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.

Quantitative and qualitative aspects of selenium utilization in pregnant and nonpregnant women: an application of stable isotope methodology.

Selenium utilization of women in early and late pregnancy was compared to that of nonpregnant controls. A defined diet providing about 150 micrograms Se/day was fed for 20 days, and selenium balance was measured during the last 12 days. Net selenium retentions of the women in early and late pregnancy were 10 and 23 micrograms/day, respectively, but probably are inflated estimates of the increased selenium requirement during pregnancy. Apparent absorption of selenium was 80% for all three groups. Pregnant women tended to conserve selenium by decreasing urinary selenium excretion. Those observations were corroborated by monitoring the urinary and fecal excretion of 40 micrograms of a stable isotope of selenium (76Se) from intrinsically labeled egg. The isotope data also indicated that recent selenium intake was incorporated into a long-term selenium pool. Mean glutathione peroxidase activity was lower in plasma and higher in platelets in the pregnant women as compared to controls, but the physiological significance of those observations is unknown.

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.

Biochemical indices of selected trace minerals in men: effect of stress.

Plasma zinc, iron, copper, and selenium and selected blood proteins were measured in 66 men before (BHW) and after (AHW) a 5-d period of sustained physical and psychological stress called Hell Week. Recovery blood samples were obtained from 26 men 7 d after Hell Week. Dietary intakes were determined BHW and during Hell Week; zinc, iron, copper, and selenium intakes during Hell Week averaged 23.6 +/- 3.4 mg/d, 35.4 +/- 3.9 mg/d, 3.0 +/- 0.5 mg/d, and 92.5 +/- 26.7 micrograms/d, respectively. C-reactive protein was detected in only five subjects BHW and in all subjects AHW. Zinc, iron, selenium, and albumin decreased by 33%, 44%, 12%, and 9%, respectively, whereas ferritin, ceruloplasmin, and creatine kinase concentrations increased AHW by 59%, 8%, and 266%, respectively. Haptoglobin concentrations increased 57% in 30 subjects but decreased 32% in 23 subjects AHW. The biochemical changes were transitory because protein (except ferritin) and mineral concentrations were similar to BHW values 7 d after Hell Week. Hell Week induced changes characteristic of an acute-phase response in physically active men.

Sodium and potassium intake and balance in adults consuming self-selected diets.

Twenty eight adults, 12 men and 16 women, participated in a 1-yr study designed to assess daily nutrient intake accurately. All subjects lived at home, consumed self-chosen diets, and maintained a detailed daily dietary record throughout the year. During four 7-day balance studies, one in each season of the year, meals, beverages, urine, and feces were analyzed for sodium and potassium content by atomic absorption spectrometry. Total intakes averaged 3.4 g/day for sodium and 2.8 g/day for potassium. The Na:K ratio for all diets analyzed averaged 1.3. Nutrient densities of sodium and potassium were 1.8 and 1.5 g/1000 kcal, respectively. Apparent absorptions of sodium and potassium were 98 and 85%, respectively, and did not change significantly over the wide range of intakes. Average urinary excretions of sodium and potassium were 86 and 77% of total intake, respectively. Mean metabolic balances were positive for sodium, +0.47 g/day, and potassium, +0.28 g/day. The data of this study provide useful information concerning the dietary intakes, excretions, and balances of sodium and potassium for adults based on analytic determination.

Zinc, copper, and manganese intake and balance for adults consuming self-selected diets.

Twenty-eight adult men and women participated in a year-long study designed to determine accurately the nutrient intake of adults who lived at home and consumed self-selected diets. During four metabolic balance periods, 7 days each, corresponding to the seasons (spring, summer, fall, winter), duplicates of the diet, and all urine and feces were collected. Daily mean intakes for zinc and copper were 9.9 and 1.2 mg, respectively. These levels were less than the recommended daily intakes of 15 mg for zinc and 2 to 3 mg for copper. In contrast, the mean dietary intake of manganese was 3.0 mg/day which is within the suggested safe and adequate range of 2.5 to 5.0 mg. Metabolic balances were negative for all three elements possibly due to a reduction in food intake during the collection periods compared to the noncollection intervals. The collection of the duplicate diets apparently influenced the food intake during the collection weeks.

Human [74Se]selenomethionine metabolism: a kinetic model.

A study was undertaken to investigate the pharmacokinetics of an organically bound form of selenium. Six adults received a single oral 200-micrograms dose of 74Se as L-selenomethionine. A kinetic model was developed to simultaneously account for the appearance and disappearance of the tracer in plasma, urine, and feces. The model included absorption distributed along the gastrointestinal tract, uptake by the liver-pancreas subsystem, enterohepatic recirculation, distribution to two large tissue pools, and transport through four components of the plasma pool. Average turnover time of the plasma components varied from 0.01 to 1.1 d. The turnover time in the liver-pancreas subsystem ranged from 1.6 to 3.1 d. Turnover time ranged from 61 to 86 d in the peripheral tissues with the slowest turnover. The whole-body residence time was approximately five-fold greater than the turnover time of the tissue pool with the slowest turnover, reflecting substantial reutilization of labeled material.

Distribution of a stable isotope of chromium (53Cr) in serum, urine, and breast milk in lactating women.

To determine the fate and distribution of chromium during lactation, six lactating women (25-38 y old) were given three doses of the tracer 53Cr (7.55 micromol/d, or 400 microg/d) on days 1, 2, and 3 of the study. Diet records, blood samples taken while subjects were fasting, and 24-h composite milk and urine samples were collected from day 0 to day 6. Fasting blood samples, morning milk samples, and 24-h urine samples were also collected on days 8, 10, 15, 30, 60, and 90. 53Cr and natural and total chromium concentrations in biological fluids were measured with gas chromatography-mass spectrometry and total urinary chromium was measured with atomic-absorption spectrometry. 53Cr was detectable in serum 2 h after dosing and continued to be detected from day 30 to day 60. Changes in total serum chromium concentration in response to the oral dose suggested that chromium concentrations in blood were not tightly regulated. 53Cr was not detected in breast milk and no significant changes in natural chromium concentration in milk were observed in response to the oral doses, suggesting that breast-milk chromium concentrations are independent of intake. The estimated chromium intake of exclusively breast-fed infants was 2.5 nmol/d (0.13 microg/d), below the lower end of the range of estimated safe and adequate daily dietary intakes (10-40 microg/d) for infants 0-6 mo of age. The baseline chromium concentration in urine and the minimum 53Cr absorption in lactating women were comparable with values for nonpregnant, nonlactating subjects. Chromium losses in breast milk do not appear to be compensated for via increased absorption or decreased excretion.

Breast milk chromium and its association with chromium intake, chromium excretion, and serum chromium.

Chromium metabolism of lactating women was evaluated by measuring diet, breast milk, urine, and serum chromium in 17 subjects 60 d postpartum. Breast milk chromium concentration was similar for the 3 d of collection with a mean +/- SE concentration of 3.54 +/- 0.40 nmol/L (0.18 ng/mL). Dietary intake and urinary chromium values were also similar for each of the 3 collection days. Total chromium intake of lactating mothers (0.79 +/- 0.08 mumol/d) was greater than that of reference female subjects (0.48 +/- 0.02). There was a significant correlation (r = 0.84) between serum chromium and urinary chromium excretion. If a breast milk volume of 715 mL is assumed, chromium intake of exclusively breast-fed infants is < 2% of the estimated safe and adequate daily intake of 10 micrograms. In summary, breast milk chromium content is independent of dietary chromium intake and serum or urinary chromium values. Chromium intake also did not correlate with serum or urine chromium but there was a significant relationship between serum and urinary chromium concentrations.

Urinary chromium excretion of human subjects: effects of chromium supplementation and glucose loading.

The utilization of inorganic chromium by free-living human subjects was studied in 76 volunteers (male, 48; female, 28) who were supplemented with 200 micrograms of inorganic chromium as chromic chloride or a placebo tablet for 3 months in a double-blind, cross-over experiment. For all subjects, initial mean +/- SEM urinary chromium (Cr) level was 0.20 +/- 0.01 (range, 0.05 to 0.58) ng/ml and did not differ by sex. Initial chromium/creatinine ratio (Cr/Ct) was 0.15 +/- 0.01 (range 0.03 to 0.36) ng Cr/mg creatinine for females and was significantly lower, 0.10 +/- 0.01 (range 0.03 to 0.36) for males. Mean urinary Cr level increased to 1.0 +/- 0.12 after 2 and to 1.13 +/- 0.08 ng/ml after 3 months' supplementation. The Cr/Ct ratio increased to 0.69 +/- 0.10 for females and to 0.50 +/- 0.04 for males after 2 months' supplementation; values were similar after 3 months. An increase in urinary Cr excretion in response to a glucose load was demonstrated for nonsupplemented normal free-living subjects but not for subjects supplemented daily with trivalent chromium. Urinary Cr excretion after a glucose challenge was not predictable and did not depend on Cr status.

Selenium in diet, blood, and toenails in relation to human health in a seleniferous area.

To determine whether high dietary selenium intake was associated with adverse effects, selenium in diet, blood, and toenails was studied in relation to human health in adults residing in western South Dakota and eastern Wyoming. Over a 2-y period 142 subjects were recruited from households selected at random and from ranches where unusually high selenium intakes were suspected. Subjects completed health questionnaires, underwent physical examinations, provided blood samples for clinical assessment, and provided blood, urine, toenails, and duplicate-plate food collections for selenium analysis. About half of the 142 free-living subjects had selenium intakes greater than 2.54 mumol/d (200 micrograms/d) (range 0.86-9.20 mumol/d, or 68-724 micrograms/d). Physical findings characteristic of selenium toxicity were not present nor were clinically significant changes in laboratory tests or frequency of symptoms related to selenium in the blood, toenails, or diet. We found no evidence of toxicity from selenium in subjects whose intake was as high as 9.20 mumol/d (724 micrograms/d).

Selenium intake, age, gender, and smoking in relation to indices of selenium status of adults residing in a seleniferous area.

Duplicate meals, serum, whole blood, and toenails were collected every 3 mo for 1 y from a group of 44 free-living adults residing in high-selenium areas of South Dakota and Wyoming to assess the relation of selenium intake to indices of selenium status. The average selenium values for the group were as follows: dietary intake, 174 +/- 91 micrograms/d (mean +/- SD), 2.33 +/- 1.08 micrograms/kg body wt; serum, 2.10 +/- 0.38 mumol/L; whole blood, 3.22 +/- 0.79 mumol/L; and toenails, 15.2 +/- 3.0 nmol/g. Selenium intake (micrograms/kg body wt) was strongly correlated (all values, P less than 0.01) with selenium concentration of serum (r = 0.63), whole blood (r = 0.62), and toenails (r = 0.59). Men and women had similar mean values of serum, whole blood, and toenail selenium despite higher selenium intakes in men. Smokers had lower tissue selenium concentrations than did nonsmokers due, at least in part, to lower selenium intake. Age was not associated with tissue selenium content. Of the variables examined selenium intake was clearly the strongest predictor of tissue selenium concentration.

Stable isotopes of minerals as metabolic tracers in human nutrition research.

Enriched stable isotopes used as tracers have proven to be valuable in studies of the absorption and metabolism of minerals. Unlike radioisotopes, they can be used in high-risk population groups such as infants, children, and pregnant or lactating women. Estimates of mineral absorption can be made from the oral administration of a single tracer or from two tracers, one given orally and the other intravenously (IV). It is possible to determine the metabolism of the mineral with modeling based on the amount of the tracer or tracers in different biological samples. One of the key decisions in studies of this type is determining which enriched isotope and what amount to use. An example is given of calculations to estimate and compare the amounts of tracers needed for an absorption study. Methods for calculating the amounts of tracer in oral and IV doses are presented, and limits of detection and quantitation are discussed in terms of percent of enrichment and related to isotope ratio measurement precision. A general review of the use of mass spectrometric instruments for quantifying various stable isotopes is given.

Red cell volume determination using a stable isotope of chromium.

OBJECTIVE: To validate and improve a method of red cell volume determination by use of a stable isotope of chromium. METHODS: Twelve subjects were sequentially injected with red blood cells labeled with a stable isotope of chromium (53Cr) and red blood cells labeled with radioisotopic chromium (51Cr). Measurement of 53Cr dilution was by gas chromatography/mass spectrometry. Measurement of 51Cr dilution was by gamma counter. RESULTS: Comparison of the two methods led to results that differed on average by 34.5 +/- 45.0 mL (1.8 +/- 2.2%), 0.3 to 3.2%, 95% confidence interval. CONCLUSION: Measurement of red cell volume by use of a stable isotope of chromium is accurate, with potential applications including measurement in pregnant women and children or other groups in whom exposure to radioisotopes is undesirable.

Bone-related mineral content of water samples collected on the Navajo reservation.

Although dairy food intake is low among the Navajo people, hip fracture rates are lower than in Caucasians. Genetic differences in bone density have been cited as the reasons for low fracture rates among Native Americans and other segments of the population. However, more detailed examination of mineral intakes suggests that environmental factors may provide part of the explanation for the lower fracture rates. Cultural practices such as the addition of ash to traditional foods and the high mineral content of water may provide much higher intakes of bone-related minerals than food intake surveys have previously reported. As part of a larger study to assess overall intake of minerals related to bone health and other conditions, water samples were collected from the Navajo reservation. Duplicates were collected at least one week apart from 53 sites including wells, springs, taps, and storage barrels and analyzed by atomic absorption and inductively coupled plasma spectrometry for a number of minerals. For average intakes of 2 l/day, water could provide up to 212 mg of calcium, 150 mg of magnesium and 8 mg of zinc. The combined contribution of mineral intakes provided by the addition of juniper ash to traditional foods, not genetic differences, may partially explain the lower fracture rates of the Navajo people. Further research in this area is required to confirm this hypothesis.

Mild peripheral neuropathy but biochemical chromium sufficiency during 16 months of "chromium-free" total parenteral nutrition.

A 6-yr, 4-month-old boy was started on total parenteral nutrition (TPN) because of chronic diarrhea. The TPN regimen (3 liter/day) initially included supplemented Cr (3 micrograms/day) in addition to standard components (including FreAmine III). At age 8 yr, 8 months, the serum Cr level was elevated: 3.7 ng/ml (normal 0.03-0.85). A repeat level at the same time by another commercial laboratory was also high (7.0). Cr supplementation was stopped. At age 10 yr, he was noted to have mild peripheral neuropathy although glucose tolerance was excellent (alpha-linolenic acid was undetectable in the plasma). Cr status was reevaluated in a research lab. The serum level was 1.4 ng/ml (normal 0.05-0.4). The urine chromium excretion was 1.27 micrograms/day (normal 0.22). The TPN regimen (unsupplemented with Cr) provided 4 micrograms/day. Normal Cr intake is about 60 micrograms/day with 0.4% absorption (net 0.24 microgram/day). We conclude that Cr contamination of standard PN fluid may prevent biochemical evidence of low Cr status. In addition, alpha-linolenic acid-free parenteral nutrition for 46 months was not associated with clinically significant neurological dysfunction.

Analytical chemistry of chromium.

The determination of chromium in most biological materials is extremely difficult because of the very low levels present. Easily accessible samples, for example biological fluids such as serum, urine, etc., usually have chromium concentrations well below 1 ng g-1. The only widely available analytical method with sufficient sensitivity is graphite furnace atomic absorption spectrometry (GFAAS), yet values reported in the early literature were wildly divergent. Man appeared to be excreting several times the amount of chromium he was absorbing from his diet. This dilemma was resolved in 1978 when it was shown that all previous chromium analytical results were probably wrong (too high), due to limitations of the instrumentation used up until then. Subsequent instruments with improved background correction capabilities have removed this limitation. However, making determinations at the sub-parts-per-billion level remains a formidable task in terms of contamination control.

EDTA chelation effects on urinary losses of cadmium, calcium, chromium, cobalt, copper, lead, magnesium, and zinc.

The efficacy of a chelating agent in binding a given metal in a biological system depends on the binding constants of the chelator for the particular metals in the system, the concentration of the metals, and the presence and concentrations of other ligands competing for the metals in question. In this study, we make a comparison of the in vitro binding constants for the chelator, ethylenediaminetetraacetic acid, with the quantitative urinary excretion of the metals measured before and after EDTA infusion in 16 patients. There were significant increases in lead, zinc, cadmium, and calcium, and these increases roughly corresponded to the expected relative increases predicted by the EDTA-metal-binding constants as measured in vitro. There were no significant increases in urinary cobalt, chromium, or copper as a result of EDTA infusion. The actual increase in cobalt could be entirely attributed to the cobalt content of the cyanocobalamin that was added to the infusion. Although copper did increase in the post-EDTA specimens, the increase was not statistically significant. In the case of magnesium, there was a net retention of approximately 85% following chelation. These data demonstrate that EDTA chelation therapy results in significantly increased urinary losses of lead, zinc, cadmium, and calcium following EDTA chelation therapy. There were no significant changes in cobalt, chromium, or copper and a retention of magnesium. These effects are likely to have significant effects on nutrient concentrations and interactions and partially explain the clinical improvements seen in patients undergoing EDTA chelation therapy.