Long-term selenium supplementation of humans: Selenium status and relationships between selenium concentrations in skeletal muscle and indicator materials

Supplementation with elevated doses of l-selenomethionine (SeM) or selenium-enriched yeast that contains SeM as the main selenium species is frequently used as a protective or therapeutic measure. Information on the effects of long-term selenium supplementation on the body selenium status is, however, rather scarce. We therefore investigated fifteen male test persons who had taken selenium yeast and/or SeM supplements in medium doses of 62.5–125 μg Se/day or high doses of 200–262.5 μg Se/day for periods ranging from 1 year to 24 years. Seven non-supplemented men served as controls. As skeletal muscle is the main selenium pool, thigh muscle biopsy samples were taken. The selenium concentrations in these biopsies and in samples of the indicator materials blood, blood plasma, blood cells, head hair and toenails were determined by neutron activation analysis. Compared with the controls, the muscle selenium level was raised with additional selenium supplementation, but the relative increase in the mean muscle selenium concentration (by factors of about 1.6 and 2 for the medium and high doses, respectively) was lower than that in the selenium intake. Highly significant correlations found between the selenium concentrations in muscle and whole blood (R=0.90), red blood cells (R=0.91), blood plasma (R=0.87), head hair (R=0.89) and toenails (R=0.85) show that in humans supplemented in this way the selenium status can be assessed in a relatively easy way by analysis of the selenium retention in these indicator materials.     

Supplementation with selenium and human immune cell functions

Biological Trace Element Research - This study examined the effect of dietary (200 μg/d for 8 wk) supplementation with selenium (as sodium selenite) on the ability of human peripheral blood...     

Selenium Downregulates RAGE and NFκB Expression in Diabetic Rats

Diabetes is one of the leading causes of death in developed and developing countries. Oxidative stress has been proposed to play a crucial role in the pathogenesis of diabetic vascular complications. In recent years, selenium has been shown to mediate a number of insulinlike actions in a dose-dependent fashion both in vitro and in vivo. In this study, the effect of selenium as sodium selenite was investigated in streptozotocin-induced diabetic rats at the dose of 1?μg sodium selenite/kg body weight. Selenium supplementation restored the streptozotocin-induced alterations in the activities of antioxidant enzymes, decreased the serum glucose level, glycated hemoglobin content as well as the levels of lipid peroxidation products, and downregulated the expressions of both NFkB and RAGE. The histopathological studies also reinforce our findings. Hence, selenium has a protective role in streptozotocin-induced diabetes mellitus.     

The bioavailability of various selenium compounds to a marine wading bird

The uptake of dietary selenium (about 3.5 mg/kg AF dry wt) as selenomethionine, selenocystine, selenite, selenate, and fish selenium in the plasma and red blood cells (RBC) of the oystercatcher has been investigated. The birds received the various selenium compounds subsequently, for at least 9 wk. After dietary supplementation of selenocystine, selenite, and selenate, plasma selenium was about 350 μg/L and RBC selenium 2.1 mg/kg dry wt. After supplementation of selenomethionine, the plasma concentration increased to 630 μg/L, and the RBC concentration to 4.1 mg/kg dry wt. When the fodder contained 3.1 mg/kg fish Se, an average plasma and RBC concentration of 415 μg/L and 14.4 mg/kg dry wt, respectively, was measured. The maximal increase of the selenium concentration in the plasma was attained at first sampling, 14 d after a change in dietary selenium (selenomethione or fish Se); the uptake seemed to be a concentration-regulated process. RBC concentrations (γ in mg/kg dry wt) increased with time (X in d) according toY=a?be^?cX . Fifty percent of the total increase was attained within 17d, suggesting that diffusion into the RBC played a role. The selenium concentration in the plasma was positively correlated with the (fish) Se concentration in the fodder; the RBC concentration (60 d after the change in diet) was positively correlated with the plasma concentration. When the diet contained fish Se, the blood selenium concentrations of the captive birds were similar to the concentrations measured in field birds. Fish Se is a yet undetermined selenium compound. The present experiment showed that fish Se differed from selenomethionine, selenocystine, selenite, or selenate in uptake from the food and uptake in the RBC.     

Evaluation of nutritional availability and anti-tumor activity of selenium contained in selenium-enriched Kaiware radish sprouts

We estimated the nutritional availability of selenium (Se) in Se-enriched Kaiware radish sprouts (SeRS) by the tissue Se deposition and glutathione peroxidase (GPX) activity of rats administered the sprouts, and examined the effect of SeRS on the formation of aberrant crypt foci (ACF) in the colon of mice administered 1,2-dimethylhydrazine (DMH) to evaluate anti-tumor activity. Male weanling Wistar rats were divided into seven groups and fed a Se-deficient basal diet or the basal diet supplemented with 0.05, 0.10, or 0.15 microg/g of Se as sodium selenite or SeRS for 28 d. Supplementation with Se dose-dependently increased serum and liver Se concentrations and GPX activities, and the selenite-supplemented groups showed a higher increase than the SeRS-supplemented groups. The nutritional availability of Se in SeRS was estimated to be 33 or 64% by slope ratio analysis. Male 4-week-old A/J mice were divided into seven groups and fed a low Se basal diet or the basal diet supplemented with selenite, SeRS, or selenite + non-Se-enriched radish sprouts (NonSeRS) at a level of 0.1 or 2.0 microg Se/g for 9 weeks. After 1 week of feeding, all mice were given six subcutaneous injections of DMH (20 mg/kg) at 1-week intervals. The average number of ACF formed in the colon of mice fed the basal diet was 4.3. At a supplementation level of 0.1 mug Se/g, only SeRS significantly inhibited ACF formation. At a supplementation level of 2.0 microg Se/g, both selenite and SeRS significantly inhibited ACF formation. The addition of NonSeRS to the selenite-supplemented diets tended to inhibit ACF formation, but this was not statistically significant. These results indicate that SeRS shows lower nutritional availability but higher anti-tumor activity than selenite.     

Comparison of whole blood selenium values and erythrocyte glutathione peroxidase activities of normal individuals on supplementation with selenate,selenite,l-selenomethionine,and high selenium yeast

The selenium levels and the glutathione peroxidase activity GSH-PX of whole blood and of erythrocytes, respectively, were determined in 139 normal Danes and related to sex and smoking habits. No differences were found in relation to sex apart from a higher GSH-PX activity of females when assayed with tertiary butyl hydroperoxide. Smokers showed significantly lower selenium values than non-smokers (p<0.05), but the two groups had identical GSH-PX activities. Individuals from the above-mentioned group were divided into four groups, receiving daily oral doses of 200 μg of selenium in the form of selenite, selenate, L-selenomethionine, and selenium as contained in yeast. Whole blood selenium values and the erythrocyte glutathione peroxidase activities were determined during three months of supplementation followed by a withdrawal period of four months. Both the inorganic selenium compounds and the organic derivatives gave rise to steady state levels of GSH-PX after one month of supplementation. However, the selenium levels in the groups receiving organic selenium showed a steady rise during the whole period, whereas those supplemented with inorganic selenium leveled off after a period of one to three months. The data for smokers and non-smokers revealed identical results when organic selenium was supplemented. However, selenite gave rise to significantly higher selenium levels and GSH-PX activities in smokers than in non-smokers. Less significant (p<0.08) elevations of both parameters were also observed among the smokers in the selenate group. By taking both the selenium level and the GSH-PX activity into consideration, organic selenium (i.e.,l-(+) selenomethionine) was judged to be more bioavailable than selenite and selenate.     

Recent advances in clinical studies of selenium supplementation in radiotherapy

BackgroundRadiotherapy is one of the most important and common therapies for cancer patients. Selenium has been shown to be capable of reducing the side effects of radiotherapy because selenoproteins have anti-oxidative functions against reactive oxygen species that are induced by the radiation. They also function in DNA-repair and cytokine control.PurposeWe explored the benefits and risks of selenium supplementation in radiotherapy in our previous review to establish guidelines. In the current study, we expanded the search to cover recent advances in clinical studies of selenium supplementation in radiotherapy.MethodsWe conducted an initial screening in the PubMed using the MeSH terms and keywords “selenium”, “radiation”, “therapy”, and “radiotherapy” using the same methodology applied in our previous review. We identified 121 articles published between January 2013 and December 2019. We then identified eight articles (six studies) on selenium and radiotherapy by excluding 113 articles.ResultsIn selenium supplementation studies, selenium doses of 300−500 μg/day with duration of 10 days to 6 months were used. Selenium supplementation improved the selenium nutritional conditions of the patients and reduced the side effects of radiotherapy. Selenium supplementation did not reduce the effectiveness of radiotherapy, and no toxicities were reported.ConclusionThe results of our previous and current reviews showed that selenium supplementation offers specific benefits for several cancer types treated with radiotherapy. Here, we suggest a new guideline for selenium supplementation in radiotherapy. We recommend determining the selenium status of the patients before radiotherapy, and in cases of deficiency (<100 μg/L serum selenium level), selenium supplement can be beneficial.     

Distribution of Selenium in Bovine Milk and Selenium Deficiency in Rats Fed Casein-based Diets,Monitored by Lipid Peroxide Level and Glutathione Peroxidase Activity

A major proportion of selenium in bovine milk was found in fluorometric analysis to be associated with the casein fraction, largely in alkali-labile form, and the rest with the whey fraction mostly in free selenite form. This uneven distribution of milk selenium seems to provide an explanation for selenium deficiency in purified caseins. The activity of glutathione peroxidase, a selenoprotein, in the liver of growing male rats fed ad libitum low-selenium diets containing either vitamin-free casein or Torula yeast 0.065 ± 0.012 or 0.015 ± 0.004 μ g Se/g diet, respectively) for 3 weeks decreased to 4 to 6% of that of the control rats fed a commercial stock diet (0.185 ± 0.092 μ g Se/g diet). Selenium status was evaluated by three different parameters for the rats assigned under pair-feeding regimen to those vitamin-free casein-based diets which were supplemented with graded levels of selenium as sodium selenite. The hepatic levels of the thiobarbituric acid-reactive substance, an indication of lipid peroxidation, decreased to control level with selenium supplementation per g diet of 0.1 μ g and over. The hepatic glutathione peroxidase activity reached a plateau above a 0.1 μ g/g diet of selenium supplementation, whereas the erythrocyte enzyme activity increased with increasing levels of supplementary selenium. These results support the notion that semi-purified diets containing vitamin-free casein as a prime protein source would not satisfy the selenium requirement of growing animals unless deliberately supplemented with additional selenium.     

The effects of organic selenium supplementation on the rumen ciliate population in sheep

The effect of selenium supplementation on the rumen protozoan population of sheep was demonstrated. Both the total and generic counts of rumen ciliates in sheep fed a diet with basal Se content (70 microg/kg dry matter) were compared to those of animals given feed supplemented with inorganic (disodium selenite) or organic Se (selenized yeast) (310 microg/kg dry matter). The genera of Entodinium, Isotricha, Dasytricha, Ophryoscolex, Diploplastron and Polyplastron occurred in all sheep except for the control, in which Ophryoscolex was not observed. The population of Ophryoscolex caudatus f. tricoronatus was significantly higher in sheep supplemented with organic Se than in animals given inorganic Se (by 160 %). Supplementation of feed with selenized yeast induced significant growth in the Diploplastron population (by 63 %) while no change occurred in sheep given selenite. The populations of Dasytricha ruminantium and Polyplastron multivesiculatum were higher than control in both Se-supplemented groups. The ciliate population of Entodinium spp. was not influenced by Se supplements. Our results suggest a protective effect of Se feed supplementation on the development of some rumen ciliate species in young ruminants.     

Effects of high doses of selenium,as sodium selenite,in septic shock patients a placebo-controlled,randomized, double-blind,multi-center phase II study – Selenium and sepsis

Selenium has a double action. (i) Seleno-compounds, among them sodium selenite have a direct pro-oxidant action leading to acute toxicity but may be also beneficial as drug. (ii) Selenium is an essential anti-oxidant required for anti-oxidant seleno-enzymes. Septic shock is a common severe syndrome leading to endothelium damage and multiple organ failure, with increased data suggesting the principle role of oxidative stress. Selenoprotein P, main selenium constituent of the plasma, may decrease dramatically and specifically in septic shock patients and may be involved in the endothelium protection. A prospective, multi-center placebo-controlled, randomized, double-blind study in severe septic shock patients with documented infection has been preformed. Patients received, for 10 days, selenium as sodium selenite (4000 μg on the first day, 1000 μg/day on the 9 following days) or matching placebo using continuous intravenous infusion. Mortality rates did not significantly differ between groups at any time point. Adverse events rates were similar in the two groups. However, high-dose selenium administration has been associated with a tendency to decrease the mortality in septic shock animal and patients, especially when using a bolus administration, whereas studies using a continuous administration failed to find any benefit on mortality. The interest of the successive use of pro-oxidant action of seleno-compounds, followed by anti-oxidant action need to be the further studied in cellular and animal models, preceding new dose–effect phase II. The interest of the selenoprotein-P as a marker of septic shock and for endothelium protection needs also to be studied further.     

Influence of selenium (antioxidant) on gliclazide induced hypoglycaemia/anti hyperglycaemia in normal/alloxan-induced diabetic rats

Oxidative stress is involved in diabetes mellitus and its complications. Since diabetes is a stress-related disorder, supplementation with antioxidants may improve the condition. The purpose of this study is to know the effect of oral administration of selenium on blood glucose and its influence on gliclazide induced hypoglycaemia/antihyperglycaemia in normal and alloxan-induced diabetic rats. Albino rats of either sex were divided into three groups of six each. Group-I/II/III were treated with selenium 1/2 TD (0.9 μg/200 g rat)/TD (1.8 μg/200 g rat)/2TD (3.6 μg/200 g rat), respectively. Later group II was treated with gliclazide TD (1.44 mg/200 g rat)/selenium TD + gliclazide TD with a washout period of 1 week between the treatments. Diabetes was induced by alloxan monohydrate 100 mg/kg body weight i.p. A group of six rats showing fasting blood glucose levels ranging from 175–250 mg/dl were selected for the study. Rats were treated with selenium TD, gliclazide TD and selenium TD + gliclazide TD with a washout period of 1 week between the treatments. Selenium 1/2 TD and TD produced hypoglycaemia while 2TD produced hyperglycaemia. The combination of selenium TD with gliclazide TD, significantly enhanced the glucose lowering effect of gliclazide in normal and diabetic rats.     

Influence of Selenium on Mast Cell Mediator Release

Selenium supplementation still enhanced the immune response even in individuals who, according to current standards, would be considered as not being overtly selenium deficient. Mast cells are granulated cells that play a pivotal role in allergic reactions. In this study, we investigated the modulatory effect of sodium selenite on mediator release and degranulation of murine mast cell line (MC/9). Cells were pre-treated with selenium selenite (1, 2, 3 μg/ml) for 24 h and controls left untreated. Then, cells were sensitized overnight with anti-dinitrophenyl (DNP) IgE and challenged with DNP/HSA for degranulation induction. The histamine and prostaglandin D2 (PGD2) were measured by ELISA, and β-hexosaminidase was measured by spectrophotometery method. Selenium-treated cells revealed significant decrease in concentration of PGD2 (P?=?0.019) and β-hexosaminidase (P?=?0.009). In addition, a slight reduction of histamine release by the selenium-treated cells was observed, based on our intracellular and extracellular assessments. The most inhibitory effect of selenium supplementation on mediator release of MC/9 cells was obtained in the presence of 3 μg/ml of sodium selenite. The results of the present study demonstrate beneficial effects of supplemental selenium in attenuating clinical manifestations of allergy and asthma.     

High dose antioxidant supplementation to MS patients. Effects on glutathione peroxidase, clinical safety, and absorption of selenium.

High-dose antioxidant supplementation has recently been recommended for multiple sclerosis (MS) patients. This study tests the clinical safety, the glutathione peroxidase (GSH-px) activity, and the absorption of selenium during such supplementation. Eighteen MS patients were given 6 tablets especially made for this study, equivalent to 6 mg sodium selenite, 2 g vitamin C, and 480 mg vitamin E a day for five wk. GSH-px, which was lower than in non-MS controls before the start of treatment, increased fivefold during 5 wk of treatment. Side effects were scarce. Ten MS patients were subjected to a 24-h selenium absorption study after ingestion of 2 active tablets, equivalent to 2 mg sodium selenite. Selenium, which was low initially, increased 24% during the first 3 h and then stabilized. It is concluded that the tested antioxidant treatment seems to be safe and that MS patients have low GSH-px, which may be increased by the tested antioxidant treatment.     

Effect of selenium on the immunocompetence of patients with head and neck cancer and on adoptive immunotherapy of early and established lesions

Supplementation with 200 microg/day of sodium selenite during therapy for squamous cell carcinoma (SQCC) of the head and neck, e.g., surgery, radiation, or surgery and radiation, resulted in a significantly enhanced cell-mediated immune responsiveness. The enhanced responsiveness was evident during therapy and following conclusion of therapy. In contrast, patients in the placebo arm of the study showed a decline in immune responsiveness during therapy. The results from studies on mice inoculated with SQCC cells expressing the receptor for interleukin-2 (IL-2) and supplemented with Se (2.00 ppm) indicated that Se significantly retards the clinical appearance of tumors; peritumoral injections of 2,000 IU of IL-2 resulted in 50% reduction in the size of established tumors and 72% of early tumors. The combined data suggested that local immunotherapy with IL-2 in hosts supplemented with Se may represent an effective modality of treatment for the prevention of recurrences at the site of conventionally treated primary tumors.     

Selenium and immunocompetence in patients with head and neck cancer

This randomized double-blind placebo-controlled study aimed to determine whether oral intake of 200 μg/d of sodium selenite, a dose within the safe and adequate daily intake (50–200 μg/d) recommended by the U.S. Food and Nutrition Board, will abrogate depressed or enhance normal-level immune functions of patients receiving therapy for squamous cell carcinoma of the head and neck. Subjects were given one selenium/placebo tablet/d for 8 wk, beginning on the day of their first treatment for the disease (e.g., surgery, radiation, or surgery and radiation) and their immune functions were monitored. Supplementation with selenium (Se) during therapy resulted in a significantly enhanced cell-mediated immune responsiveness, as reflected in the ability of the patient’s lymphocytes to respond to stimulation with mitogen, to generate cytotoxic lymphocytes, and to destroy tumor cells. The enhanced responsiveness was evident during therapy and following conclusion of therapy. In contrast, patients in the placebo arm of the study showed a decline in immune responsiveness during therapy, which was followed, in some patients, by an enhancement, but the responses of the group remained significantly lower than baseline values. The data also show that at baseline, patients entered in the study had significantly lower plasma Se levels than healthy individuals, and patients in stage I or II of disease had significantly higher plasma selenium levels than patients in stage III or IV of disease.     

Revised reference values for selenium intake

The German, Austrian and Swiss nutrition societies are the joint editors of the ‘reference values for nutrient intake’. They have revised the reference values for the intake of selenium and published them in February 2015. The saturation of selenoprotein P (SePP) in plasma is used as a criterion for the derivation of reference values for selenium intake in adults. For persons from selenium-deficient regions (China) SePP saturation was achieved with a daily intake of 49 μg of selenium. When using the reference body weights the D-A-CH reference values are based upon, the resulting estimated value for selenium intake is 70 μg/day for men and 60 μg/day for women. The estimated value for selenium intake for children and adolescents is extrapolated using the estimated value for adults in relation to body weight. For infants aged 0 to under 4 months the estimated value of 10 μg/day was derived from the basis of selenium intake via breast milk. For infants aged 4 to under 12 months this estimated value was used and taking into account the differences regarding body weight an estimated value of 15 μg/day was derived. For lactating women compared to non-lactating women a higher reference value of 75 μg/day is indicated due to the release of selenium with breast milk. The additional selenium requirement for pregnant women is negligible, so that no increased reference value is indicated.     

Effect of Zinc and Selenium Supplementation on Serum Testosterone and Plasma Lactate in Cyclist After an Exhaustive Exercise Bout

Zinc and selenium are essential minerals and have roles for more than 300 metabolic reactions in the body. The purpose of this study was to investigate how exhaustive exercise affects testosterone levels and plasma lactate in cyclists who were supplemented with oral zinc and selenium for 4?weeks. For this reason, 32 male road cyclists were selected equally to four groups: PL group, placebo; Zn group, zinc supplement (30?mg/day); Se group, selenium supplement (200?μg/day); and Zn-Se group, zinc-selenium supplement. After treatment, free, total testosterone, and lactate levels of subjects were determined before and after exhaustive exercise. Resting total, free testosterone, and lactate levels did not differ significantly between groups, and were increased by exercise (P?>?0.05). Serum total testosterone levels in Zn group were higher than in Se group after exercise (P??0.05). The results showed that 4-week simultaneous and separately zinc and selenium supplementation had no significant effect on resting testosterone and lactate levels of subjects who consume a zinc and selenium sufficient diet. It might be possible that the effect of zinc supplementation on free testosterone depends on exercise.     

Supplemental selenium and coenzyme Q10 reduce glycation along with cardiovascular mortality in an elderly population with low selenium status – A four-year,prospective, randomised,double-blind placebo-controlled trial

BackgroundA low intake of selenium has been shown to increase the risk of cardiovascular mortality, and supplementation of selenium and coenzyme Q10 influences this. The mechanism behind is unclear although effects on inflammation, oxidative stress and microRNA expression have been reported.Fructosamine, a marker of long-term glycaemic control, is also a marker of increased risk of heart disease and death, even in non-diabetics.ObjectiveTo analyse the impact of selenium and coenzyme Q10 supplementation on the concentration of fructosamine. Also, the relation between pre-intervention serum selenium concentration and the effect on fructosamine of the intervention was studied.MethodsFructosamine plasma concentration was determined in 219 participants after six and 42 months of intervention with selenium yeast (200 μg/day) and coenzyme Q10 (200 mg/ day) (n = 118 of which 20 had diabetes at inclusion), or placebo (n = 101 of which 18 had diabetes at inclusion). Pre-intervention, the serum selenium levels were 67 μg/L (active treatment group: 66.6 μg/L; placebo group: 67.4 μg/L), corresponding to an estimated intake of 35 μg/day. Changes in concentrations of fructosamine following intervention were assessed by the use of T-tests, repeated measures of variance, and ANCOVA analyses.ResultsPost-intervention selenium concentrations were 210 μg/L in the active group and 72 μg/L in the placebo group. A lower concentration of fructosamine could be seen as a result of the intervention in the total population (P = 0.001) in both the males (P = 0.04) and in the females (P = 0.01) in the non-diabetic population (P = 0.002), and in both the younger (<76 years) (P = 0.01) and the older (≥76 years) participants (P = 0.03). No difference could be demonstrated in fructosamine concentration in the diabetic patients, but the total sample was small (n = 38). In subjects with a low pre-intervention level of serum selenium the intervention gave a more pronounced decrease in fructosamine compared with those with a higher baseline selenium level.ConclusionA significantly lower concentration of fructosamine was observed in the elderly community-living participants supplemented with selenium and coenzyme Q10 for 42 months compared to those on the placebo. As oxidative mechanisms are involved in the glycation of proteins, less glycoxidation could be a result of the supplementation of selenium and coenzyme Q10, which could have contributed to lower cardiac mortality and less inflammation, as has earlier been reported.This study was registered at Clinicaltrials.gov, and has the identifier NCT01443780.     

The antimutagenic effect of selenium on 7,12-dimethylbenz(a)anthracene and metabolites in the amesSalmonella/microsome system

The antimutagenic effect of selenium as sodium selenite, sodium selenate, selenium dioxide, and seleno-methionine was studied in the AmesSalmonella/microsome mutagenicity test using 7,12-dimethylbenz(a)anthracene (DMBA) and some of its metabolites. Selenium (20 ppm) as sodium selenite reduced the number of histidine revertants on plates containing up to 100 μg DMBA/plate. Increasing concentrations of selenium as sodium selenite, sodium selenate, and selenium dioxide up to 40 ppm Se progressively decreased the number of revertants caused by 50 μg DMBA. DMBA and its metabolites 7-hydroxymethyl-12-methylbenz(a)anthracene, 12-hydroxymethyl-7-methylbenz(a)anthracene, and 3-hydroxy-7,12-dimethylbenz(a)anthracene were mutagenic forSalmonella typhimurium TA100 in the presence of an S-9 mixture. Selenium supplementation as Na₂SeO₃ reduced the number of revertants induced by these metabolites to background levels. The antimutagenic effect of inorganic selenium compounds cannot be explained by toxicity of selenium as determined by viability tests withSalmonella typhimurium TA100. Selenium supplementation in all forms examined, except sodium selenate, decreased the rate of spontaneous reversion. Selenium as sodium selenate was slightly mutagenic at concentrations of 4 ppm or less. Higher concentration of Na₂SeO₄ inhibited the mutagenicity of DMBA. The present studies support the anticarcinogenic potential of selenium and indicate that form and concentration are important factors in this trace element's efficacy.