Isotope Ratios of Trace Elements in Samples from Human Nutrition Studies Determined by TIMS and ICP-MS: Precision and Accuracy Compared

Stable isotopes are used with increasing frequency to trace the metabolic fate of minerals in human nutrition studies. The precision of the analytical methods used must be sufficient to permit reliable measurement of low enrichments and the accuracy should permit comparisons between studies. Two methods most frequently used today are thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS). This study was conducted to compare the two methods. Multiple natural samples of copper, zinc, molybdenum, and magnesium were analyzed by both methods to compare their internal and external precision. Samples with a range of isotopic enrichments that were collected from human studies or prepared from standards were analyzed to compare their accuracy. TIMS was more precise and accurate than ICP-MS. However, the cost, ease, and speed of analysis were better for ICP-MS. Therefore, for most purposes, ICP-MS is the method of choice, but when the highest degrees of precision and accuracy are required and when enrichments are very low, TIMS is the method of choice.

Stable Isotope Techniques in Human Nutrition Research: Concerted Action is Needed

Use of stable isotopes in applied nutrition has been a developing field for over 60 years. The past 20 years has seen improvements in computer and mass-spectrometer technology that has opened up even greater possibilities in the understanding of human metabolism. While improvements in technology can bring great opportunities, it can also cause problems if there is no consensus among the stable isotope user-community on standardization of new techniques and methods. Users of stable isotopes have traditionally been split into two groups; those who work with heavy isotopes (e.g., 58Fe, 70Zn) and those who work with light isotopes (e.g., 2H218O). Standardization issues have been addressed by the light isotope users and awareness of this type of problem is starting to emerge within the heavy isotope community.

Molybdenum intake influences molybdenum kinetics in men

The objectives of this study were to determine physiologic adaptations that occur when humans are exposed to a wide range of molybdenum intake levels and to identify the pathways that are influenced by dietary intake. Four males consumed each of 5 daily molybdenum intakes of 22, 72, 121, 467, and 1490 microg/d (0.23, 0.75, 1.3, 4.9, and 15.5 micromol/d) for 24 d each. During each treatment period, oral and intravenous doses of (100)Mo and (97)Mo were administered. Serial plasma, urine, and fecal samples were analyzed for labeled and unlabeled molybdenum. Compartmental modeling was used to determine rates of distribution and elimination at each dietary intake level. Three pathways were sensitive to daily molybdenum intake. With increasing intake, absorption and urinary molybdenum excretion increased, whereas the fraction deposited in tissues decreased. Kinetic analysis suggested a daily intake of 115-120 microg/d (1.20-1.25 micromol/d) would maintain initial plasma molybdenum levels at their prestudy values and that their prestudy dietary intake was well above the Recommended Dietary Allowance of 45 microg/d. The physiological adaptations to changing intake that the model demonstrated may help prevent molybdenum deficiency and toxicity.

Mineral bioavailability and metabolism determined by using stable isotope tracers1

Definitive data on mineral bioavailability in humans and animals can be obtained by using isotopic tracers. The use of stable isotope tracers to study important issues in mineral nutrition has expanded rapidly in the past two decades, particularly in human nutrition studies. Stable isotopes have a number of advantages over radioisotopes. There is no exposure to radiation with stable isotopes, and some minerals have no radioisotope that can be used satisfactorily as a tracer. Multiple stable isotopes of one mineral and isotopes of multiple minerals can be administered simultaneously or sequentially. The analytical methods of choice for stable isotopes are thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry (ICPMS). Thermal ionization mass spectrometry offers the greatest precision and accuracy, but it is slower, more labor intensive, and more costly than ICPMS. Bioavailability data are critical to establishing reliable dietary mineral requirements and recommendations. Combined with a computer program for compartmental modeling, mineral kinetics can be studied, including mineral turnover, pool sizes, and transfer rates between compartments. Our laboratory conducts studies using stable isotopes of Zn, Cu, Fe, Ca, Mg, and Mo. We have studied the effect of the amount of dietary intake of minerals on bioavailability and use, pregnancy and aging, and interactions among minerals. The research resulted in establishing new dietary recommendations for Cu and Mo and developing compartmental models for these minerals. Although stable isotopes have been used more extensively to date in humans than in animals, the techniques applied to humans can be used to study a number of issues important to optimizing feeding strategies for animal production.

Molybdenum kinetics in men differ during molybdenum depletion and repletion

In this study we developed an expanded compartmental model of molybdenum (Mo) kinetics to determine rates of molybdenum distribution during molybdenum depletion and repletion. The model was based on a clinical study in which 4 men consumed a low-molybdenum diet of 22 microg/d (0.23 micromol/d) for 102 d, followed by a high molybdenum diet of 467 microg/d (4.9 micromol/d) for 18 d. Stable isotopes 100Mo and 97Mo were administered orally and intravenously, respectively, at several time points during the study, and serial samples of plasma, urine, and feces were analyzed for 100Mo, 97Mo, and total Mo. Based on plasma, urine, and fecal molybdenum levels, kinetic parameters of distribution and elimination were determined. The rates of molybdenum distribution and elimination were different during depletion and repletion. During high intake, urinary molybdenum excretion was greater than during low intake. In addition, fractional tissue storage of molybdenum was lower during high intake than during low intake. This suggests that low intake results in an adaptation to conserve body Mo, and that high intake results in an adaptation to eliminate Mo. The model also suggested that food-bound molybdenum was approximately 16% less bioavailable than purified Mo. Finally, under the conditions of this study, the model suggested that an intake of 43 microg/d (0.45 micromol/d) would be sufficient to maintain plasma molybdenum levels at steady state. This is a minimum estimate because subjects in this study were in a molybdenum-sparing state. These findings provide an understanding of the adaptations in molybdenum metabolism that take place during depletion and repletion.

Long-term high copper intake: effects on copper absorption, retention, and homeostasis in men

BACKGROUND

Numerous studies have examined the effect of low and adequate intakes of copper on absorption and retention, but little information is available on the regulation of absorption and retention of copper when intake is high.

OBJECTIVE

A study was conducted in men to determine the effect of long-term high copper intake on copper absorption, retention, and homeostasis.

DESIGN

Nine men were confined to a metabolic research unit (MRU) for 18 d and were fed a 3-d rotating menu containing an average of 1.6 mg Cu/d. They continued the study under free-living conditions for 129 d, supplementing their usual diets with 7 mg Cu/d. They then returned to the MRU for 18 d and consumed the same diet as during the first period, except that copper intake was 7.8 mg/d. The stable isotope (63)Cu was fed to 3 subjects and infused into the other 6 on day 7 of each MRU period, and complete urine and stool collections were made throughout the study. Total copper and (63)Cu were determined by inductively coupled plasma mass spectrometry. Copper absorption, excretion, and retention were calculated on the basis of dietary, urinary, and fecal copper and (63)Cu.

RESULTS

Results were as follows when comparing the high copper intake with the usual intake: fractional copper absorption was significantly lower, but the amount absorbed was significantly higher; excretion of the infused (63)Cu was significantly faster; and total retention was significantly higher.

CONCLUSIONS

Homeostatic regulation of copper absorption and retention helped to minimize the amount of copper retained with high copper intake but was not sufficient to prevent retention of >0.6 mg Cu/d.

Effect of long-term, high-copper intake on the concentrations of plasma homocysteine and B vitamins in young men

OBJECTIVE

We evaluated the effect of a long-term, high-copper intake on plasma total homocysteine, folate, vitamin B12, and pyridoxal-5'-phosphate concentrations in humans.

METHODS

Nine healthy young men were confined in a metabolic research unit for 18 d and fed 3-d rotation diets supplying an average of 1.6 mg of copper per day followed by 129 d of free-living conditions when they received 7 mg of copper per day in addition to their usual diets. The subjects returned to the metabolic research unit for the second 18-d period and were given the same diets as during the first 18 d with the exception that the copper intake was 7.8 mg/d. There was no apparent biochemical indication that the subjects were deficient in copper before the large-dose copper intake. Blood samples were obtained at the end of the first and second 18-d periods at the metabolic research unit, and plasma concentrations of total homocysteine, folate, vitamin B12, and pyridoxal-5'-phosphate were measured.

RESULTS

The long-term, high-copper intake resulted in small but significant decreases in plasma concentrations of total homocysteine and folate. There was no effect of the high-copper intake on plasma concentrations of vitamin B12 and pyridoxal-5'-phosphate.

CONCLUSIONS

These findings can be explained by our previous observation in rats suggesting that methionine synthase is copper dependent and that the metabolism of homocysteine and folate is regulated in part by copper nutriture. It may be necessary to consider copper nutriture for the interpretation of plasma concentrations of total homocysteine in humans.

Kinetic parameters and plasma zinc concentration correlate well with net loss and gain of zinc from men

The search for a reliable, convenient indicator of Zn status was the focus of research for several decades. Plasma Zn concentration is still the most widely used clinical measurement, despite the known problems of interpretation. More recently, researchers suggested that isotopically determined kinetic parameters, such as the exchangeable Zn pool (EZP), may more accurately and reliably reflect body Zn status. The objective of this study was to examine the relationship between net body Zn loss and gain during acute changes in dietary Zn intake with biochemical and kinetic indices of Zn status. Five men participated in an 85-d Zn depletion/repletion study. Net body Zn loss and gain were determined from the difference between dietary plus intravenously administered Zn and Zn excretion. Biochemical indicators of Zn status included plasma Zn, plasma alkaline phosphatase activity, and plasma retinol binding protein concentration. Following intravenous administration of (70)Zn or (67)Zn, a compartmental model was used to determine EZP mass, fractional Zn absorption, endogenous zinc excretion (EZE), and plasma Zn flux. The changes in total body zinc correlated best with changes in plasma Zn (r(2) = 0.826, P < 0.001), EZE (r(2) = 0.773, P < 0.001), and plasma Zn flux (r(2) = 0.766, P < 0.001). This study confirms that plasma Zn concentration is a valid indicator of whole-body Zn status in the absence of confounding factors; however, further research is needed to determine how kinetic parameters respond to conditions where plasma Zn concentration is known to be unreliable.

Long-term high copper intake: effects on indexes of copper status, antioxidant status, and immune function in young men

BACKGROUND

Short-term high copper intake does not appear to affect indexes of copper status or functions related to copper status, but the effects of long-term high copper intake are unknown.

OBJECTIVE

A study was conducted in men to determine the effect of long-term high copper intake on indexes of copper status, oxidant damage, and immune function.

DESIGN

Nine men were confined to a metabolic research unit (MRU) for 18 d and were fed a 3-d rotating menu providing an average of 1.6 mg Cu/d. The men continued the study under free-living conditions for 129 d and supplemented their usual diets with 7 mg Cu/d. The men then returned to the MRU for 18 d of the same diet as during the first period, except that copper intake was 7.8 mg/d. Plasma copper, ceruloplasmin activity, ceruloplasmin protein, plasma malondialdehyde, benzylamine oxidase activity, erythrocyte superoxide dismutase, hair copper, urinary copper, and urinary thiobarbituric acid-reactive substances were measured during each MRU period.

RESULTS

Ceruloplasmin activity, benzylamine oxidase, and superoxide dismutase were significantly higher at the end of the second MRU period than at the end of the first. Urinary copper excretion, hair copper concentrations, and urinary thiobarbituric acid-reactive substances were significantly higher during the second MRU period than during the first. Polymorphonuclear cell count, the percentage of white blood cells, lymphocyte count, and interleukin 2R were affected by copper supplementation. Antibody titer for the Beijing strain of influenza virus was significantly lower in supplemented subjects after immunization than in unsupplemented control subjects.

CONCLUSIONS

Under highly controlled conditions, long-term high copper intake results in increases in some indexes of copper status, alters an index of oxidant stress, and affects several indexes of immune function. The physiologic implications of these changes are unknown.

Plasma molybdenum reflects dietary molybdenum intake

The relationship between plasma molybdenum (Mo) and dietary intake has not been investigated in humans. We developed an isotope dilution method to determine molybdenum in 0.5 mL blood plasma by ICP-MS and conducted a study to determine the effect of dietary intake on plasma molybdenum. Twelve young men consumed a very low Mo diet (22 microg/day) for 24 days while confined to the WHNRC metabolic research unit and plasma molybdenum was monitored. (97)Mo was infused in four of the subjects (Group 1) to follow its clearance from the blood. The other eight remained in unit for 120 days (an additional 96 days). Four consumed the 22 microg/day molybdenum diet for 102 days followed by 467 microg/day for 18 days (Group 2). and four consumed five levels of dietary molybdenum for 24 days each (Group 3). (100)Mo was added to the diet one or more times at each dietary level. Total plasma molybdenum and (100)Mo were monitored throughout the study. Plasma molybdenum in the 12 subjects decreased from 8.2 +/- 0.5 to 6.1 +/- 0.5 nmol/L after 13 days of low molybdenum intake and was 5.1 +/- 0.5 nmol/L after 24 days. In Group 2, average plasma molybdenum was 7.8 +/- 0.9 nmol/L at the beginning of the study, 5.4 +/- 0.4 nmol/L during the 102 days low molybdenum period, and 16.5 +/- 0.6 nmol/L during the high molybdenum period. Plasma molybdenum in Group 3 was 4.2 +/- 2.1 nmol/L at 22 microg/day; 5.8 +/- 2.5 nmol/L at 72 microg/day; 6.6 +/- 2.3 nmol/L at 121 microg/day; 19.7 nmol/L +/-2.1 at 467 microg/day; and 43.9 +/- 2.1 nmol/L at 1490 microg/day. The results demonstrate that, in contrast to most other essential minerals, plasma molybdenum reflects low and high dietary molybdenum intakes within 14 days and may a useful indicator of low and high dietary intakes.

A compartmental model of magnesium metabolism in healthy men based on two stable isotope tracers

The aim of this study was to build a compartmental model of magnesium (Mg) kinetics by using data collected from six healthy adult men after oral administration of 26Mg and intravenous administration of 25Mg. Blood, urine, and feces were collected for 12 days after administration of the isotopes. Isotopic ratios were determined by inductively coupled plasma-mass spectrometry. Data were analyzed for each subject using SAAMII. We began with a compartmental model previously proposed (Avioli LV and Berman M. J Appl Physiol 21: 1688-1694, 1966) and developed an alternative approach to resolve the discrepancy between model-predicted curves and experimental data. This analysis enables the exploration of 25% of total body Mg that exchanges rapidly from plasma compartment with two extraplasma pools. One of the extraplasma compartments contains 80% of the exchangeable Mg with a transport rate of 48 +/- 13 mg/h. The second exchanges 179 +/- 88 mg of Mg/h. The model permitted estimation of kinetic parameters as well as fractional Mg absorption and fecal endogenous excretion.

Comparison of stable-isotope-tracer methods for the determination of magnesium absorption in humans

BACKGROUND

The double-labeling (DL) method for determining magnesium absorption is less cumbersome than is the fecal monitoring method, which has been used most often, but it has not been validated.

OBJECTIVE

The aim of this study was to compare methods and several sampling protocols for determining magnesium absorption to establish a simple and reliable alternative to the fecal monitoring approach. Fecal monitoring was used as the standard against which the DL methods based on urine data (DLU), plasma data (DLP), and plasma kinetics with the use of a deconvolution analysis (DP) were compared.

DESIGN

Six healthy adult men received 70 mg (26)Mg orally and 30 mg (25)Mg intravenously. Multiple blood samples and complete urine and fecal samples were collected over 12 d. Stable-isotope ratios were determined by inductively coupled plasma mass spectrometry.

RESULTS

Results from DLU were not significantly different from the fecal monitoring reference value (0.48 +/- 0.05; +/- SD) when based on 3-d urine pools from 72 to 144 h (0.54 +/- 0.04) and when based on the 24-h urine pools from 48 to 72 h (0.49 +/- 0.06), 72 to 96 h (0.51 +/- 0.11), and 96 to 120 h (0.50 +/- 0.06). Results with the DLP method 72 h after isotope administration also compared well with those with the fecal monitoring method (0.54 +/- 0.09). Magnesium absorption was 0.47 +/- 0.06 with the DP method, which also agreed with the fecal monitoring value.

CONCLUSIONS

The DL methods are an alternative to fecal monitoring when applied within the appropriate time intervals. Therefore, DLU-the simplest and least invasive approach-is recommended for determining magnesium absorption.

Stable isotope techniques in human nutrition research: concerted action is needed

Use of stable isotopes in applied nutrition has been a developing field for over 60 years. The past 20 years has seen improvements in computer and mass-spectrometer technology that has opened up even greater possibilities in the understanding of human metabolism. While improvements in technology can bring great opportunities, it can also cause problems if there is no consensus among the stable isotope user-community on standardization of new techniques and methods. Users of stable isotopes have traditionally been split into two groups; those who work with heavy isotopes (e.g., 58Fe, 70Zn) and those who work with light isotopes (e.g., 2H218O). Standardization issues have been addressed by the light isotope users and awareness of this type of problem is starting to emerge within the heavy isotope community.

Isotope ratios of trace elements in samples from human nutrition studies determined by TIMS and ICP-MS: precision and accuracy compared

Stable isotopes are used with increasing frequency to trace the metabolic fate of minerals in human nutrition studies. The precision of the analytical methods used must be sufficient to permit reliable measurement of low enrichments and the accuracy should permit comparisons between studies. Two methods most frequently used today are thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS). This study was conducted to compare the two methods. Multiple natural samples of copper, zinc, molybdenum, and magnesium were analyzed by both methods to compare their internal and external precision. Samples with a range of isotopic enrichments that were collected from human studies or prepared from standards were analyzed to compare their accuracy. TIMS was more precise and accurate than ICP-MS. However, the cost, ease, and speed of analysis were better for ICP-MS. Therefore, for most purposes, ICP-MS is the method of choice, but when the highest degrees of precision and accuracy are required and when enrichments are very low, TIMS is the method of choice.

Effect of acute zinc depletion on zinc homeostasis and plasma zinc kinetics in men

BACKGROUND

Zinc homeostasis and normal plasma zinc concentrations are maintained over a wide range of intakes.

OBJECTIVE

The objective was to identify the homeostatic response to severe zinc depletion by using compartmental analysis.

DESIGN

Stable zinc isotope tracers were administered intravenously to 5 men at baseline (12.2 mg dietary Zn/d) and after 5 wk of acute zinc depletion (0.23 mg/d). Compartmental modeling of zinc metabolism was performed by using tracer and mass data in plasma, urine, and feces collected over 6-14 d.

RESULTS

The plasma zinc concentration fell 65% on average after 5 wk of zinc depletion. The model predicted that fractional zinc absorption increased from 26% to essentially 100%. The rate constants for zinc excretion in the urine and gastrointestinal tract decreased 96% and 74%, respectively. The rate constants describing the distribution kinetics of plasma zinc did not change significantly. When zinc depletion was simulated by using an average mass model of zinc metabolism at baseline, the only change that accounted for the observed fall in plasma zinc concentration was a 60% reduction in the rate constant for zinc release from the most slowly turning over zinc pool. The large changes in zinc intake, excretion, and absorption-even when considered together-only explained modest reductions in plasma zinc mass.

CONCLUSION

The kinetic analysis with a compartmental model suggests that the profound decrease in plasma zinc concentrations after 5 wk of severe zinc depletion was mainly due to a decrease in the rate of zinc release from the most slowly turning over body zinc pool.

Molybdenum absorption and utilization in humans from soy and kale intrinsically labeled with stable isotopes of molybdenum

BACKGROUND

Stable-isotope studies of molybdenum metabolism have been conducted in which molybdenum was added to the diet and was assumed to be absorbed and utilized similarly to the molybdenum in foods.

OBJECTIVE

Our objective was to establish whether the molybdenum in foods is metabolized similarly to molybdenum added to the diet.

DESIGN

We first studied whether sufficient amounts of molybdenum stable isotopes could be incorporated into wheat, kale, and soy for use in a human study. Enough molybdenum could be incorporated into soy and kale to study molybdenum absorption and excretion. Two studies were then conducted, one in women and one in men. In the first study, each meal contained approximately 100 microg Mo from soy, kale, and extrinsic molybdenum. In the second study, soy and extrinsic molybdenum were compared; the meal contained approximately 300 microg Mo.

RESULTS

In the first study, molybdenum was absorbed equally well from kale and an extrinsic source. However, the molybdenum in soy was less well absorbed than the molybdenum in kale or that added to the diet. In the second study, absorption of molybdenum from soy was less than from the extrinsic label. Urinary excretion of soy molybdenum was also lower than urinary excretion of the extrinsic label, but excretion as a percentage of the absorbed dose was not significantly different between treatments.

CONCLUSIONS

The molybdenum in soy is less available than molybdenum added to the diet, but the molybdenum in kale is as available as molybdenum added to the diet. Once absorbed, excretion is not significantly different for soy, kale, and extrinsic molybdenum.

The effects of zinc depletion on peak force and total work of knee and shoulder extensor and flexor muscles

In this study we tested the effect of zinc (Zn) on muscle function in human. After receiving 12 mg Zn/day for 17 days, 8 male subjects received 0.3 mg Zn/day for either 33 or 41 days. Subjects were divided into two groups for repletion. Group A subjects received overnight infusions of 66 mg Zn on Days 1 and 10 and then were fed 12 mg Zn/day for another 16 days. Group B subjects were fed 12 mg Zn/day for 3 weeks. Peak force and total work capacity of the knee and shoulder extensor and flexor muscle groups were assessed using an isokinetic dynamometer at baseline, at two points during depletion, and at repletion. Plasma Zn declined significantly during depletion and remained below baseline levels after repletion. The peak force of the muscle groups tested was not affected by acute Zn depletion, however, total work capacity for the knee extensor muscles and shoulder extensor and flexor muscles declined significantly. The data suggest that acute Zn depletion alters the total work capacity of skeletal muscle.

Copper absorption, excretion, and retention by young men consuming low dietary copper determined by using the stable isotope 65Cu

A study was conducted in young men to evaluate the effect of a low-copper diet on copper absorption, excretion, and retention. Eleven young men were confined to a metabolic research unit for 90 d. The study was divided into three periods, with dietary copper as the only variable. Dietary copper intake was 0.66 mg/d for 24 d, 0.38 mg/d for 42 d, and 2.49 mg/d for 24 d. The stable isotope 65Cu was fed to five of the subjects once during the first and last dietary period and twice, early and late, in the second period to determine copper absorption. 65Cu was infused into an arm vein of the other six subjects once during each dietary period to estimate excretion of endogenous copper. Total copper and 65Cu were determined by isotope dilution with thermal-ionization mass spectrometry. Fractional absorption was significantly higher during the low-copper period than in either period with higher dietary copper and excretion of the infused isotope was significantly lower in the low-copper period. Subjects were in negative balance early in the first two periods but achieved balance by the end of those periods. They retained copper during the highest dietary copper period (third period). The results suggest that endogenous copper excretion is a major point of regulation of the body's copper stores. Regulation of absorption and of endogenous excretion in response to dietary copper intake helps to protect against deficiency and toxicity. However, this regulation was not sufficient to maintain copper status at the lowest intake of dietary copper, 0.38 mg/d.

Human whole-body copper metabolism

Whole-body copper metabolism is difficult to study in human subjects. However, the use of isotopic tracers and kinetics modeling has added a dimension beyond what can be learned in humans by direct measurement. Mechanisms regulating total body copper seem to be strong, given the relatively small and constant body pool, but they are not yet well understood. The efficiency of copper absorption varies greatly, depending on dietary intake. Changes in efficiency of absorption help to regulate the amount of copper retained by the body. In addition, endogenous excretion of copper into the gastrointestinal tract depends heavily on the amount of copper absorbed. When dietary copper is high and more is absorbed, endogenous excretion increases, protecting against excess accumulation of copper in the body. When intake is low, little endogenous copper is excreted, protecting against copper depletion. Regulation is not sufficient with very low amounts of dietary copper (0.38 mg/d) and appears to be delayed when copper intake is high. The use of isotopic tracers and kinetic modeling should aid in elucidating the regulatory mechanisms.

A longitudinal study of calcium homeostasis during human pregnancy and lactation and after resumption of menses

To clarify the role of the intestine, kidney, and bone in maintaining calcium homeostasis during pregnancy and lactation and after the resumption of menses, a longitudinal comparison was undertaken of 14 well-nourished women consuming approximately 1200 mg Ca/d. Measurements were made before conception (prepregnancy), once during each trimester of pregnancy (T1, T2, and T3), early in lactation at 2 mo postpartum (EL), and 5 mo after resumption of menses. Intestinal calcium absorption was determined from the enrichment of the first 24-h urine sample collected after administration of stable calcium isotopes. Bone mineral of the total body and lumbar spine was measured by dual-energy X-ray absorptiometry and quantitative computerized tomography, respectively. Twenty-four-hour urine and fasting serum samples were analyzed for calcium, calcitropic hormones, and biochemical markers of bone turnover. Despite an increase in calcium intake during pregnancy, true percentage absorption of calcium increased from 32.9+/-9.1% at prepregnancy to 49.9+/-10.2% at T2 and 53.8+/-11.3% at T3 (P < 0.001). Urinary calcium increased from 4.32+/-2.20 mmol/d at prepregnancy to 6.21+/-3.72 mmol/d at T3 (P < 0.001), but only minor changes in maternal bone mineral were detected. At EL, dietary calcium and calcium absorption were not significantly different from that at prepregnancy, but urinary calcium decreased to 1.87+/-1.22 mmol/d (P < 0.001) and trabecular bone mineral density of the spine decreased to 147.7+/-21.2 mg/cm3 from 162.9+/-25.0 mg/cm3 at prepregnancy (P < 0.001). Calcium absorption postmenses increased nonsignificantly to 36.0+/-8.1% whereas urinary calcium decreased to 2.72+/-1.52 mmol/d (P < 0.001). We concluded that fetal calcium demand was met by increased maternal intestinal absorption; early breast-milk calcium was provided by maternal renal calcium conservation and loss of spinal trabecular bone, a loss that was recovered postmenses.

Opioid Peptides, Adrenocorticotrophic Hormone and Dietary Copper Intake in Humans

Copper plays an important role in cardiac and brain function possibly through endocrine and neuroendocrine systems. The syndrome of copper deficiency is worsened by dietary fructose and other trace metals such as zinc. We investigated the effect of a low copper diet on plasma opioid peptides in 11 healthy young volunteers who were fed foods low in copper but adequate in all other nutrients. The study was divided into three dietary periods. Copper was added to the diet so that the diet contained 0.66 mg/day for 24 days (marginal Cu), 0.38 mg/day for 42 days (low Cu) and 2.49 mg/day for 24 days (adequate Cu). The indices of copper status, ceruloplasmin and plasma copper concentrations, declined and were significantly lower (p < 0.05) at the end of the low Cu period than at the beginning of the study and the end of the marginal Cu period. They increased significantly at the end of the adequate Cu diet to the levels of the marginal Cu diet. Plasma β-endorphin (BEN), Leu-enkephalin (LE), Met-enkephalin (ME) and Adrenocorticotropic hormone (ACTH) were measured by radioimmunoassay at the beginning of the study and at the end of each dietary period. No significant differences were observed in BEN, LE or ME during any of the periods. There were only small increases in LE and ME at the end of marginal and low copper diet periods and no significant changes were observed on copper repletion. Plasma ACTH was significantly lower at the end of low copper compared to baseline value but was not lower after marginal copper. Copper repletion had no significant effect on ACTH. The data show that plasma opioid peptides did not respond significantly to differential copper intake.

In vitro copper stimulation of plasma peptidylglycine alpha-amidating monooxygenase in Menkes disease variant with occipital horns

We determined the concentrations of copper, the activities of ceruloplasmin and peptidylglycine alpha-amidating monooxygenase (PAM), and the stimulation index of PAM by the in vitro addition of copper in plasma samples obtained from three male patients with occipital horns and a milder Menkes disease phenotype, having severe copper deficiency due to the defect in copper transport. We found a decreased plasma ceruloplasmin activity and an increased copper stimulation index of plasma PAM in these patients compared with healthy control subjects. The combination of these two determinations may provide a means for the assessment of copper nutriture in humans using blood samples obtained in a single microhematocrit tube. Further investigation is warranted to evaluate whether these noninvasive measurements can be used for the diagnosis of mild copper deficiency in humans with sufficient specificity and sensitivity.

Copper status of young men consuming a low-copper diet

A study was conducted in 11 young men to evaluate the effect of a low-copper diet on indexes of copper status and to define an amount of dietary copper at which adequate copper status could not be maintained. The young men were confined to a metabolic research unit for 90 d. The study was divided into three periods, with dietary copper as the only variable. Dietary copper was 0.66 mg/d for 24 d, 0.38 mg/d for 42 d, and 2.49 mg/d for 24 d. Plasma copper, ceruloplasmin activity, ceruloplasmin concentration, and erythrocyte superoxide dismutase (SOD) were measured at selected time points during each dietary copper period. Urine was collected throughout the study. Plasma copper, ceruloplasmin concentration and activity, and urinary copper declined significantly during the lowest dietary copper period. Plasma copper, ceruloplasmin concentration, and urinary copper increased in response to repletion. The average erythrocyte SOD concentration was lower during the depletion period than in the periods before or after depletion, but it did not decline significantly over time in the depletion period. The results suggest that these indexes are sensitive to copper depletion; that 0.38 mg Cu/d is not sufficient to maintain copper status in normal, healthy young men; and that the minimum dietary copper requirement is between 0.4 and 0.8 mg/d.

Deliberations and evaluations of the approaches, endpoints and paradigms for manganese and molybdenum dietary recommendations

The background of the current dietary recommendations for manganese and molybdenum are described. This article reviews how the previous and current estimated safe and adequate daily dietary intakes (ESADDI) were set, shortcomings in the methods used, concerns about the current recommendations, and brief summaries of new research reports. New approaches, endpoints and paradigms to use for the development of useful recommendations are given.

Molybdenum metabolism in men with increasing molybdenum intakes: changes in kinetic parameters

Molybdenum metabolism was studied in four young men to determine the effect of the amount of dietary molybdenum on molybdenum kinetics. A compartmental model, developed by using data from a study with low dietary molybdenum, was adapted to accommodate five levels of molybdenum. Each level, ranging from 22 to 1,470 micrograms molybdenum/day, was fed for 24 days. Kinetics of absorption and excretion were traced by using 97Mo (intravenous) and 100 Mo (oral) stable-isotope tracers at selected intervals. Urinary and fecal isotope excretion data for 6-day pooled collections were fit to a kinetic model by using SAAM/CONSAM software. Residence times for molybdenum were estimated at 2.4 days in the gastrointestinal tract, 40 min in plasma, from 3.3 to 0.3 days in fast-turnover tissue, and from 63 to 237 days in slow-turnover tissue. As dietary molybdenum increased, residence time decreased in fast-turnover tissue and increased in slow-turnover tissue. The model closely approximated the highly efficient homeostatic mechanisms of molybdenum metabolism over a wide range of intakes.

Kinetic model of molybdenum metabolism developed from dual stable isotope excretion in men consuming a low molybdenum diet

The aim of this study was to develop a compartmental model of molybdenum metabolism based on stable isotope excretion patterns. Molybdenum (Mo) is an essential trace element in humans, with an estimated safe and adequate daily dietary intake (ESADDI) of 75-250 micrograms Mo/d. Four adult men were fed low molybdenum diets, 22 micrograms Mo/d for a period of 102 d. 97Mo+ and 100Mo stable isotopes, in intravenous and oral doses, respectively, were administered at selected intervals. The resulting 6-d cumulative urinary and fecal isotope excretion data were used to model molybdenum metabolism using SAAM/CONSAM software. A kinetic model, including gastrointestinal (GI), plasma, slow-turnover tissue and fast-turn-over tissue compartments, accurately simulated the observed pattern of urinary and fecal excretion for both stable isotopes in all four subjects. Residence time for molybdenum in the GI tract was estimated at 1.7 +/- 0.4 d. Predicted residence time for plasma molybdenum was 22 +/- 4 min, whereas slow-turnover tissue (possible hepatic) retention averaged 58 +/- 16 d. The model thus permitted estimation of kinetic parameters for molybdenum metabolism in tissues not readily accessible or measurable in humans.

Effect of calcium supplements and stage of lactation on the calcium absorption efficiency of lactating women accustomed to low calcium intakes

The effect of calcium intake on the calcium absorption efficiency from 100 mL cow milk was measured in lactating Gambian mothers habituated to a low-calcium diet [mean intake 7.08 mmol (283 mg)/d], and compared with UK lactating mothers consuming high-calcium diets [mean intake 29.2 mmol (1168 mg)/d] by using a double stable-isotope technique (oral 44Ca and intravenous 42Ca). In a double-blind trial starting 9 d postpartum, Gambian mothers were given a calcium supplement [17.85 mmol (714 mg)/d] or placebo for 12 mo. At 3 and 12 mo postpartum, mean (+/- SEM) calcium absorption from isotopically enriched milk was 52.3 +/- 3.1% (n = 25) and 47.2 +/- 4.8% (n = 24) in the unsupplemented Gambian mothers and 48.8 +/- 2.8% (n = 28) and 42.9 +/- 3.7% (n = 24) in the supplemented mothers, respectively. There was no effect of supplementation or stage of lactation on the efficiency of calcium absorption. At 3 mo postpartum the UK mothers absorbed 32.2 +/- 3.8% of the isotopically enriched calcium added to milk, which was significantly less than that of the Gambian mothers (P < 0.01).

Molybdenum absorption, excretion, and retention studied with stable isotopes in young men at five intakes of dietary molybdenum

A study of molybdenum absorption, excretion, and balance was conducted in four young men fed five amounts of dietary molybdenum, ranging from 22 to 1490 micrograms/d, for 24 d each. The study was conducted to obtain scientific data on which to base a recommendation on dietary molybdenum intake for healthy young men. Stable isotopes of molybdenum were used as tracers. 100Mo was fed five times during the study and 97Mo was infused three times. 94Mo was used to quantify the molybdenum isotopes and total molybdenum in urine, fecal collections, and diets by isotope dilution. Adverse effects were not observed at any of the dietary intakes. Molybdenum was very efficiently absorbed, 88-93%, at all dietary molybdenum intakes, and adsorption was most efficient at the highest amounts of dietary molybdenum. The amount and percentage of molybdenum excreted in the urine increased as dietary molybdenum increased, suggesting that molybdenum turnover is slow when dietary molybdenum is low and increases as dietary molybdenum increases. We conclude from these results that dietary intakes between 22 and 1500 micrograms/d by adult men are safe for > or = 24 d and that molybdenum retention is regulated by urinary excretion. Molybdenum is conserved at low intakes and excess molybdenum is rapidly excreted in the urine when intake is high.

Effects of low-copper diets on human immune response

We examined the effects of low-copper diets on indexes of immune response of 11 healthy men (aged 21-32 y) during a 90-d metabolic suite study. Daily copper intake for the first 24 d, next 42 d, and the last 24 d of the study was 0.66, 0.38, and 2.49 mg, respectively. Feeding the diet with 0.38 mg Cu/d was associated with a significant (P < or = 0.05) decrease in the proliferation of peripheral blood mononuclear cells cultured with phytohemagglutinin, Concanavalin A, or pokeweed, and an increase in the percentage of circulating B cells (CD 19+), but had no effect on the concentration of serum interleukin 2 receptor, the percentage of peripheral monocytes, neutrophils, CD3+, CD4+, or CD8+ T cells; or on the neutrophil phagocytic activity. Feeding 2.49 mg Cu/d for 24 d prevented further decreases in the indexes affected by the low-copper diet but did not restore them to the prestudy concentrations, even though plasma copper and ceruloplasmin concentrations were restored to normal.

Molybdenum absorption, excretion, and retention studied with stable isotopes in young men during depletion and repletion

A study of molybdenum absorption, excretion, and balance was conducted in four young men fed a low-molybdenum diet (22 micrograms/d) for 102 d followed by 18 d of the same diet supplemented to contain 467 micrograms/d. The study was conducted to determine the minimum dietary molybdenum requirement of healthy young men. Stable isotopes of molybdenum were used as tracers. 100Mo was fed four times during the study, 97Mo was infused twice, and 94Mo was used as an isotopic diluent to quantify the molybdenum isotopes and total molybdenum in complete urine and fecal collections and in the diets. The study demonstrated that subjects could not consistently attain balance with the low-molybdenum diet, but balance improved with time, and no signs of molybdenum deficiency were observed. Molybdenum was very efficiently absorbed at both intakes of dietary molybdenum and urinary excretion increased as dietary molybdenum increased. Molybdenum turnover was significantly slower when dietary molybdenum was low. We estimate from these results that the minimum dietary molybdenum requirement is approximately 25 micrograms/d or possibly less. This suggests that the lower end of the recommended range could be less than the current recommended amount of 75 micrograms/d.

Future directions for establishing mineral/trace element requirements

The amount of an element needed to prevent frank deficiency may not be sufficient to support optimal nutrition, but amounts to support optimal nutrition have not been established. Minerals and trace elements are toxic in excess and the interval between the required and toxic amount of some elements is narrow. Thus, lower and upper limits of an optimal range must be established. Before establishing dietary recommendations to support optimal nutriture for minerals, we need (1) sensitive and reliable methods for assessing status of most elements and (2) a better understanding of the influence of nutrient and non-nutrient components of diets upon requirements. Functions such as immune function, anti-oxidant status, muscle strength, glucose metabolism, and blood clotting can be affected by inadequate or excessive amounts of an element and may be more sensitive than specific status indices. Since such functions are not specific, studies must be designed so that a cause and effect relationship between the mineral and the functional index can be established. Two approaches to mineral status assessment may be both sensitive and specific: (1) tests of metalloenzyme function and (2) tracer studies using stable isotopes of minerals. Not only can stable isotopes be used to follow the metabolic fate of a mineral without exposure to radioactivity, they can be used in conjunction with compartmental modeling to predict kinetics and pool sizes in tissues not accessible in humans.

A compartmental model of zinc metabolism in adult men used to study effects of three levels of dietary copper

The effect of dietary copper level on zinc metabolism in five healthy adult men was studied using compartmental modeling. The subjects were fed one level of dietary zinc (11.2 +/- 1.6 mg/day) and three levels of dietary copper (1.68, 0.785, and 7.52 mg/day). The stable isotope tracers 70Zn and 67Zn were administered intravenously (3 times) and orally (4 times), respectively. Plasma, urinary, and fecal 67Zn and 70Zn levels were measured. An existing model of zinc metabolism was used as a basis for our model. Model rate constants were modified to simulate multiple oral and intravenous doses of stable isotope tracers given during the 90-day study. Most rate constant changes occurred in the absorption and excretion pathways. The model predicted that movement of zinc through the gastrointestinal tract increased when the highest level of copper was fed and the urinary zinc excretion rate tended to increase. The fractional rate constant representing absorption of zinc decreased slightly when the highest level of copper was fed. Total body zinc was predicted to be approximately 2,000 mg, with 95% in tissues other than the liver, red blood cells, or plasma and approximately 2.25% in the blood (with 95% of that in the red blood cells), which follows expected zinc masses within the body and those predicted by previous models.

Isotope ratios of molybdenum determined by thermal ionization mass spectrometry for stable isotope studies of molybdenum metabolism in humans

Methods were developed to separate and purify Mo from biological samples and to measure isotopic ratios in 1 microgram of Mo. A magnetic sector, thermal ionization mass spectrometer was used with simultaneous collection of five isotopes. Isotopic ratios were corrected for mass fractionation by iterative normalization using the 96/98 ratio. Ion beam intensity was enhanced by using a double-filament configuration, loading samples onto evaporation filaments with silica gel and boric acid. A triple-isotope-dilution approach was used, so the method could be applied to two-tracer studies of Mo metabolism in human subjects. 94Mo was added to samples prior to purification to quantify the total Mo content of samples and to determine the amounts of enriched 97Mo and 100Mo appearing in urine and fecal samples of study participants. The three ratios, 94/98, 97/98, and 100/98, were determined with within-run precision of from 0.06 to 0.10% (RSD). Precision of the ratios between replicates was from 0.05 to 0.08%.

Vitamin B-6 depletion followed by repletion with animal- or plant-source diets and calcium and magnesium metabolism in young women

An 84-98-d study was conducted in young women to determine the effect of vitamin B-6-deficient diets on calcium and magnesium metabolism. A vitamin B-6-deficient formula diet fed initially was followed by either animal- or plant-source protein food diets containing four increasing amounts of vitamin B-6. Calcium balance was negative during vitamin B-6 depletion. Serum calcium was higher and calcium balance negative with the plant protein diets. Magnesium balance was negative during vitamin B-6 depletion due to increased urinary magnesium excretion. Urinary calcium decreased during vitamin B-6 depletion and increased as dietary vitamin B-6 increased. Urinary oxalate was significantly higher at the end than at the beginning of vitamin B-6 depletion and was higher with plant than animal protein diets. The results suggest that vitamin B-6 depletion may alter calcium and magnesium metabolism and that dietary components associated with the protein source may influence calcium retention.

A stable-isotope study of zinc, copper, and iron absorption and retention by young women fed vitamin B-6-deficient diets

A 98-d study was conducted in young women to determine the effect of vitamin B-6-deficient diets on zinc, copper, and iron metabolism. Young women were fed vitamin B-6-deficient formula initially, followed by food diets containing four increasing amounts of vitamin B-6. Zinc, copper, and iron absorption, retention, and status were determined at intervals throughout the study. Zinc absorption and retention were greater during vitamin B-6 depletion but serum zinc declined, suggesting that absorbed zinc was not available for utilization. Copper absorption was lower during vitamin B-6 depletion but serum copper was not affected and balance was positive. Iron absorption was not impaired significantly by vitamin B-6-deficient diets but status may have declined. The results suggest that vitamin B-6 depletion of young women may alter zinc metabolism, inhibit copper absorption, and affect iron status. The effects of vitamin B-6 depletion differ markedly among these elements.

Bioavailability of dietary minerals to humans: the stable isotope approach

A number of minerals contained in foods are essential nutrients for humans, animals, and/or plants. While most vitamins are very well absorbed, most essential minerals are not. Usual absorption of minerals ranges from less than 1% to over 90%. The bioavailability of dietary minerals must be considered when determining whether the diet contains enough, too little, or too much. By using stable isotope tracers as labels, the metabolic fate of minerals in a specific day's diet, a specific meal, or a food can be distinguished from minerals from other sources and followed. A number of mass spectrometric methods have been used to measure stable isotopes. Magnetic sector, thermal ionization mass spectrometry (TIMS) is used routinely in our laboratory to study bioavailability of Zn, Cu, and Fe. Other mass spectrometric methods that are less precise, but useful for many applications requiring isotopic determinations include quadrupole TIMS, inductively coupled plasma mass spectrometry (ICP/MS), and fast atom bombardment mass spectrometry (FAB/MS). One of the major advantages of stable isotope studies is that multiple isotopes of the same mineral can be used simultaneously and multiple minerals can be studied simultaneously. The use of stable isotopes for studies of bioavailability of minerals in foods has gained widespread interest in recent years. The approach is expected to be applied to an increasing number of food science and nutrition problems in the future.

Milk's effect on the bioavailability of iron from cereal-based diets in young women by use of in vitro and in vivo methods

A study was conducted in healthy young women to measure and compare the availability of iron from cereal-based diets with and without milk by use of in vivo and in vitro methods. In vitro iron-bioavailability tests demonstrated that the amounts of soluble and ionizable iron in cereal-based diets were increased two- and three-fold, respectively, when milk was added. 54Fe, a stable isotope of iron, and fecal monitoring were used to determine iron absorption in eight young women. Iron absorption was higher with milk than without milk in seven of the eight subjects but did not differ significantly between the two treatments. The results suggest that in vivo and in vitro effects differ and that the absorption of iron from cereal-based diets is neither enhanced nor inhibited by the addition of milk.

Copper status and urinary and salivary copper in young men at three levels of dietary copper

Eleven young men were confined to a metabolic research unit for 90 d to determine the effect of the amount of dietary copper on copper nutriture. The study was divided into three metabolic periods (MP): 1) with an adequate-copper diet (1.68 mg/d) for 24 d, 2) with a low-copper diet (0.79 mg/d) for 42 d, and 3) with a high-copper diet (7.53 mg/d) for 24 d. Three indices of copper status, urinary copper, and salivary copper were determined at intervals throughout the study. Neither copper status, urinary copper, nor salivary copper differed among MPs. Sweat collections from three subjects suggested that losses of copper through sweat were very low and would not contribute significantly to copper balance. These results suggest that an amount of dietary copper slightly less than 0.8 mg/d is adequate to maintain copper status for greater than or equal to 42 d in normal, healthy men and that neither urinary nor salivary copper is affected by the amount of Cu in the diet.

Copper absorption and retention in young men at three levels of dietary copper by use of the stable isotope 65Cu

Eleven young men were confined to a metabolic research unit for 90 d to determine the effect of the level of dietary copper on absorption and retention. Cu absorption was determined by feeding the stable isotope 65Cu. Absorption and retention averaged 36.3 +/- 1.3% and 0.17 mg/d, respectively, with an adequate-Cu diet (1.68 mg/d). Absorption averaged 55.6 +/- 0.9% and retention averaged -0.316 mg/d for 6 d and 0.093 mg/d for the next 36 d of a low-Cu diet (0.785 mg/d). Absorption averaged 12.4 +/- 0.9% with a high-Cu diet (7.53 mg/d) and retention was strongly positive at first, decreasing linearly with time. The study demonstrated that Cu absorption is strongly dependent on dietary Cu level and that Cu balance can be achieved by most young men from a diet of 0.8 mg Cu/d. These results suggest that current dietary Cu recommendations may be higher than necessary. The apparent regulation of Cu absorption and endogenous losses would tend to protect humans from Cu deficiency and toxicity.

Protein utilization by young women consuming animal or plant protein diets at various levels of vitamin B-6 intake

Eight young women consumed a vitamin B-6 depletion diet (egg albumen formula, less than 0.05 mg vitamin B-6/d) for 11-28 d. Subjects (n = 4) then consumed either animal protein (AP, mainly dairy and poultry products) or plant protein (PP, mainly various types of beans) diets with increasing vitamin B-6 intake (0.5, 1.0, 1.5, and 2.0 mg/d) for periods of 14-21 d. All diets provided 1.55 g protein/kg body wt. Apparent protein digestibility of AP (94.6%) was significantly higher than that of PP (88.4%) diets (p less than 0.001). Protein digestibility was not significantly affected by vitamin B-6 intake. Apparent nitrogen balance of subjects consuming AP diets was slightly, though not significantly, higher than that of subjects fed PP diets. N balance was not influenced by vitamin B-6 intake. Data suggest that short-term low vitamin B-6 intake does not affect protein utilization in humans as determined by digestibility and N balance.

The use of stable isotopes in mineral nutrition research

Stable isotopes are valuable tools for research on mineral bioavailability and metabolism. They can be used as tracers with no exposure to radiation and they do not decay over time. Attempts to use stable isotopes of minerals as metabolic tracers were first described only 25 years ago. There were relatively few reports of their use over the next 15 years, but interest in stable isotopes has expanded markedly in the last 10 years. The advantages of stable isotope tracers are so great that scientists have been willing to accept the laborious and costly nature of mineral isotope analysis, and substantial progress has been made in the field. New applications for stable isotopes and new analytical methods have been introduced recently. However, limitations to the approach and methodological problems remain to be resolved. This review describes early work in the field and discusses the advantages and disadvantages of stable isotope tracers and of the various methods of analysis. Information discovered with stable isotopes is reviewed, and probable future applications are discussed.

Copper absorption in young men fed adequate and low zinc diets

Copper absorption was measured at two levels of dietary zinc in six healthy young men who were confined to a metabolic unit for a 75 d study of zinc utilization. A diet of conventional foods was fed, providing either 16.5 or 5.5 mg zinc and 1.3 mg copper daily. Copper absorption was determined by feeding 65Cu, a stable isotope of copper, once during the 16.5 mg Zn diet and near the beginning and end of the 5.5 mg Zn diet. Apparent copper absorption averaged 48.1% when the 16.5 mg Zn diet was fed. This was significantly higher than the averages of 37.2 and 38.5% when the 5.5 mg Zn diet was fed. Absorption also differed significantly among subjects. Fecal copper did not differ between diets or among subjects. All subjects were in positive copper balance at both levels of dietary zinc. These results suggest that a dietary zinc intake slightly above the Recommended Dietary Allowance of 15 mg/d does not increase fecal copper loss and does not interfere with copper absorption.