acid and a reduction in the adverse changes associated with exposure to iAs (decreased oxidative stress, reduced risk of As-related skin lesions, and increased cognitive abilities). In turn, low intake (choline, B vitamins, folic acid, and zinc) was associated with the deterioration of cardiovascular outcomes and an increased risk of skin lesions. The differences in these results may be due to the levels of nutrient intake (higher intake was connected with a reduction in the negative effects associated with iAs exposure, while a low intake may exacerbate these effects).
However, in several studies, there are conflicting data that require clarification. No consistent effect was observed between B vitamins and folic acid intake, and risk of diabetes and bladder cancer, as well as cognitive performance and achievements in children. In these studies, the author suggested that the lack of, or unclear, effect may be due to the low variability in vitamin intake, or that intake was above a sufficient level in the majority of participants. Additionally, the results could also be influenced by additional metabolic differences (e.g., genetic variation or differences in iAs metabolism-the study was conducted on children, adults and patients with bladder cancer). In one of these studies, a high intake of vitamin B 6 was associated with diabetes-related outcomes, but this effect was unclear because, in most studies, low consumption contributed to the severity of the negative effects and, therefore, this aspect requires further analysis.        Table 3 includes results from studies in which folic acid (eight studies) and zinc (one study) were supplemented in a population exposed to As.

Folic Acid and Zinc Supplementation-iAs Metabolism
Five studies analyzed the protective effect of folic acid and zinc supplementation on iAs metabolism. Changes in iAs metabolite concentrations (inter alia, decreased concentrations of MMA in the blood and increased urinary excretion of DMA) were observed in people taking folic acid supplementation [45]. In a study by Bozack et al. [46], folic acid supplementation in subjects with low and normal blood concentrations of this compound also altered iAs metabolism profile (by increased percentage dimethyl-arsenical species and decreased %iAs and monmethylarsenical species in the urine). In the study by Peters et al. [47] conducted on a Bangladeshi population, folic acid supplementation at a higher dose (800 µg/db) reduced the concentration of tAs in the blood. This effect persisted even after 12 weeks from the end of supplementation, but no such effect was observed with a lower dose of this vitamin (400 µg/db). Folic acid supplementation in participants with betaine concentrations below the median affected iAs metabolism (inter alia, through an increased percentage of dimethyl-arsenical species and decreased percentage of monomethyl arsenical species in the urine) [48]. In contrast, in a group of children, zinc supplementation decreased urinary %DMA, but did not affect the concentration of other As biomarkers in the urine [49]. In a group of Bangladeshi residents exposed to iAs in drinking water supplementation with folic acid, there was an influence on parameters related to iAs metabolism through increased plasma choline and betaine concentration and a percentage decrease in dimethylglycine in the plasma. There were no significant differences between groups receiving lower (400 µg/day) and higher (800 µg/day) doses of folic acid [50].

Folic Acid Supplementation-Toxicity of As
In one study, symptoms of chronic arsenic intoxication were analyzed in an adolescent girl who was taking globules (containing iAs). Treatment with thiamine and folic acid, among others, reduced the adverse symptoms and the concentration of iAs in urine and hair [51]. In contrast, folic acid supplementation did not alter post-translational histone modifications in peripheral blood mononuclear cells from Bangladeshi adults [52]. Ghose et al. [53] showed that folic acid supplementation reduced adverse symptoms in patients with chronic As toxicity.

Folic Acid and Zinc Supplementation-Summary
In four studies, folic acid supplementation (in doses 400 and 800 µg/day for 12 or 24 weeks) improved iAs metabolism in adults (mainly through increased urinary DMA concentration and decreased tAs concentration in the urine and the blood). In one of these studies, only a higher dose of folic acid improved iAs metabolism. In turn, zinc supplementation (30 mg/day for 6 months) in children resulted in a different effect (decreased urinary DMA concentration).

Blood Nutrient Concentration-iAs Metabolism
Eight studies analyzed the relationship between blood micronutrient concentrations and urinary As metabolites in children. No relationship was observed between plasma vitamin B 6 levels and urinary excretion of As metabolites (%MMA, %DMA, and %iAs) [24]. The results of three studies [24,25,54] did not demonstrate the relationship between vitamin B 12 concentration in the plasma/serum and As metabolites in the urine. In turn, the results of two studies demonstrated an increase in the %DMA and a decrease in the %MMA, %iAs in the urine of the group of children with high concentrations of vitamin B 12 and folate in their plasma [55] in the second study showed a positive association between serum vitamin B 12 concentration and urinary concentrations of DMA [56]. Additionally, results of three studies showed no relationship between the concentration of plasma/serum folate and urinary %MMA [24,54,57]. Plasma/serum folate concentration was positively associated with the percentage and concentration of DMA in the urine [56][57][58] and negatively with urinary %iAs [54,57]. In contrast, in the study by Kurzius-Spencer et al. [24], folate concentration in the serum had no relationship with urinary %DMA and %iAs, and in the study by Desai et al. [25], no relationship was observed with any urinary iAs metabolites.
Family iAs metabolism was only analyzed in one study. A strong pattern of metabolism was shown between siblings. Additionally, in siblings, positive correlations between urinary iAs/methylated As, monomethylarsonate/dimethylarsinate a methionine, vitamin B 6 , B 12 folate in the blood were observed. These correlations between parents and children were much lower [59].
The relationship between nutritional status and iAs metabolism has also been analyzed in the adult population (12 studies).
One study found that plasma vitamin B 6 concentration was not associated with the urinary excretion of As metabolites (%MMA, %DMA, and %iAs) [24].
A lower %MMA and higher %DMA in the urine were observed in adults with a low concentration of vitamin B 12 in the plasma compared to individuals with a higher concentration of this vitamin. The study also noted that the concentration of vitamin B 12 in the plasma was positively associated with urinary %MMA, but inversely associated with urinary %iAs. Such a relationship was stronger in the group of subjects with an adequate folate concentration in the plasma [60]. The study by Zhu et al. [56] demonstrated that serum vitamin B 12 concentration was positively associated with urinary concentrations of DMA. However, the results of other studies showed no relationship between plasma and serum vitamin B 12 concentration and urinary As metabolites [24,61].
In contrast, three studies found no relationship between plasma/serum vitamin B 12 concentrations and urinary excretion of As metabolites (%MMA, %DMA, and %iAs) in pregnant women [62][63][64]. Only the study by Laine et al. [62] demonstrated a negative correlation between maternal serum vitamin B 12 with tAs in the urine. Moreover, in women, the concentration of vitamin B 12 in the plasma was inversely associated with arsenate [65] and positively associated with %iAs [62] in cord blood.
In one study, serum/plasma folate concentration was positively associated with urinary %MMA [58], while in the study by Gamble et al. [61], it was negatively associated with urinary %MMA. Two studies demonstrated a positive association between folate concentration in the serum/plasma and urinary %DMA [56,61]. In turn, the results of another study did not report such a relationship [58]. Moreover, in the study by Gamble et al. [61] plasma folate concentration was negatively associated with urinary %iAs. In turn, in the study by Kurzius-Spencer et al. [24] no relationship was shown between serum folate levels and urinary iAs metabolites.
In a Bangladeshi population, individuals with low plasma folate concentrations showed a correlation between decreased ratio of glutathione-to-glutathione disulfide and higher blood levels of tAs, as well as having increased %MMA and decreased %DMA in the urine [66].
The study by Chung et al. [67] analyzed the relationship between gene polymorphisms, iAs metabolism and plasma folate. Abnormal iAs metabolism and decreased plasma folate levels were observed in patients with urothelial carcinoma. Subjects with the 5,10-methylenetetrahydrofolate reductase CT or TT genotype had lower percentage dimethylarsenic acid in the urine and a lower folate concentration in the plasma than those with the CC genotype. A positive correlation was also observed between plasma folate concentration and percentage urinary dimethylarsenic acid in the control group.
In Bangladeshi women, folate concentration in the plasma/serum was inversely associated with arsenate in the maternal blood [65], and in pregnant women negatively correlated with percentage monomethyl arsenicals in the cord serum [62]. In contrast, three studies did not demonstrate significant correlations between plasma folate levels and urinary As metabolites in pregnant women [62][63][64]. Only in women with both higher As exposure level and plasma folate concentration was a reduced iAs level in the urine noted [63].
Two studies reported no significant relationship between plasma zinc concentrations and urinary As metabolites in pregnant women [62,64]. Only in subjects with the highest exposure level were higher plasma zinc concentrations associated with increased %MMA and %iAs, and decreased %DMA [63].

Blood and Tissue Nutrient Concentration-Toxicity of As
Many studies have focused on analyzing the relationship between nutritional status and As concentrations in various tissues and adverse health effects in individuals exposed to As.
The relationship between As exposure, nutritional status, and DNA methylation was analyzed in three studies. In elderly men, no association was observed between the plasma concentrations of vitamin B 6 and B 12 and DNA methylation. However, in men with a low concentration of folate in the plasma, a positive association with one repetitive element (increased Alu DNA methylation) was observed, whereas in men with a higher concentration of plasma folate, the effect was the opposite [68]. Similarly, the study in Bangladeshi adults with high plasma folate concentration (above 9 nmol/L) showed negative correlation between urinary or plasma tAs and methylation of peripheral blood leukocyte DNA (lower [3H]-methyl incorporation) [69]. In turn, in women with folate deficiency in the plasma, an inversely association was observed between the tAs concentration in toenail and total histone 3 levels in the plasma [70].
Howe et al. [52] demonstrated positive associations between the plasma concentration of vitamin B 12 (among women) and choline (among men) and global levels of post-translational histone modifications.
Several studies have reported the relationship between blood concentrations of nutrients and the risk of disease development or the severity of adverse body changes associated with an exposure to As.
A higher odds ratio of type 1 diabetes by the percentage of monomethylates As was observed in individuals with a higher concentration of plasma folate (stronger relationship between percentage monomethylated As and type 1 diabetes). No significant relationship was shown for type 2 diabetes between percentage monomethylates As and plasma folate or vitamin B 12 concentration [71]. It was observed in the study by Chung et al. [72] that low a concentration of folate in the plasma, global 5-methyl-2 -deoxycytidine levels, and a high concentration of total urinary As levels was related to an increased risk of urothelial carcinoma. The study conducted in a West Bengal population, did not demonstrate any relationship between the concentration of folate, methionine, vitamin B 6 , vitamin B 12 in the blood and the susceptibility to develop skin lesions [73]. Subjects with low plasma folate concentrations had an increased risk of skin lesions, whereas no such relationship was observed in subjects with lower vitamin B 12 concentrations [74].
Low concentrations of folate and vitamin B 12 in the plasma were indirectly (through association with decreased iAs metabolism) related to an increased childhood developmental delay (increase in odds ratio of developmental delay) [55]. In the study by Desai et al. [43], higher broad math scores were observed in subjects with higher vitamin B 12 in the serum and tAs concentration in the urine. In subjects exposed to As, zinc and tAs concentrations in different tissues were analyzed. In pregnant women living in Wuhan (the largest industrial city), a positive correlation between tAs and zinc concentration in blood was observed [75]. On the other hand, in patients with blackfoot disease, decreased zinc concentration in the hair, urine, blood as well as increased tAs concentration in hair and urine were observed [76][77][78]. On the other hand, in groups of workers, a higher concentration of tAs in the lung tissue and in the blood was observed, but no decreased concentration of zinc in these tissues was shown [79,80]. Moreover, in the study by Tutkun et al. [80], the correlation between Zn and tAs was negative and in the workers group higher levels of inflammatory markers (interleukins 6, 10 and tumor necrosis factor-α) was also observed, but no correlation was observed between these cytokines and Zn.

Concentration of Nutrients in Blood and Other Tissues-Summary
The results of human studies on the relationship between the blood concentrations of dietary compounds and iAs metabolism are also inconclusive. They indicate that the concentration of dietary compounds in the blood may be related to iAs metabolism and contribute to its elimination from the body. This is indicated by several studies in which it was observed that the blood concentration of vitamin B 12 and folate correlated with iAs metabolism. However, some of the correlations between the blood concentration of these nutrients (folate, B 6 , B 12 , and zinc) and the urinary content of various forms of iAs suggest that these nutrients may or may not impair iAs methylation.
Three studies showed that nutritional status (choline, vitamin B 12 , folate-higher plasma/serum concentrations) may be related to a decrease in adverse health effects (affected methylation DNA, histone modification, and children achievement) in individuals exposed to As. The results of this study are in line with expectations, since the appropriate status of these nutrients can alleviate the adverse health effects associated with exposure to As. Numerous studies also observed a relationship between concentration of these nutrients (folate-low and higher plasma concentration; vitamin B 12 -low plasma concentration) and increased risk of disease and adverse body changes associated with an exposure to As (increased DNA methylation, risk of type 1 diabetes and urothelial carcinoma, skin lesions, and developmental delay). Folate deficiency (five studies) and vitamin B12 deficiency (one study) enhanced adverse effects of iAs exposure, and only in one study higher concentration of folate exacerbated them. These results indicate that both a deficiency and an excess of these compounds may contribute to enhancing the toxicity of As. The level of exposure to iAs does not appear to be a differentiating factor, as the adverse effects associated with folic acid deficiency occurred in populations with low (including elderly men, preschool children, and women) and high exposure (cases with skin lesions and urothelial carcinoma). In several studies, no relationships (or the relationship was unclear) between blood/plasma concentration of methionine, vitamins B 6 , B 12 , folate a DNA methylation, risk of type 2 diabetes and skin lesions were found. Moreover, in pregnant women, patients with blackfoot disease and workers exposed to As, elevated tAs concentrations in many tissues were connected with disturbances in zinc concentrations.   n = 135 Ankara, men-workers (exposed to As) and control group serum: Zn blood: tAs↑, Zn↔, IL-6↑, IL-10↑, TNF-α↑ (in the workers group) correlation tAs-Zn (−) (in the workers group) ↑-significant increase; ↓-significant decrease; ↔-no significant changes; (+)-positively association; (−)-negatively association; (NS)-no significant association; As-arsenic; DMA-dimethylarsinic acid; GSH/GSSG-glutathione/glutathione disulfide; H3-histone 3; H3K36me2-histone 3 lysine 36 dimethylation; H3K36me3-histone 3 lysine 36 trimethylation; H3K79me2-histone 3 lysine 79 dimethylation; iAs-inorganic arsenic; IL-6-Interleukin 6; IL-10-interleukin 10; MMA-monomethylarsonic acid; MMA/DMA-monomethylarsonate/dimethylarsinate; MMAs-monomethyl arsenical species; ORs-odds ratios; SMI-secondary methylation index; tAs-total arsenic species; TNF-α-tumor necrosis factor-α; Zn-zinc; 5-MedC-5-methyl-2 deoxycytidine.

Conclusions
Many studies have also focused on analyzing the relationship between dietary compounds (intake, supplementation mainly folic acid, and blood concentrations), and iAs metabolism, as well as exposure-related disorders. The intake and blood concentrations of certain dietary compounds (methionine, choline, vitamin B 2 , B 6 , B 12 , folic acid, and zinc) showed a relationship with an improvement in iAs metabolism and were associated with reduction in adverse health effects. It was also shown that not only the deficiency (folate), but also the excess of some dietary compounds (vitamin B 12 , folic acid, zinc) may impair iAs metabolism and may increase adverse health effects. Human studies, to date, are inconclusive, because many factors influenced the results. This signals the need for a more detailed analysis of the relationship between the nutrient intake of involved in iAs metabolism, nutritional status as well as the severity and source of iAs exposure. Considering the promising results of the studies conducted so far, it seems reasonable that individuals exposed to iAs should consume natural products rich in methionine, choline, folic acid, B vitamins (B 2 , B 6 , B 12 ) and zinc. Products rich in methionine (meat: turkey, beef, pork and milk, tofu, and Brazil nuts), choline (eggs, beans, and broccoli; meat: chicken, pork, and beef), folic acid (green leafy vegetables), B-group vitamins (meat, eggs, dairy product, leafy greens, and legumes), and zinc (meat, nuts, and cereal products). Nutrition education focusing on an adequate dietary intake of methionine, choline, zinc, folic acid, and B vitamins (B 2 , B 6 , and B 12 )-nutrients which potentially have modulating effects in iAs metabolism and toxicity-should be used in prevention efforts for populations exposed to iAs.