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.

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.

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.

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.

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.

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%.

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.

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 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.

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.

A stable isotope study of copper absorption in young men: effect of phytate and alpha-cellulose.

A sixty-three day study was conducted with young men confined to a metabolic unit to study the effects of alpha-cellulose and phytate on copper absorption. Copper absorption was determined with 65Cu, a stable isotope of copper, during each of 3 dietary treatments (basal diet, basal diet + alpha-cellulose, or basal diet + phytate). The addition of alpha-cellulose or phytate to the basal diet did not affect copper absorption. Average copper absorption was 35.0% from the basal diet, 34.1% from the diet with 0.5 g alpha-cellulose per kg body weight added, and 31.4% from the diet with 2.34 g of phytate as sodium phytate added to the diet. Copper absorption was significantly different between subjects and averaged 44.1%, 26.8%, 33.4%, and 29.5% in individual subjects. The results suggest that high levels of either alpha-cellulose or phytate do not have marked effects on copper absorption, but copper absorption differs between individuals.

A stable isotope study of zinc absorption in young men: effects of phytate and alpha-cellulose.

A 63-day study was conducted with young men confined to a metabolic unit to study the effects of a alpha-cellulose and phytate on zinc absorption. A liquid formula diet was used throughout the experiment. During each of three dietary treatments, (basal diet, basal diet + alpha-cellulose, or basal diet + phytate), 67Zn, a stable isotope of zinc, was added to the diets to measure zinc absorption. Zinc absorption was determined by fecal monitoring of 67Zn, measured by thermal ionization mass spectrometry. Analytical precision of all measurements was within 1%. Average zinc absorption was 34.0 +/- 6.2% from the basal diet and 33.8 +/- 2.9% from the diet with 0.5 g alpha-cellulose per kg body weight added. Zinc absorption fell to 17.5 +/- 2.5% when 2.34 g of phytate as sodium phytate were added to the basal diet. The phytate/zinc molar ratio of the diet was 15. The decrease in zinc absorption was accompanied by increased fecal zinc and decreased urinary zinc. The results suggest that phytate inhibits zinc absorption and high levels of dietary phytate could result in zinc deficiency in man.

Copper absorption in elderly men determined by using stable 65Cu.

The apparent absorption of copper was determined in seven elderly men using the stable isotope 65Cu. Analysis of isotopic ratios of copper was done using thermal ionization, magnetic sector mass spectrometry after separation of copper from fecal samples by ion exchange chromatography. Mean apparent copper absorption during the first metabolic period in five men was 23.9 +/- 1.4% (mean +/- SEM) when fed a semipurified diet containing 3.3 mg of copper daily and 9 to 19 g nitrogen from egg albumin. Mean apparent copper absorption in five men during the second metabolic period was slightly but significantly higher, 27.7 +/- 0.31% (mean +/- SEM) when their diet contained 3.3 mg of copper per day and 4.0 to 5.7 g of nitrogen from egg albumin. The quantity of copper absorbed was equivalent to 0.8 mg per day in the first metabolic period and 0.9 mg during second metabolic period and was sufficient to maintain copper balance in these elderly men.

Use of enriched stable isotopes to determine zinc and iron absorption in elderly men.

The absorption of zinc and iron was determined in seven elderly men using the enriched stable isotopes 70Zn and 58Fe. Analyses of isotopic ratios were done using thermal ionization magnetic sector mass spectrometry on chloride solutions of zinc and of iron after separation of zinc from fecal samples by ion exchange and of iron by solvent extraction. Mean apparent zinc absorption in the seven subjects was 17.3 +/- 3.1% (mean +/- SEM) and mean iron absorption was 7.9 +/- 2.5% (mean +/- SEM) when fed a semipurified diet containing 15 mg of zinc and 10 mg of iron daily. Quantities absorbed were equivalent to an average of 2.6 mg of zinc and 0.8 mg of iron per day. These average zinc and iron values are close to previous estimates of endogenous losses of zinc and iron.