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.