Ascorbic acid-selenite interactions in humans studied with an oral dose of 74SeO3(2-)

The interaction between dietary ascorbic acid at extremes of ascorbic acid intake and selenium in young adult male humans was investigated with a stable-isotope approach using 74Se-selenite. Measurements were made of 74Se in plasma, urine, and feces with neutron-activation analysis after oral administration of 74SeO3(2-). Urine excretion and total body retention of isotope and the selenite-exchangeable metabolic pool (Se-EMP) were calculated. Limiting dietary ascorbic acid to about 20 mg/d appeared to reduce the time-related retention of absorbed selenite and the size of Se-EMP. Compared with a diet providing 1 g ascorbic acid/d the low ascorbic acid intake was associated with a lower fractional absorption of the isotope, a reduced retention of the label, and a smaller Se-EMP. These data and those previously obtained in subjects with more usual ascorbic acid intakes point to a possible important role for ascorbic acid in the maintenance of Se homeostasis.     

Experimental selenium restriction in healthy adult humans: changes in selenium metabolism studied with stable-isotope methodology

Mechanisms responsible for selenium homeostasis were investigated in healthy adult men receiving diets adequate or low in Se (eight subjects per group). The appearance of a stable isotope of Se, 74Se, in plasma, urine, and feces was measured after oral administration of 74Se-selenite. One group received a restricted level of Se (18 +/- 1 micrograms/d) for 30 d, which resulted in a decrease in urinary, fecal, and plasma Se content compared with the group that consumed 119 +/- 1 micrograms/d. Low Se intake also resulted in decreased urinary 74Se excretion (27.2 +/- 1.4% vs 32.5 +/- 2.3% of the absorbed dose for the adequate intake), increased body retention of 74Se (74.8 +/- 3.1% vs 67.6 +/- 3.8% of the absorbed dose for the adequate group), and a contracted selenite-exchangeable metabolic pool (Se-EMP) (9782 micrograms for adequate Se and 6314 micrograms for the low-Se group; p less than or equal to 0.05). Measurement of Se-EMP may provide an additional and sensitive approach for assessing Se nutriture in human subjects.     

Dynamics of selenite metabolism in young men: studies with the stable isotope tracer method

Dynamics of selenite metabolism in young adult North American men were studied using an amino acid diet and the stable isotope tracer methodology during a short-term selenium replete-restriction phase. During the initial 10 days subjects consumed the diet providing a total daily selenium intake of 107.7 +/- 0.1 micrograms mostly as selenite. This was followed by selenium restriction at 11.4 +/- 0.1 micrograms/day (as impurities from diet components) for the next 34 days. Kinetic studies with the stable isotope tracer 74SeO32- were carried out on days 4 and 39 of the study. Kinetics of excretion of the tracer in feces and urine were followed from which body retention curves were constructed. The retention curves were resolved into two exponential decay components with half-lives of 2.4 +/- 0.3 and 162 +/- 9 days (mean +/- 1 SEM), respectively. Retention data and urine isotope enrichment curves were combined to determine dynamics of changes in the apparent body pool size for selenite (10.4 mg at t----infinity) as well as rates of turnover for this parameter.     

Dose-response relations in urinary excretion of trimethylselenonium in the rat

75Se-labeled selenite was administered to fasting rats by orogastric intubation (1.5-3000 micrograms/kg body wt). Urine was collected and characterized for total radioactivity as well as for radiolabeled trimethylselenonium (TMSe). At lower doses of selenite (up to 500 micrograms/kg body wt), 30% of the administered dose was excreted. At higher doses of selenite, fractional urine excretion decreased as a function of the dose. The observed decrease in fractional urine excretion was not caused by changes in the absorption of the administered radiolabel. There was a direct relationship between the amount of the administered dose of selenite (up to 1500 micrograms/kg body wt) and the proportion of urinary [75Se] excreted as TMSe. Pretreatment with seleno compounds (10 or 100 micrograms Se/kg body wt as selenite, or selenomethionine) for 35 d before a challenge dose of [75Se]selenite did not influence the excretion of total [75Se] or of [75Se]TMSe in urine. Ingestion of a choline-deficient diet, which should deplete the availability of methyl groups, did not have any effect on excretion of total [75Se] or of [75Se]TMSe in urine after a challenge dose of [75Se]selenite (500 micrograms/kg body wt). The data presented here permit the following conclusions: 1) Production of TMSe is dose dependent, 2) production of TMSe from a single acute dose does not depend on the history of selenium intake and 3) rats fed a methyl-deficient diet are able to eliminate Se via formation of TMSe.     

Absorption and retention of selenium from intrinsically labeled egg and selenite as determined by stable isotope studies in humans

A study was carried out with four healthy young adult men, consuming a self-selected diet, to investigate quantitative aspects of the gastrointestinal absorption, urinary excretion, and body retention of egg selenium in comparison with selenite. The approach involved simultaneous consumption of egg biologically labeled (intrinsic) with the stable isotope 74Se and a dose of selenite labeled with 76Se (extrinsic label). Four labeled test diets, given on days 6, 16, 26, and 36 of the study were employed, each differing in their protein source: test diet I, 74Se-labeled egg white; diet II, 74Se-labeled egg yolk (high labeling dose) plus balanced L-amino acid mixture; diet III, 74Se-labeled egg yolk (low labeling dose) plus balanced amino acid mixture; and diet IV, balanced amino acid mixture extrinsically labeled with both 74SeO32- and 76SeO32-. The latter diet was included to assess the magnitude of any cross-isotope methodological bias. Fractional absorption (means +/- SEM) for Diet IV was 0.771 +/- 0.010 for the 74SeO32- and 0.656 +/- 0.021 for the 76SeO32- (ratio: 0.851 +/- 0.020); reflecting a small overall cross-isotope bias. Accepting the measurements made with 74SeO32- as the more accurate, experimentally determined values of absorption for the extrinsic tag were adjusted accordingly. The corrected absorption for these diets (% of dose) was (mean +/- SEM; first value for intrinsic label, second value for extrinsic label): diet I, 54.1 +/- 0.7 and 55.4 +/- 2.2; diet II, 76.7 +/- 0.8 and 83.0 +/- 1.8; diet III, 79.0 +/- 1.5 and 85.2 +/- 4.0.(ABSTRACT TRUNCATED AT 250 WORDS)     

Labeling hen's egg with 74Se for use in human metabolic experiments

Experiments were conducted with laying hens to explore quantitative aspects of incorporation of 75Se ( radioselenium ) at various dosing levels and for different chemical forms of an orally administered tracer. Quantitative distribution of the incorporated isotope in egg white and egg yolk was strongly influenced by both the chemical form of the label and the dosing level. The ratio of egg yolk:egg white selenium decreased with increased level of administered dose of selenite. In addition, the rate of incorporation and the amount of selenium in whole egg were higher when [75Se]selenomethionine was given as compared to [75Se]selenite. Characterization of the chemical form of selenium in egg white and egg yolk labeled biologically by giving hens radioactive selenite or selenomethionine was performed by classification as: selenite, selenoprotein and fat-bound selenium. Studies were then undertaken to achieve intrinsic labeling of egg white and egg yolk with stable isotope 74Se for purposes of exploring selenium bioavailability in humans. Enrichments of 74Se in egg white and egg yolk of hens given high dose selenite (54.4 micrograms 74Se ) were 20- and 28-fold, whereas in egg white and egg yolk of hens given low dose (10.9 micrograms 74Se ) they were 4- and 10-fold the level of natural abundance, respectively. The stable isotope-labeling studies indicated that a 7-day sequential dosing protocol with 20-100 micrograms Se per dose permitted sufficient enrichment of egg white (only high dose) and of yolk with the stable isotope 74Se for use in human metabolic studies.     

Comparison of the magnitude of the selenite-exchangeable metabolic pool and whole body endogenous selenium in adult rats

The quantitative relationship between the size of the selenite-exchangeable metabolic pool (WSe-EMP) and whole body endogenous selenium (Seend) was investigated in adult male rats. Two experiments based on multiple labeling with stable isotopes were performed. One focused on short-term (7 d, Expt. 1) and the other on long-term (60 d, Expt. 2) relationships. Rats were fed a Torula yeast diet and water supplemented with [76Se]selenite at 0.1 micrograms Se/mL; the in vivo [74Se]selenite tracer was administered orally. Groups of three or four animals were killed at timed intervals and whole carcass or selected organs were analyzed for the stable isotopes 74Se, 77Se and 82Se with hydride generation/inductively coupled plasma mass spectrometry. The value of WSe-EMP was determined from plasma or urine isotope ratios. In Experiment 1, with plasma as the sampling compartment, WSe-EMP at 24 h was 36.5 +/- 1.2% of the baseline value of whole body endogenous selenium (Seend) and 36.3 +/- 1.8% at 7 d. When urine was the sampling compartment, the corresponding values were 3.9 +/- 0.3% and 43.1 +/- 2.8%, respectively. In Experiment 2, WSe-EMP (plasma) was 38.9 +/- 1.3% of Seend at 7 d, increasing to 45.5 +/- 1.6% at 60 d. The corresponding values for urine as the sampling compartment were 45.5 +/- 2.0% (7 d) and 61.5 +/- 1.7% (60 d), respectively.     

Effect of chronic selenite supplementation on selenium excretion and organ accumulation in rats

We examined the effect of chronic selenite supplementation on whole body and selected organ selenium (Se) accumulation, urine excretion of total Se and trimethylselenonium ion, and Se balance in adult male rats. Animals were housed in metabolic cages and given either deionized water or water containing 4 micrograms of Se/mL as selenite for 30 d. Absorption of selenite was nearly complete, with only approximately 10% of ingested Se appearing in feces. There was a rapid rise in urinary Se that reached a plateau within a few days and accounted for 54 +/- 2% of the intake. Excretion of trimethylselenonium ion (TMSe) in urine increased rapidly, representing 35-40% of urinary Se in the supplemented animals compared with only 2% for the control group. In one experiment, rats were killed at 30 d and total carcass Se was measured using isotope dilution analysis. Supplemented rats had only a modest increase in whole body Se (94 +/- 4 micrograms Se vs. 66 +/- 3 in controls). Calculation of Se balance in the supplemented rats showed that approximately 35% of ingested Se could not be accounted for by urine plus fecal losses combined with the portion retained in the carcass. The results from this study demonstrate that under the condition of supplementation at 4 micrograms of Se/mL of drinking water, pathways other than urinary and fecal excretion may account for a substantial portion of Se loss.     

Metabolism of selenite in men with widely varying selenium status.

OBJECTIVE: This study was undertaken to investigate the metabolism of selenite in men with life-long intakes of deficient, adequate and excess selenium. METHODS: Stable isotopes of selenium were infused for five hours into Chinese men living in deficient, adequate or excessive selenium areas, and 24-hour urine and blood samples were collected daily for the next seven days. Stable isotopic selenium excretion was determined in urine and in whole plasma and plasma fractions. RESULTS: Even though there was a positive correlation of selenium intake with the urinary excretion of this element, this relationship was not linear over the entire range (deficient, adequate, excessive) of selenium intake. When the urine excretion was normalized internally within each group, a sharp increase in the slope of this relationship was found when long-term intake increased to adequate amounts, but the slope reached a plateau when the daily intake exceeded the adequate group. The plasma selenoprotein P fraction was labeled initially, but the incorporation in the glutathione peroxidase fraction subsequently increased by a small amount. A two-month dietary restriction of selenium of the subjects from the excess area did not result in a reduction of urinary excretion of infused selenite. CONCLUSION: A complex relationship exists between long-term intake of selenium and selenium status, and subjects living in the excess area are more saturated with selenium than anticipated. More than two months of depletion are required to affect urinary excretion of selenium.     

Selenium utilization in humans--a long-term, self-labeling experiment with stable isotopes

A stable (nonradioactive) isotope of selenium in a chemical form common in foods (selenomethionine) or inorganic selenite was taken orally (200 micrograms/d) for 3 wk to label deep body pools. By deep body pools we mean selenium compartments that are large and/or have a slow turnover (exchange) rate. Blood plasma was removed, stored for 11 mo, and later reinfused as a labeled tracer dose with the selenium label in all of the biologically significant chemical forms. Accessible tissues such as red blood cells were highly labeled (20-25%) in the subjects receiving selenomethionine. Selenium from deep body pools is excreted primarily via the urine (80%). Reexcretion of previously absorbed selenium back into the gastrointestinal tract can be measured, avoiding a major source of error in conventional balance studies used to estimate nutrient absorption.     

The retention and distribution by healthy young men of stable isotopes of selenium consumed as selenite, selenate or hydroponically-grown broccoli are dependent on the isotopic form

Twenty-seven healthy young men were randomly assigned to diets that supplied low (32.6 microg/d) or high (226.5 microg/d) levels of selenium for a 105-d study. After consuming the diets for 85 d, subjects were fed a test meal that contained 74Se in the form of selenite or selenate and 82Se incorporated into hydroponically-raised broccoli. Urine, fecal and blood samples were collected daily. Isotope absorption was not different (P > 0.05) for selenate and Se in broccoli; Se absorption from selenite was highly variable and was not included in statistical analyses. Significantly more isotope was absorbed by subjects fed the high Se diet (P = 0. 015). Urinary isotope excretion was greater when selenate was fed than when broccoli was fed (P = 0.0001), and consequently more Se from broccoli (as compared to selenate) was retained (59.2 +/- 2.4 and 36.4 +/- 4.6% for Se in broccoli and selenate, respectively; P = 0.0001). Despite the higher retention, less isotope from broccoli than from selenate was present in the plasma. Plasma proteins separated by gel permeation chromatography showed that most of the isotopes were distributed between two medium molecular weight peaks. Less isotope was found in plasma proteins of subjects fed the high Se diet, but the form of Se had no effect on isotope distribution. These results show that dietary Se intake alters the retention of stable isotopes of Se and that humans retain and distribute Se from broccoli in a different manner than Se from inorganic salts.     

Organ and subcellular distribution of mercury in rats as dependent on the time of exposure to sodium selenite.

Sodium selenite was administered to rats employing different sequence patterns: before, after, and simultaneously with mercuric chloride. All animal groups were given ²⁰³HgCl₂ intravenously at a dose of 0.5 mg Hg/kg, every other day for 2 weeks. Na₂SeO₃ was administered intragastrically, either as a single dose of 7.0 mg Se/kg or by repeated doses of 0.1 mg Se/kg each. Administration of sodium selenite after saturation of the organism with mercury did not change essentially the mercury level in the kidneys while bringing about a decrease of the level of this metal in the liver and a considerable accumulation of mercury in the blood. In the case of other forms of exposure, selenium decreased the level of mercury in the kidneys, the highest changes of the binding of this metal being observed for the soluble fraction. In the nuclear fraction of this organ the level of mercury did not change irrespective of the sequence of administration and of the selenium dose. In the liver, an increased retention of mercury was found, especially in the nuclear and mitochondrial fractions. The highest interaction effect was attained only in the case of simultaneous administration of equimolar amounts of both elements.     

A novel dual radio- and stable-isotope method for measuring calcium absorption in humans: comparison with the whole-body radioisotope retention method

BACKGROUND: Dietary calcium absorption can be determined only with the use of isotope techniques. Currently used isotope techniques require exclusive equipment or are not true tracer approaches. OBJECTIVE: The objective was to compare a dual-isotope method combining radioisotopes and stable isotopes with a whole-body radioisotope retention method for measuring calcium absorption. DESIGN: Seven healthy adults aged 21-27 y consumed a test meal containing 63 +/- 14 (macro x +/- SD) mg Ca together with a water solution of (47)Ca (0.11 MBq). One hour after ingestion, 18 mg (44)Ca was administered intravenously. All feces and urine were collected for 5 and 6 d, respectively. Calcium absorption was estimated from whole-body retention of the radioisotope 12 times over 3 wk after ingestion and from the excretion of (47)Ca and (44)Ca in a 24-h urine sample collected on day 2. (44)Ca in urine was determined by inductively coupled plasma mass spectrometry. RESULTS: Mean (+/- SD) calcium absorption was 75 +/- 9% with the dual-isotope method and was 74 +/- 8% with the whole-body radioisotope retention method. There was a high degree of agreement between the methods. CONCLUSION: The dual-isotope method is a valid approach for measuring calcium absorption from a single meal.     

The analysis of stable isotopes in urine to determine the fractional absorption of zinc.

We measured isotopic enrichment in urine after oral and intravenous administration of stable isotopes of zinc to determine fractional absorption (FA). 68Zn and 70Zn were administered orally and intravenously to four normal adults. Subsequently, urine and fecal samples were collected for 7 and 14 d, respectively, ashed, and passed through ion-exchange columns to separate zinc from other elements. Samples were analyzed by fast-atom-bombardment mass spectrometry. From 32 h onwards the enrichment of 68Zn and 70Zn in urine declined proportionately so that FA could be determined as follows: FA = enrichment (oral/iv) x dose (iv/oral). FA determinations from urine and feces (cumulative excretion) were, respectively, for subject ZK1, urine 0.79 +/- 0.03 and feces 0.70 +/- 0.01; ZK2, 0.79 +/- 0.05 and 0.69 +/- 0.02; ZK3, 0.26 +/- 0.01 and 0.25 +/- 0.01; and ZK4, 0.41 +/- 0.02 and 0.37 +/- 0.02. ZK1 and ZK2 received the oral isotope while fasting whereas ZK3 and ZK4 received the oral isotope with meals. FA of zinc can be determined by measurement of isotope enrichment in urine.     

Absorption and retention of 75SeO3 in relation to soybean protein intake in the rat

The absorption and retention of ⁷⁵Se, given as an oral dose of ⁷⁵SeO₃, was studied in young male rats receiving different levels and sources of soy protein, with and without selenite and methionine supplementation. Rats fed a protein-free diet had a higher cumulative urine ⁷⁵Se excretion and a sligtly lower ⁷⁵Se absorption and ⁷⁵Se retention than rats fed diets containing 10% protein supplied as soya flour. Results indicated that supplementation with selenite decreased the fractional absorption and retention of selenium, but the overall effect was a marked increase in the total amount of selenium ingested, absorbed and retained. Methionine supplementation of a diet based on soya increased growth and PER: it also decreased slightly cumulative feces ⁷⁵Se excretion and increased ⁷⁵Se absorption, but only in rats fed diets supplemented with selenium. The present findings are consistent with the view that selenium homeostasis in the rat is maintained largely through changes in the urinary excretion of selenium and they show that an inadequate protein diet reduces the efficiency of retention of absorbed selenite.     

Measurement of magnesium absorption and retention in type 2 diabetic patients with the use of stable isotopes

BACKGROUND: Magnesium deficiency has been associated with type 2 diabetes and may reduce insulin sensitivity and impair glucose tolerance. The etiology of magnesium depletion in diabetes is unclear. Animal studies suggest that diabetes may impair magnesium absorption; however, there are no published data on magnesium absorption in humans with diabetes. OBJECTIVE: Magnesium absorption from a test meal and the excretion and retention of magnesium were compared between patients with type 2 diabetes and healthy control subjects. DESIGN: A meal labeled with 10 mg (26)Mg isotopic label was administered, and stool and urine samples were collected for 10 and 6 d, respectively. Apparent absorption was calculated as the difference between the oral dose of (26)Mg isotopic label and the total amount of the isotopic label excreted in the feces. Magnesium retention was calculated from the apparent absorption and urinary excretion of (26)Mg isotopic label in the 6 d after administration. RESULTS: Mean (+/- SD) values for fractional magnesium absorption in the diabetic patients and the control subjects were 59.3 +/- 7.0% and 57.6 +/- 8.5%, respectively (NS). Mean (+/- SD) urinary magnesium excretion values in the diabetic patients and the control subjects were 11.2 +/- 2.6% and 11.7 +/- 3.8%, respectively (NS); retention values were 54.2 +/- 7.1% and 51.4 +/- 6.1%, respectively (NS). CONCLUSION: Dietary magnesium absorption and retention are not impaired in patients with reasonably well-controlled type 2 diabetes.     

Excretion and Body Distribution of Promethium 147

Rats were injected intramuscularly with 1 microcurie of promethium chloride Pm 147. Distribution of the isotope in body tissue and relative rate of excretion in urine and feces were different from previously reported studies in which the isotope was administered intravenously. The major route of isotope excretion both in rats given a diet and those fasted was via the intestinal tract, with the largest amount being excreted the second day after injection. Urinary excretion of the isotope was greatest during the first 24 hours after injection and then declined markedly in subsequent days in both fed and fasted rats. Within 24 hours, the liver showed the highest activity (20% of the administered dose) as compared to blood, kidney, and bone. Activity of the isotope in injected muscle for the first three days was greater than in noninjected muscle.     

Absorption, distribution and excretion of selenium from beef and rice in healthy North American men

Previous metabolic studies of selenium used pure selenium compounds with pharmacologic activities unrelated to selenium nutrition. Healthy men were fed foods naturally high or low in selenium while confined to a metabolic research unit. Selenium intake was 47 microg/d (595 nmol/d) for 21 d while energy intakes and body weights were stabilized and selenium excretion and intake came into metabolic balance. On d 22, selenium intake was changed to either 14 microg/d (177 nmol/d, low selenium) or 297 microg/d (3.8 micromol, high selenium) for the remaining 99 d. The absorption, distribution and excretion of selenium in food were similar to selenomethionine, and distinctly different from sodium selenite. Daily urinary selenium excretion and selenium concentrations in plasma and RBC showed the largest responses to selenium intake relative to interindividual variation. Urinary selenium and plasma selenium responded most rapidly to changes in selenium intake, whereas RBC reflected longer-term selenium intake. Given the difficulty of 24-h urine collections outside a metabolic research unit, RBC and plasma selenium seem to be the most useful indicators of selenium intake. During the intervention period, the high selenium group retained 15 mg (190 micromol) of selenium, with approximately 5 mg (63 micromol) going into skeletal muscle. The low selenium group lost only 0.9 mg (11 micromol) of whole-body selenium but lost 3.3 mg (42 micromol) from muscle, indicating that selenium was redistributed from muscle to tissues that have a higher metabolic priority for selenium such as testes. Fecal excretion decreased by half, representing an important but previously underappreciated adaptation to selenium restriction.     

Selenium utilization during human lactation by use of stable-isotope tracers

We examined utilization of selenomethionine (SeMet) and selenite in six lactating (L) and six nonlactating (NL) women, 2-3 mo postpartum, and seven never-pregnant (NP) women by use of stable-isotope tracers. All groups had similar selenium status at the start of the study. Significantly more selenium from SeMet than from selenite was absorbed and appeared in plasma in all groups. Milk contained more selenium from apparently absorbed SeMet than from selenite. More selenium from apparently absorbed selenite than from SeMet appeared in urine of NP and NL subjects whereas L subjects had approximately the same amount of selenium from apparently absorbed selenite and SeMet in their urine. All groups retained significantly more selenium from SeMet than from selenite; L women retained more selenium from selenite than did the other two groups. Absorption and retention of selenium from SeMet in L women did not appear to be significantly different from that in other women, suggesting that selenium requirements during lactation are increased mainly because of milk losses.     

Organ and subcellular distribution of selenium in rats exposed to cadmium,mercury, and selenium

Three groups of rats were given sodium selenite (Se), sodium selenite and cadmium chloride (Se + Cd), or sodium selenite, cadmium chloride, and mercuric chloride (Se + Cd + Hg), respectively. All animals received subcutaneous doses of ¹¹⁵CdCl₂ (0.3 mg Cd/kg) every other day for a fortnight. Mercuric chloride was administered intravenously at doses of 0.5 mg Hg/kg every other day and Na₂⁷⁵SeO₃ intragastrically at doses of 0.1 mg Se/kg every other day for 2 weeks. The whole-body retention of selenium was slightly elevated by cadmium and increased threefold by cadmium with mercury (mainly blood, liver, and kidneys). Cadmium did not affect subcellular levels of selenium in the kidneys and slightly increased the selenium content in the soluble fraction of the liver. On the other hand, combined administration of mercury and cadmium induced a significant elevation of the selenium content in all subcellular fraction of the kidneys and in the nuclear and mitochondrial fractions of the liver. In all animal groups selenium was bound in the soluble fractions of both the liver and kidneys by high-molecular-weight proteins.