Measurement of zinc absorption from meals: comparison of extrinsic zinc labeling and independent measurements of dietary zinc absorption.

BACKGROUND: Extrinsic labeling techniques are typically used to measure fractional absorption of zinc (FAZ(extrinsic)) but none have been adequately evaluated. OBJECTIVE: To compare determination of the quantity of zinc absorbed (TAZ(extrinsic)) using measurements of FAZ(extrinsic) with results of simultaneous determinations of dietary zinc absorbed (TAZ(metabolic)) that are not dependent on labeling ingested food with an extrinsic tracer (modified metabolic balance technique). DESIGN: (70)Zn was administered orally with all meals for 6 consecutive days to 21 healthy, free-living adult women consuming a constant diet. (68)Zn and (67)Zn were administered intravenously. FAZ(extrinsic) was measured using a dual isotope tracer ratio technique and multiplied by dietary zinc to give TAZ(extrinsic). TAZ(metabolic) was determined by addition of net absorption of zinc and endogenous fecal zinc, the latter determined by an isotope dilution technique. RESULTS: TAZ(extrinsic) and TAZ(metabolic) were 3.0 +/- 1.1 mg/day and 3.1 +/- 1.1 mg/day respectively, paired t-test p = 0.492. The correlation coefficient for TAZ(extrinsic) and TAZ(metabolic) was 0.91, and for FAZ(extrinsic) and FAZ(metabolic) was 0.95. A Bland Altman analysis indicated a bias of 0.07, and the limits of agreement of -0.86 to 1.01 for TAZ(extrinsic) and TAZ(metabolic). CONCLUSION: These results from two independent methods provide reasonable validation of our extrinsic labeling technique for a wide range of composite diets.

Intestinal excretion of endogenous zinc in Guatemalan school children.

The intestine is the major route of excretion of endogenous zinc (Zn) and has a key role in maintaining Zn homeostasis. The principal objective of this paper is to provide an interpretative report of quantities of endogenous fecal Zn (EFZ) excreted by rural Guatemalan school children fed either normal or low phytate maize as their principal food staple. EFZ was measured by a Zn stable isotope technique. EFZ did not differ between control and low phytate maize groups. The overall EFZ (n = 53) was (mean +/- SD) 1.56 +/- 0.69 mg Zn/d or 0.07 +/- 0.03 mg Zn x kg body wt(-1) x d(-1). EFZ was not correlated with the quantity of Zn absorbed. The estimated EFZ at the level of absorption that matched the physiologic requirement (EFZ(PR)) did not differ from the above mean value. The EFZ(PR) of 0.07 +/- 0.03 mg Zn/kg body wt is twice the value currently used in the estimation of Dietary Reference Intakes. Supported by other recent childhood data, these results suggest that the current estimates of EFZ(PR) used in the calculation of Zn requirements for children are misleadingly low.

Zinc absorption during late pregnancy in rural southern Ethiopia.

BACKGROUND: Little is yet known about zinc absorption in late pregnancy, and no information on absorption from the total diet is available. OBJECTIVE: The objective was to measure the fractional absorption of zinc (FAZ) and to estimate the total quantity of absorbed zinc (TAZ) each day during the third trimester of pregnancy in poor rural southern Ethiopian women. DESIGN: The participants (n = 17) were a convenience sample from a larger study population. The third stage of pregnancy was estimated from fundal height by the Bushulo Health Center prenatal outreach program. FAZ was determined with a dual-isotope tracer ratio technique that uses measurements of urine enrichment with zinc stable isotopes administered intravenously and orally, as an extrinsic label, with all meals in 1 d. Total dietary zinc (TDZ) was calculated from weighed diet records and Ethiopian food-composition tables supported by zinc and phytate analyses of major food items for individual meals. Plasma zinc and exchangeable zinc pool size were also estimated. RESULTS: Mean (+/-SD) FAZ was 0.35 +/- 0.11, TDZ was 6.0 +/- 3.2 mg/d, TAZ was 2.1 +/- 1.0 mg/d, phytate intake was 1033 +/- 843 mg/d, plasma zinc was 44.1 +/- 6.0 microg/dL, and the exchangeable zinc pool size was 142 +/- 39 mg. The molar ratio of phytate to zinc was 17:1. CONCLUSIONS: Women from a poor rural population who were dependent on a moderately high-phytate diet had low TDZ and low plasma zinc concentrations in the third trimester of pregnancy. TAZ was modestly higher than that predicted but did not meet physiologic requirements.

Zinc absorption in Guatemalan schoolchildren fed normal or low-phytate maize.

BACKGROUND: Poor bioavailability of zinc from high-phytate diets is an important contributory factor to zinc deficiency in low-income populations. OBJECTIVE: The objective of this study was to determine the effect of low-phytate maize consumption on zinc absorption. DESIGN: The participants were apparently healthy children from the Central Highlands of Guatemala. Sixty children (20 per group) were randomly assigned to be fed only the low-phytate maize or 1 of 2 control maizes, the isohybrid wild-type maize or a local maize, for a 10-wk period. During the final week, the fractional absorption of zinc for all meals was measured during 1 d with the use of zinc stable isotopes and a dual isotope ratio technique based on urine enrichment data. RESULTS: Mean (+/-SD) phytate intakes for the low-phytate, wild-type, and local maize groups were 1536 +/- 563, 2056 +/- 517, and 2253 +/- 687 mg/d, respectively. Corresponding zinc intakes were 8.6 +/- 2.5, 8.1 +/- 2.0, and 9.7 +/- 2.6 mg/d, and the dietary phytate:zinc molar ratios were 18 +/- 5, 26 +/- 6, and 23 +/- 5. Corresponding fractional absorptions of zinc were 0.32 +/- 0.07, 0.28 +/- 0.07, and 0.29 +/- 0.06. The respective values for total absorbed zinc were 2.72 +/- 0.88, 2.30 +/- 0.96, and 2.78 +/- 1.04 mg/d. No significant differences in either the fractional absorption of zinc or total absorbed zinc were seen between the maize groups. CONCLUSION: Under the conditions of the present study, zinc absorption was not increased by the long-term use of low-phytate maize in children whose major dietary staple is maize.

Dysprosium as a nonabsorbable fecal marker in studies of zinc homeostasis.

BACKGROUND: Dysprosium is a nonabsorbable rare earth element that has had successful application as a marker for fecal excretion of unabsorbed zinc. OBJECTIVE: Our goals were 1) to evaluate the efficacy of administering dysprosium with all meals over several days as a method of determining the completeness of fecal collections, 2) to determine the similarity of gastrointestinal transit kinetics and excretion patterns of dysprosium and zinc tracer administered simultaneously over several days, and 3) to evaluate alternative methods of using the data for fecal excretion of orally administered zinc tracer and dysprosium to measure the fractional absorption of zinc. DESIGN: 70Zn and dysprosium were administered orally with all meals for 5 consecutive days to 7 healthy, free-living adults consuming a constant diet based on habitual intake. Additional tracers, 67Zn and 68Zn, were administered intravenously. Urine and fecal samples were collected during tracer administration and for 8 d after the last dose. Isotope ratios were measured in urine and feces, and total zinc and dysprosium were measured in fecal samples. RESULTS: The mean recovery of dysprosium was 101.3 +/- 2.4%. The zinc oral tracer and dysprosium had similar fecal excretory patterns; the correlation coefficient for 70Zn and dysprosium in fecal samples exceeded 0.99 (P < 0.0001) for each subject. Fractional zinc absorption measurements using various dysprosium methods correlated well (r > 0.95) with those from the fecal monitoring and dual-isotope-tracer ratio methods. CONCLUSION: Administration of dysprosium is a useful means of determining the completeness of fecal collections and of measuring zinc absorption.

Zinc absorption as a function of the dose of zinc sulfate in aqueous solution.

BACKGROUND: Zinc supplements are used extensively in medicine and research and for public health purposes in the prevention and treatment of zinc deficiency. However, little is known about the efficiency of zinc utilization after different doses. OBJECTIVE: The objective was to determine the relation between dose of aqueous zinc and absorbed zinc (AZ) in healthy adults. DESIGN: Eight healthy adults (3 men and 5 women) aged 33.8 +/- 9.8 y (x +/- SD) received 3 pairs of zinc doses (2 and 5, 10 and 15, and 20 and 30 mg) in random order in 3 phases (1 pair per phase). There was a 3-wk washout between phases. Aqueous zinc sulfate labeled with 70Zn or 68Zn was orally administered in the postabsorptive state on days 1 and 6, respectively; intravenous 67Zn was administered 1 h after the first oral zinc dose. Two urine samples were collected daily from days 3 to 15; zinc isotopic ratios were determined by inductively coupled plasma mass spectrometry. Fractional absorption of zinc (FAZ) was determined by dual-isotope-tracer ratio; AZ was calculated by multiplying FAZ by dose. RESULTS: Mean (+/-SD) AZ values at doses of 2.2, 5.2, 10.4, 15.2, 20.3, and 30.1 mg ingested Zn were 1.6 +/- 0.4, 3.5 +/- 1.3, 7.4 +/- 1.0, 9.5 +/- 2.2, 11.0 +/- 4.4, and 11.2 +/- 2.1 mg, respectively. A saturable dose-response model, the Hill equation, was selected to model the relation of AZ to ingested zinc. Parameter estimation by nonlinear regression predicted a maximum zinc absorption of 13 mg for larger doses. CONCLUSIONS: Increases in aqueous zinc doses >20 mg result in relatively small and progressively diminishing increases in AZ postabsorptively in healthy adults.

Serum lithium concentration can be used to assess dietary compliance in adults.

With dietary intervention studies, it is important to ensure that subjects adhere to the test diet. Current methods to monitor adherence have substantial limitations. Therefore, a dose-response test curve was constructed to determine whether small differences in serum Li could be detected in response to ingestion of variable Li doses indicative of full or partial dietary compliance. During 3 separate weeks, subjects consumed a test meal that included a single food containing Li citrate daily for 4 d. Doses of 250, 213, or 175 micromol Li were used each week to approximate compliance levels of 100, 85, and 70%. On d 4, blood samples were taken before and 1, 2, 3, 5, 7, 9, and 24 h after ingesting the test meal. Compared with the 100% dose, serum Li was significantly lower at all times after the 70% dose and at most times after the 85% dose. Data were analyzed to determine a cutoff value so that if a subject's serum Li was below that value, they would be declared noncompliant. The probability that a subject was declared to be noncompliant when in fact they were compliant was set to 0.05 (specificity was set to 0.95) and the probability of noncompliance (sensitivity) was estimated. Test performance was best in the 3- to 9-h range, for which 90-95% of subjects "noncompliant" at the 70% dose were identified. Li can be used as a biomarker to determine dietary compliance. Measuring serum Li 3-9 h after the last dose provides the highest sensitivity and specificity for noncompliance at doses <70%.