AJCN Tufts Nutrition Symposium, Boston & Online Sept 2009
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American Journal of Clinical Nutrition, Vol. 87, No. 5, 1336-1345, May 2008
© 2008 American Society for Nutrition

ORIGINAL RESEARCH COMMUNICATION

Adaptation in human zinc absorption as influenced by dietary zinc and bioavailability1,2,3,4

Janet R Hunt, Jeannemarie M Beiseigel and LuAnn K Johnson

1 From the US Department of Agriculture–Agricultural Research Service Grand Forks Human Nutrition Research Center, Grand Forks, ND (JRH and JMB), and the University of North Dakota, Grand Forks, ND (LKJ)

2 Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

3 Supported by the US Department of Agriculture–Agricultural Research Service.

4 Reprints not available. Address correspondence to JR Hunt, USDA-ARS GFHNRC, 2420 2nd Avenue N STOP 9034, Grand Forks, ND 58202-9034. E-mail: jhunt{at}gfhnrc.ars.usda.gov.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: An understanding of the relations among dietary zinc intake, bioavailability, and absorption is necessary for making dietary intake recommendations.

Objectives: We aimed to assess adaptation in human zinc absorption to controlled differences in zinc and phytate intakes and to apply the results to predictive models.

Design: In 3 experiments, radiotracers were used to assess zinc absorption by healthy adults (n = 109) from controlled diets, before and after 4 or 8 wk of dietary equilibration. Subjects consumed 4–29 mg Zn/d from 1 of 10 diets, 5 with molar ratios of phytate to zinc from 2 to 7 and 5 with ratios from 15 to 23.

Results: Absorptive efficiency was inversely related to dietary zinc from both low- and high-phytate diets. In response to low zinc intakes (<11 mg/d) for 4–8 wk, zinc absorption was up-regulated to as high as 92%, but only if the diets were low in phytate. The results help validate and refine a published saturable transport model that predicts zinc absorption from dietary zinc and phytate. Possible biomarkers of impaired zinc status, including erythrocyte osmotic fragility, in vitro erythrocyte 65Zn uptake, and leukocyte expression of the zinc transport proteins Zip1 and ZnT1, were unresponsive to dietary zinc content.

Conclusions: Humans absorbed zinc more efficiently from low-zinc diets and adapted to further increase zinc absorption after consuming low-zinc, low-phytate diets for several weeks. Such adaptation did not occur with higher phytate diets. Zinc absorption can be predicted from dietary zinc and phytate after allowing for dietary equilibration.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Animal studies have established that body zinc is controlled across a broad range of zinc intakes, from marginal to excessive, by adaptive adjustments in both zinc absorption and excretion (1-3). Humans absorb zinc more efficiently when dietary zinc is low (4-6), but this at least partly reflects a concomitant effect of the dose ingested, rather than a greater expression of transport proteins in response to inadequate zinc intake (7, 8).

An understanding of the relations among dietary zinc intake, bioavailability, and absorption is necessary for making dietary intake recommendations. Current dietary recommendations for zinc rely on factorial estimates of the amounts of absorbed zinc needed to replace zinc excreted from the body daily and the amount of dietary zinc that needs to be absorbed from practical diets to provide that amount of absorbed zinc (9). The relation between dietary zinc and zinc absorption, required for the second part of such analyses, is currently based on mean absorption data from a few small studies in humans conducted by multiple investigators (9-11). As discussed by Miller et al (11), the use of such data may introduce variability related to multiple laboratories, analytic methods, and experimental protocols. Data gathered by using a single method to determine isotopic zinc absorption from practical diets with different zinc and phytic acid contents can help validate models of the relation between dietary zinc and zinc absorption.

The main objectives of the present investigation were to assess adaptation in human zinc absorption under conditions of varied zinc intake and bioavailability and to apply these data to evaluate models predicting zinc absorption from dietary zinc and phytic acid. In addition, we evaluated the effects of dietary zinc intake on several possible biochemical indexes of zinc status.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
Men and women were recruited with local advertising; they met eligibility requirements if they were 21–51 y old and had no apparent disease as indicated by brief questionnaires, interviews, and measures of blood pressure, blood glucose, hemoglobin, creatinine, and liver transaminases. Weight for height [body mass index (BMI; in kg/m2)] was between the 5th and 95th percentiles for the US population of similar age. The women had not been pregnant within the previous year, and they were not lactating. Subjects had not used zinc supplements exceeding 20 mg/d for ≥6 mo, and they agreed to discontinue all nutrient supplements when their applications were submitted, generally 6–12 wk before the beginning of the study. None routinely used medication; however, routine use of hormonal contraceptives or hormone replacement therapy was allowed. The subjects’ characteristics are shown in Table 1Go.


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  		TABLE 1. Subject characteristics1

 
All procedures and potential risks were explained, and all subjects gave written informed consent. This study received approval for human subjects from the University of North Dakota's Institutional Review Board and its Radioactive Drug Research Committee; further approval was obtained from the US Department of Agriculture's Radiological Safety Committee.

Experimental protocols
Zinc absorption was measured initially and again after 4 or 8 wk of equilibration to experimental diets in 3 separate experiments. Subjects did not participate in more than one experiment. Within an experiment, each subject was randomly assigned to consume a single experimental diet, and this diet was used for both zinc absorption measurements. A 4-wk period was required for each measurement of zinc absorption from a 2-d rotating menu with the use of 65Zn radiotracer and whole-body scintillation counting methodology. Accordingly, each subject was scheduled as follows: 2 d of radiolabeled diets, 4 wk of self-selected diets to complete the first absorption measurement, 4 or 8 wk of equilibration with controlled diets ending in 2 d of radiolabeled diets, and 4 wk of self-selected diets to complete the second absorption measurement.

Experiment 1
Zinc absorption by healthy men (n = 19) and women (n = 20) (see Table 1Go) was tested before and after 4 wk equilibration to one of 5 diets differing in zinc concentration. The expected zinc bioavailability of these 5 diets (diets A, B, C, D, and E) was relatively high.

Experiment 2
Zinc absorption by healthy men (n = 23) and women (n = 21) (see Table 1Go) was tested before and after 4 wk equilibration to 1 of 5 diets differing in zinc concentrations. The expected zinc bioavailability of these 5 diets (diets F, G, H, I, and K) was relatively low.

Experiment 3
Zinc absorption by healthy women only (n = 26) (see Table 1Go) was tested before and after 8 wk equilibration to 1 of the same 5 higher-bioavailability diets tested in experiment 1. Experimental diets were randomly assigned; 8 subjects were assigned to each of the 2 diets with the lowest zinc concentrations (diets A and B), and 4 subjects were assigned to each of the 3 diets with higher zinc concentrations (diets C, D, and E). Two of the subjects assigned to diet B dropped out.

Experimental diets
For the higher-bioavailability diets in experiments 1 and 3 (diets A through E; Table 2Go and Table 3Go), different zinc concentrations (4.3–17.0 mg/2500 kcal) were obtained by modifying the types and, to a lesser degree, the serving sizes of meat, poultry, and fish. For diets with the lowest zinc concentrations (diets A and B), a primary source of animal protein was fish, selected from species with relatively low mercury content. Emphasis was on refined sources of carbohydrate that were low in phytic acid and cereal fiber, with the goal of molar phytate-to-zinc ratios (phytate:zinc) of ≤5; however, with both minimal zinc and phytate content, phytate:zinc was slightly >5 at the lowest dietary zinc concentration (Table 2Go).


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  		TABLE 2. Diet composition (per 2500 kcal)

 

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  		TABLE 3. Menus for 5 higher-bioavailability diets (A, B, C, D, and E) varying in zinc content1

 
For the lower-bioavailability diets in experiment 2 (diets F through J; Table 2Go and Table 4Go), different zinc concentrations (6.3–18.2 mg/2500 kcal) were obtained by modifying the amounts of legumes, nuts, and whole grains while limiting animal protein. Soy-based products intended as meat substitutes were used for some meals. Red meat was used only at the highest zinc intake. No more than one-third of protein was from animal sources. Because of the strong positive association between zinc and phytic acid contents of plant-based foods, it is difficult to plan such menus that are both low in zinc and high in phytic acid. Accordingly, the zinc contents were somewhat greater for the lower- than for the higher-bioavailability menus, and a general goal of phytate:zinc ≥15 was barely met at the lowest zinc concentration (Table 2Go).


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  		TABLE 4. Menus for 5 lower-bioavailability diets (F, G, H, I, and J) varying in zinc content1

 
The experimental diets were planned as 2-d rotating menus, using ordinary foods that were not fortified with zinc. During the 4 or 8 wk of equilibration with controlled diets, the subjects ate breakfast at the research center on weekday mornings and took away food for all other meals and weekend days, which was to be consumed elsewhere with minimal re-heating as necessary. Foods were weighed to 1% accuracy and consumed quantitatively. To meet individual energy requirements based on body size and activity, dietary energy was individualized (in 250-kcal increments) by proportionally modifying the amounts of all foods in the diets, from 2000 to 4000 kcal/d. Energy intakes varied considerably between some male volunteers (experiments 1 and 2), depending on the physical labor required for their employment. Tap water and chewing gum were consumed as desired, after analyses indicating minimal zinc content. Limited amounts of salt, coffee, tea, and selected low-energy carbonated beverages were individualized to volunteers’ preferences, but they were not consumed on the days when meals were radiolabeled for absorption measurements.

Zinc absorption measurements
Zinc absorption was measured by using 65Zn radiotracer and whole body scintillation counting. For each measurement, 7.4 kBq (0.2 µCi) 65Zn tracer was extrinsically added to the entire menu (3 meals/d for 2 d; evening snack foods were served with the third meal). The tracer was added to one of the primary sources of zinc in each meal, and the specific activity (ratio of 65Zn to elemental zinc) was constant for all meals. Although dietary energy differed between subjects, in order to maintain body weights, the amounts of energy served with the radiolabeled meals were consistent between the 2 absorption measurements for each subject.

Absorption was determined by serial whole-body scintillation counting with the use of individual retention curves to correct for endogenous 65Zn excretion (12). Whole-body radioactivity was determined before the meals, after the second labeled meal, and twice weekly thereafter. The initial whole-body activity (representing 100% of the administered dose) was calculated from the whole-body activity after 2 meals (before any unabsorbed isotope was excreted), divided by the fraction of the total activity contained in those 2 meals. The percentage absorption was calculated by extrapolating back to the time of isotope administration along the linear portion (days 14–28 after 65Zn administration) of a semilogarithmic retention plot (the natural logarithm of the percentage of remaining radioactivity versus time). The second absorption measurement was corrected for previously administered 65Zn by subtracting the background whole-body radiation measured 1–2 d before the second measurement from all subsequent measurements.

Chemical analyses
Diets and blood were collected while precautions were taken to avoid trace mineral contamination. Diet samples were analyzed for zinc, iron, calcium, and phosphorus content by using inductively coupled argon plasma emission spectrophotometry after nitric acid and hydrogen peroxide digestion. Analytic accuracy was monitored by assaying a typical diet standard (SRM 1548a; US National Institute of Standards and Technology, Gaithersburg, MD), yielding results that were (x ± SD) 100 ± 4% of certified values for zinc, 102 ± 6% for calcium, 100 ± 4% for phosphorus, and 89 ± 7% for iron. Dietary phytate was determined by acid extraction, ion exchange separation, and phosphorus analysis (13) and quantified by assuming 6 mol phosphorus/mol phytic acid.

Fasting blood samples were obtained by phlebotomy initially, at the beginning and end of the controlled diet period, and at the end of the study (4 wk after the controlled diet period), with subsampling on 2 separate days at each of these 4 time-points. Plasma zinc was measured by using inductively coupled argon plasma emission spectrophotometry. Measurements of other possible indicators of zinc nutritional status included extracellular zinc superoxide dismutase (14), mononuclear and plasma 5'nucleotidase (15), alkaline phosphatase (16), bone-specific alkaline phosphastase (17), insulin-like growth factor-1, binding protein 3 (both as Immulite enzyme-labeled chemiluminescent immunometric assays; Diagnostic Products Corp, Los Angeles, CA), erythrocyte osmotic fragility (18), and in vitro erythrocyte 65Zn uptake (19). After RNA extraction from blood leukocytes using the QiaAMP RNA blood mini kit (Qiagen, Valencia, CA), gene expression of the zinc transport proteins Zip1 and ZnT1 was assessed by using real-time–polymerase chain reaction, using a Cepheid Smart Cycler (Sunnyvale, CA) with carboxyfluorescein (FAM)-labeled TaqMan probes and primers (Applied Biosystems, Foster City, CA). Expression of β-actin was used for normalization and relative quantification was calculated by using the 2–{Delta}{Delta}Ct method of Livak and Schmittgen (20).

Statistical analysis
The effects of dietary treatment and equilibration time were determined by using 2-factor repeated-measures analysis of variance, with individual volunteers serving as their own controls. All statistical evaluations were done with the use of SAS software (version 9.1.3; SAS Institute Inc, Cary, NC). Specifically, within each of the 5 dietary treatments, the results from each equilibration time were compared; within a given equilibration time, the 5 dietary treatments were compared pairwise, for a total of 25 Bonferroni contrasts. Analysis of variance results were considered significant if P < 0.05; however, nonsignificant P values between 0.05 and 0.10 were noted when judged to be of interest. Several models were used to evaluate the relation between dietary variables and the amount of zinc absorbed, including a change-point (broken line) model (21), a logit regression model (10), and saturation transport models (11). Univariate and multivariate linear regression models were fitted by using the Reg procedure in SAS, and nonlinear models were fitted by using the NLIN procedure in SAS.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Zinc absorption
As expected, dietary zinc significantly affected fractional zinc absorption in all 3 experiments, with lower zinc intakes resulting in higher fractional absorption (Table 5Go, Table 6Go, and Table 7Go). Greater dietary zinc increased the amount (in mg) of zinc absorbed from the higher bioavailability diets (Tables 5Go and 7Go), but this increase was not significant for the lower-bioavailability diets (Table 6Go). With the higher-bioavailability diets, mean zinc intakes ranging from 4.3 to 18.1 mg resulted in initial zinc absorption from 2.4 to 4.2 (experiment 1; Table 5Go) or 1.8 to 4.6 (experiment 3; Table 7Go) mg/d. With the lower-bioavailability diets, dietary zinc from 6.2 to 21.0 mg/d resulted in initial zinc absorption from 2.4 to 3.1 mg/d (experiment 2; Table 6Go). Comparisons (nonstatistical) between experiments 1 and 2 showed that menus with the lowest zinc content of the higher- and lower-bioavailability diets (4.8 and 6.2 mg/d, respectively) resulted in the initial absorption of similar amounts of zinc (2.4 mg/d) (Tables 5Go and 6Go).


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  		TABLE 5. Men's and women's zinc absorption from the 5 menus with higher bioavailability, before and after 4 wk of dietary equilibration: experiment 11

 

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  		TABLE 6. Men's and women's zinc absorption from the 5 menus with lower bioavailability, before and after 4 wk of dietary equilibration: experiment 21

 

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  		TABLE 7. Women's zinc absorption from the 5 menus with higher bioavailability, before and after 8 wk of dietary equilibration: experiment 31

 
Dietary equilibration for several weeks resulted in changes in fractional zinc absorption only at the lower zinc intakes of the higher bioavailability diets (experiments 1 and 3; Tables 5Go and 7Go). Fractional absorption increased significantly, from 49% to 70%, after 4 wk of consuming the diet containing 4.8 mg Zn/d (experiment 1; Table 5Go). A change in fractional absorption from 40% to 46% was not significant with 7.4 mg Zn/d, and no change was apparent with 10.9, 15.6, or 18.1 mg Zn/d. However, the equilibration-associated changes in absorption at low zinc intakes were not significant when expressed as absolute zinc absorption, because the largest increases in fractional absorption were applied to the lowest absolute zinc intakes (Table 5Go).

Individual data showed that fractional zinc absorption from the higher-bioavailability diets was highly reproducible when zinc intakes exceeded 11 mg/d and generally increased at lower intakes (experiment 1; Figure 1Go A). Some individual increases were substantial; 2 of 5 subjects (a male and a female) consuming <5 mg Zn/d nearly maximized their absorption to 90% and 92%. Similar trends were seen in experiment 3, which included only women (because of their lower energy and accompanying zincintakes) and which involved 8 wk of dietary equilibration (Table 7Go and Figure 1GoB), although 79% was the highest fractional zinc absorption observed.


Figure 1
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  		FIGURE 1.. Fractional zinc absorption by subjects before and after equilibration to controlled zinc intakes. Changes for individual subjects, before ({diamondsuit} and —) and after ({square} and - - -) dietary equilibration, are designated with vertical lines. Absorptive efficiency generally increased at lower zinc intakes, but it was quite reproducible at higher zinc intakes. Amax, maximum absorption; KR, relative binding constant between zinc and transport receptors; TDZ, total dietary zinc. A: Data from experiment 1—n = 39 male and female subjects with 4 wk of dietary equilibration. The data significantly fit the model (modified from equation 1 in the text): %Zn absorption = AMAX/(KR+TDZ), with R2 = 0.40 at week 0 and R2 = 0.65 (n = 39) at week 4. B: Data from experiment 3—n = 26 female subjects with 8 wk of dietary equilibration. The data significantly fit the model: %Zn absorption = AMAX/(KR+TDZ), with R2 = 0.36 at week 0 and R2 = 0.68 (n = 26) at week 8.

 
Neither fractional nor absolute zinc absorption changed significantly after 4 wk of equilibration with any of the lower-bioavailability diets (experiment 2; Table 6Go). Individual zinc absorption from the lower-bioavailability diets did not exceed 50% (data not shown). Although the similar initial mean absorption of {approx}2.4 mg Zn from both diet A (higher bioavailability), and diet F (lower bioavailability; see Tables 5Go and 6Go) may have provided the same biological stimulus to increase absorptive efficiency, an increase in absorption was observed only with the higher-bioavailability diet.

Modeling to predict zinc absorption from dietary intake
The relation between zinc absorption and zinc intake was modeled with the univariate saturation transport model based on the Michaelis-Menten equation:

Formula 1(1)
where TAZ is total absorbed zinc and TDZ is total dietary zinc, and the coefficients fitted by regression analysis, AMAX and KR, describe the maximum absorption and a relative binding constant between zinc and transport receptors, respectively. All are expressed in millimoles. This model significantly fit the zinc absorption measurements both before (not shown) and after dietary equilibration for each of the 3 experiments. Using the equilibrated data, model coefficients (and 95% CIs) were consistent for experiments 1 and 3, which tested the higher-bioavailability diets (Table 8Go). The model coefficients were less with the lower-bioavailability diets (Table 8Go) and were minimally affected by dietary equilibration (not shown). The models after 4 wk of equilibration with the higher- and lower-bioavailability diets (experiments 1 and 2) are plotted in Figure 2Go); the equation coefficients are expressed in millimoles, but, for all figures, zinc intake and absorption have been converted to milligrams (1 mmol = 65.4 mg Zn).


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  		TABLE 8. Univariate models of the relation between dietary zinc and absolute zinc absorption1

 

Figure 2
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  		FIGURE 2.. The relation between dietary zinc and absolute zinc absorbed, modeled separately for the higher- (; experiment 1, n = 39) and lower- (—; experiment 2, n = 44) bioavailability diets after 4 wk of dietary equilibration. Univariate saturation response models significantly fit the data, as described in equation 1 in the text. The saturation response model for the higher-bioavailability diet (after 4 wk of equilibration) is similar to the model used to derive the Dietary Recommended Intakes [–– –(9)], as described in equation 2 in the text.

 
At a dietary intake of <11 mg Zn/d, the model for the higher-bioavailability diet was nearly identical to the asymptotic regression model, as calculated by using the following equation:

Formula 2(2)
(with dietary zinc and zinc absorbed expressed in mg), which was used in the derivation of the current Dietary Recommended Intakes (DRIs) for zinc (Figure 2Go; LD Meyers, personal communication, 2006), and which was based on 10 data points from 7 studies (9). This similarity to the DRI model applied only to absorption measured after 4 wk of dietary equilibration. Without dietary equilibration, the lower zinc absorption associated with lower zinc intakes would inflate estimates of dietary requirements by {approx}2 mg/d to achieve similar zinc absorption (data not shown). The DRI model was limited to data from male subjects. The inclusion of both sexes from experiment 1 matched the DRI model better than did sex-specific modeling, which included half the sample sizes (data not shown) and also matched the model for experiment 3, which included women only (see Table 8Go for coefficients). Therefore, the current measurements of zinc absorption from higher-bioavailability diets, after 4 or 8 wk of equilibration, appeared to be fully consistent with the dietary zinc absorption model used in setting the DRIs.

The present data (all data with 4 wk of dietary equilibration, from experiments 1 and 2 combined) were also tested to obtain the best fit for 2 multivariate models that predict zinc absorption from both dietary zinc and dietary phytic acid. These were a logit regression model proposed by the International Zinc Nutrition Consultative Group [IZiNCG (10)] and a saturation model proposed by Miller et al (11), which were derived by using 15 and 21 data-points, respectively, that represented mean absorption measurements from multiple studies and investigators.

The IZiNCG logit regression model, as given in the following equation,

Formula 3(3)
initially appeared to fit the present data [R2 = 0.61, n = 83, with coefficients (95% CI) of b0 = 2.75 (2.11, 3.40), b1 = –1.26 (–1.50, –1.02), and b2 = –0.26 (–0.38, –0.14)]. However, we found this model to be unsatisfactory because the assumed linear relation between logit(fractional zinc absorption) and ln(phytate:zinc) (P:Zn in equation) was not well met with the present data. A further concern was the model's prediction, at a higher ratio of phytate to zinc, that zinc absorption maximized and then slightly decreased, rather than continuously increasing with dietary zinc intake. Compared with the published IZiNCG model, the logit regression model fitted to the present data predicted substantially less change in zinc absorption as dietary zinc increased (Figure 3GoA): 30–55% more zinc was absorbed at low zinc intakes (5 mg), and {approx}25% less zinc was absorbed at high zinc intakes (25 mg) within the present range of phytate:zinc ratios (2-20).


Figure 3
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  		FIGURE 3.. Zinc absorption measurements after 4 wk of dietary equilibration (experiments 1 and 2) were applied to 2 published multivariate models predicting zinc absorption from dietary zinc and phytic acid contents. Each model is graphed as originally published (–– –) and as fitted to the present data (—) for phytate-to-zinc (P:Zn) ratios of 2 and 20. A: The International Zinc Nutrition Consultative Group model (10); B: the model of Miller et al (11).

 
The multivariate saturation model of Miller et al (11), as given in the following equation,

Formula 4(4)
where TDP is total diet phytate (all in millimole units), also fit the present data (R2 = 0.44; n = 83). The regression coefficients of AMAX (0.11; 95% CI: 0.08, 0.14), KR (0.06; 95% CI: 0.01, 0.11), and KP (1.46; 95% CI: 0.32, 2.60) were consistent with, but more precise than those originally published. Specifically, only 1 of the 3 model coefficients derived by Miller et al (11) differed significantly from zero, whereas, with the present data, all 3 coefficients did so. Differences between the published (11) and the newly fitted model increased with phytate:zinc (Figure 3GoB). At a phytate:zinc of 20, the newly fitted model predicted {approx}25% greater absorption than did the published model. Differences between the newly fitted and the published model were modest at a phytate:zinc of 2. Also at a phytate:zinc of 2, the zinc absorption predicted by the newly fitted multivariate model (Figure 3GoB) was nearly identical to both the univariate model from experiment 1 and the current DRI model (both in Figure 2Go). The residuals of the multivariate saturation model were not associated with differences in dietary protein or calcium, and the modeling was not improved by expressing the amount of zinc absorbed per kg body wt.

Biochemical indexes of zinc status
These experiments with controlled diets and marginally-to-moderately high zinc content provided an opportunity to assess possible biomarkers of zinc status in addition to absorption. Differences in dietary zinc intake for 4 or 8 wk did not significantly affect plasma zinc concentration. To further evaluate a possible relation between plasma zinc and zinc absorption, the influence of plasma zinc and of dietary zinc as variables predictive of zinc absorption was tested by multiple regression analysis. Even after adjustment for the substantial influence of dietary zinc on zinc absorption, plasma zinc explained little of the variation in zinc absorption; it slightly but significantly contributed as a predictor of absorption for both meals in experiment 1 (partial R2 = 0.07 and 0.03, respectively), but not in experiments 2 and 3.

Dietary zinc intakes did not significantly alter other blood indexes, including extracellular zinc superoxide dismutase and mononuclear and plasma 5' nucleotidases (tested in experiments 1 and 2 only); alkaline phosphatase; bone-specific alkaline phosphatase; and insulin-like growth factor-1, binding protein 3, erythrocyte osmotic fragility, and in vitro erythrocyte 65Zn uptake (experiment 3 only for the last 4). Zinc intake also did not significantly affect gene expression of the zinc transport proteins Zip1 or ZnT1 in blood leukocytes (experiment 3 only). Moreover, neither the difference nor the ratio of change in zinc absorption between 0 and 8 wk correlated with the expression of these zinc transporters in leukocytes.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This is the first time that the same experimental diets have been used to distinguish between the shorter- and longer-term effects of the amount of ingested zinc on fractional absorption. The immediate inverse effect of the amount of ingested zinc on fractional zinc absorption was recognized by Sandstrom et al (7) and Sandstrom and Cederblad (8). This short-term effect has confounded the interpretation of longer-term observations of increased fractional absorption with zinc depletion (4-6, 22). For instance, an increase in fractional absorption from 25% of 16.4 mg dietary Zn to 49–53% after 13–42 d of 5.5 mg dietary Zn, as reported by Wada et al (4), does not necessarily indicate adaptation, because these values are similar to the results of the present study before any dietary equilibration (Tables 5Go and 7Go). The present results show that humans adaptively increase fractional zinc absorption, beyond the immediate and substantial influence of the ingested zinc dose, when consuming low-zinc diets.

The observed adaptation in zinc absorption from the low-zinc, higher-bioavailability diets appeared to occur within 4 wk (experiment 1; Figure 1Go); the degree of adaptation was no greater after 8 wk in other subjects (experiment 3; Figure 2Go). This observation is consistent with the report of Lee et al (6) that men's zinc absorption did not change between 2, 4, and 6 mo of consuming 4.1 mg dietary Zn. Istfan et al (23), who repeatedly tested men's absorption of a single dose of 1.2 mg Zn, reported consistent absorption between days 2 and 10 of consuming a formula diet with 15 mg Zn/d and, after the diet was reduced to <2 Zn mg/d on day 12, a consistently greater absorption on days 18 and 28. For the higher-bioavailability diets in the present study, dietary equilibration influenced the modeled relation between zinc intake and zinc absorption. It is fitting that the model used for the current DRI recommendations (9) best matched the present data after 4 wk (Figure 3Go), rather than after no dietary equilibration, as the DRIs are for ongoing dietary intakes.

With the higher-bioavailability diets, subjects up-regulated fractional zinc absorption after equilibration to the lowest zinc intakes. Absorptive efficiency generally increased if dietary zinc was below {approx}11 mg/d, corresponding to absorption <4.1 mg/d (Figure 1Go, Table 5Go), but remained remarkably constant with zinc intakes from 11 to 25 mg/d (Figure 1Go). For subjects with the lowest zinc intakes, the increase in absorption to as high as 92% suggests near-maximal biological adjustment to absorb zinc, and it likely indicates that intake was inadequate. These data did not suggest a cutoff <11 mg/d that was clearly indicative of inadequacy, and the more moderate observed changes may represent appropriate adaptation to zinc intakes that were slightly less than customary. In a recent study whose results are complementary to those of the the present study (24), our group observed that women adapted to down-regulate zinc absorption from low-zinc diets (3–6 mg/d) when supplemented with 42 or 27 but not with 9 mg Zn/d; absorption with the 3 diets was 9.2, 8.9, and 4.6 mg/d initially and 5.0, 4.8, and 4.8 mg/d after 16 wk of equilibration, respectively. From these 2 sets of data, we may speculate that adults on higher-bioavailability diets will biologically adjust upward or downward to absorb 4–5 mg Zn/d.

The present results add substantially to the limited data on human zinc absorption from diets (lasting ≥1 d, rather than single meals) with a phytate:zinc ≥ 15. They are consistent with reports 1) that US women absorbed 30% of 6.7 mg dietary Zn from a low meat diet with a phytate:zinc of 15 (12) and 26% of 9.1 mg dietary Zn from a vegetarian diet with a phytate:zinc of 18 (25) and 2) that pregnant Ethiopian women absorbed 35% of 6 mg dietary Zn from diets with a phytate:zinc of 17 (26). Less consistent with the present results are reports of men's zinc absorption from more unusual diets thate contain maize only (17% of 5 mg dietary Zn with a phytate:zinc of 36 and 30% of 4.3 mg with a phytate:zinc of 17) (27) or that are based on EDTA-washed soy-protein (60% absorption of 4.1 mg dietary Zn with a phytate:zinc of 21) (6).

The results suggest minimal ability for humans to adaptively increase zinc absorption from diets high in phytic acid. We are not aware of comparable findings, because most absorption studies have used a lower phytate:zinc, and none have measured zinc absorption before and after equilibration to the same diets. The results suggest that with a phytate:zinc >15–20, the unbound zinc available to absorptive transporters may be insufficient for biological up-regulation to increase zinc absorption.

Applying the multivariate saturation model of Miller et al (11) with the coefficients derived by using the present data and the requirements for absorbed zinc of 3.3 mg for women and 3.84 mg for men estimated for Canada and the United States by the Food and Nutrition Board (9), diets would meet requirements with 7.1 and 9.0 mg Zn for women and men, respectively, if the phytate:zinc is 2, which may be typical of many Canadian and US diets. However, 9.2 and 13.4 mg Zn for women and men, respectively, would be needed if the phytate:zinc increased to 8, which we estimate would occur with current US recommendations that include greater consumption of whole grains [estimated by calculations using MyPyramid menus (28)]. By extrapolation of the model, 22 and 140 mg would be needed for women and men, respectively, if the phytate:zinc is 20, as may be seen in international settings. These numbers suggest that zinc fortification or supplementation is necessary if the ratio of phytate to zinc cannot be reduced by dietary modifications, such as the addition of animal sources of zinc, the reduction of phytate by means such as fermentation, or both (29). A goal of a phytate:zinc ≤ 12 would enable the requirements for absorbed zinc to be met with 11 and 19 mg Zn/d for women and men, respectively—amounts that are achievable with unsupplemented diets.

Caution is warranted in applying predictive models. The above intake estimates are based on the absorbed amounts recommended by the Food and Nutrition Board (9), and they would be reduced by using the lower absorbed amounts (1.86 mg for women and 2.69 mg for men) recommended for international use by IZiNCG (10). With the factorial approach used by both groups, small changes in the models of zinc absorption versus zinc intake can readily change the recommended intakes by 20%. For instance, for women to absorb 3.3 mg Zn/d from a diet with a phytate:zinc of 2, the recommended dietary zinc would be 8.0 mg/d according to the model of the IZiNCG (10), 5.4 mg/d according to the IZiNCG model fitted to the present data, 8.4 mg/d according to the model of Miller et al (11), 7.1 according to the model of Miller et al fitted to the present data, or 6.8 mg/d according to the univariate model used in DRI derivations of the current estimated average requirement (9). Although the present data did not appropriately fit the IZiNCG model (10), they helped refine the coefficients and confirm the fit of the more biologically descriptive model of Miller et al (11). Considering the limitations of applying the latter, relatively simple transport model based on Michaelis-Menten kinetics to the daily average of a meal-based intermittent zinc intake, this newly fitted model appeared to provide useful predictions of zinc absorption based on zinc and phytate intakes. Our results suggest that data from both men and women can be included in such modeling.

Although they were not the primary focus of this study, controlled zinc intakes for 4–8 wk provided an opportunity to test potential biological markers of zinc intake and status. Low-zinc diets under the present conditions (ie, consumed by healthy Western volunteers for 4–8 wk) did not significantly affect any of the biochemical markers tested. Others have reported that plasma zinc is insensitive to zinc intakes within this range (4, 30), and the Food and Nutrition Board ruled out plasma zinc as a useful status indicator for evaluating human zinc requirements for Canada and the United States (9). Zinc metalloenzymes that may be influenced by more severe zinc deficiency in animal studies have not been clearly shown to be reliable, sensitive indexes of human zinc intakes (9, 10). The discrepancy between animal and human observations also applies to potential indexes of biochemical function, such as erythrocyte osmotic fragility (18) and in vitro erythrocyte 65Zn uptake (19).

The more recently identified zinc transport proteins have received less testing as potential zinc status indexes. On the basis of the responsiveness of ZnT1 and Zip1 expression to zinc concentrations in cultured lymphoblastoid cells, Andree et al (31) tested the effect of supplementation with 22 mg Zn/d for 27 d on women's lymphocyte expression of these transporters. Zip1 expression decreased by {approx}20% with zinc supplementation, but ZnT1 expression was unaffected (31). In the present study of apparently healthy volunteers, neither Zip1 nor ZnT1 expression was influenced by controlling dietary zinc between 4.3 and 17 mg/2500 kcal for 8 wk.

In conclusion, absorptive efficiency is inversely related to the amount of ingested zinc, and, if the diet is low in phytic acid, absorption is up-regulated in response to extended low zinc intakes. Daily zinc and phytate intakes are major predictors of zinc absorption, which can be quantitatively predicted with multivariate models based on saturable transport kinetics, such as that proposed by Miller et al (11). We found the IZiNCG model (10) to be less useful. The model of Miller et al (11), with coefficients modified by using the data of the present study, suggests that diets with a phytate:zinc >{approx}12 do not provide an amount of absorbed zinc that meets the mean physiologic requirement for absorbed zinc estimated by the Food and Nutrition Board (9).


   ACKNOWLEDGMENTS
 
We gratefully acknowledge the valuable contributions of other members of our human studies research team. Bonnie Hoverson supervised the planning and preparation of the experimental diets, Emily J Nielsen managed the volunteer recruitment and coordination, Sandy K Gallagher supervised the clinical laboratory analyses, Carol Zito managed the food radiolabeling, Jackie Nelson conducted the whole-body counting in the volunteers, Bill Siders conducted the body-composition testing, Lana DeMars consulted on real-time–polymerase chain reaction measurements, and Glenn I. Lykken consulted on the use of the whole-body counter. We are also grateful to Linda D Meyers of the Food and Nutrition Board for providing information about the zinc absorption model used in derivation of the current Dietary Reference Intakes for zinc and to Leland Miller for information about the derivation of his model.

The authors’ responsibilities were as follows—JRH: initiated, planned, and conducted the experiments; JMB: contributed to planning, conducting, and analyzing experiment 3, including measuring the expression of zinc transport proteins; LKJ: conducted the statistical analyses and mathematical modeling; and JRH: wrote the manuscript, with review and input from all authors. None of the authors had any personal or financial conflict of interest.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Weigand E, Kirchgessner M. Total true efficiency of zinc utilization: determination and homeostatic dependence upon the zinc supply status in young rats. J Nutr 1980;110:469–80.[Abstract/Free Full Text]
  2. Hunt JR, Johnson PE, Swan PB. Influence of usual zinc intake and zinc in a meal on 65Zn retention and turnover in the rat. J Nutr 1987;117:1427–33.[Abstract/Free Full Text]
  3. Johnson PE, Hunt JR, Ralston NVC. The effect of past and current dietary Zn intake on Zn absorption and endogenous excretion in the rat. J Nutr 1988;118:1205–9.[Abstract/Free Full Text]
  4. Wada L, Turnlund JR, King JC. Zinc utilization in young men fed adequate and low zinc intakes. J Nutr 1985;115:1345–54.[Abstract/Free Full Text]
  5. Taylor CM, Bacon JR, Aggett PJ, Bremner I. Homeostatic regulation of zinc absorption and endogenous losses in zinc-deprived men. Am J Clin Nutr 1991;53:755–63. (Published erratum appears in Am J Clin Nutr 1992;56:462.)[Abstract/Free Full Text]
  6. Lee DY, Prasad AS, Hydrickadair C, Brewer G, Johnson PE. Homeostasis of zinc in marginal human zinc deficiency—role of absorption and endogenous excretion of zinc. J Lab Clin Med 1993;122:549–56.[Medline]
  7. Sandström B, Arvidsson B, Cederblad A, Bjorn-Rasmussen E. Zinc absorption from composite meals. I. The significance of wheat extraction rate, zinc, calcium, and protein content in meals based on bread. Am J Clin Nutr 1980;33:739–45.[Abstract/Free Full Text]
  8. Sandström B, Cederblad A. Zinc absorption from composite meals. II. Influence of the main protein source. Am J Clin Nutr 1980;33:1778–83.[Abstract/Free Full Text]
  9. Food and Nutrition Board: Institute of Medicine. Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. Washington, DC: National Academy Press, 2001.
  10. Brown KH, Rivera JA, Bhutta Z, et al; International Zinc Nutrition Consultative Group. Assessment of the risk of zinc deficiency in populations and options for its control. Food Nutr Bull 2004;25(suppl):S99–203. [International Zinc Nutrition Consultative Group (IZiNCG) technical document #1.][Medline]
  11. Miller LV, Krebs NF, Hambidge KM. A mathematical model of zinc absorption in humans as a function of dietary zinc and phytate. J Nutr 2007;137:135–41.[Abstract/Free Full Text]
  12. Hunt JR, Gallagher SK, Johnson LK, Lykken GI. High- versus low-meat diets: effects on zinc absorption, iron status, and calcium, copper, iron, magnesium, manganese, nitrogen, phosphorus, and zinc balance in postmenopausal women. Am J Clin Nutr 1995;62:621–32.[Abstract/Free Full Text]
  13. Phytate in foods (method 986.11). In: Helrich K, ed. AOAC official methods of analysis. 15th ed. Arlington, VA: Association of Official Analytical Chemists, 1990:800–1.
  14. Marklund S, Marklund G. Involvement of the superoxide anion in the autooxidation of pyrogallol and a convenient assay for superoxide dismutase. Eur J Biochem 1974;47:469–74.[Medline]
  15. Arkesteijn CL. A kinetic method for serum 5'-nucleotidase using stabilised glutamate dehydrogenase. J Clin Chem Clin Biochem 1976;14:155–8.[Medline]
  16. Tietz NW, Burtis CA, Duncan P, et al. A reference method for measurement of alkaline phosphatase activity in human serum. Clin Chem 1983;29:751–61.[Abstract/Free Full Text]
  17. Gomez B Jr, Ardakani S, Ju J, et al. Monoclonal antibody assay for measuring bone-specific alkaline phosphatase activity in serum. Clin Chem 1995;41:1560–6.[Abstract/Free Full Text]
  18. O'Dell BL, Browning JD, Reeves PG. Zinc deficiency increases the osmotic fragility of rat erythrocytes. J Nutr 1987;117:1883–9.[Abstract/Free Full Text]
  19. Van Wouwe JP, Veldhuizen M, De Goeij JJM, van den Hamer CJA. In vitro exchangeable erythrocytic zinc. Biol Trace Ele Res 1990;25:57–69.
  20. Livak KJ, Schmittgen TD. Analysis of relative gene expression data using real-time quantitative PCR and the 2(-Delta Delta C(T)) method. Methods 2001;25:402–8.[Medline]
  21. Bacon DW, Watts DG. Estimating the transition between two intersecting straight lines. Biometrika 1971;58:525–34.[Abstract/Free Full Text]
  22. August D, Janghorbani M, Young VR. Determination of zinc and copper absorption at three dietary Zn-Cu ratios by using stable isotope methods in young adult and elderly subjects. Am J Clin Nutr 1989;50:1457–63.[Abstract/Free Full Text]
  23. Istfan NW, Janghorbani M, Young VR. Absorption of stable70 Zn in healthy young men in relation to zinc intake. Am J Clin Nutr 1983;38:187–94.[Abstract/Free Full Text]
  24. Beiseigel JM, Klevay LM, Hunt JR. Healthy postmenopausal women adapt to reduce zinc absorption in response to zinc supplementation. FASEB J 2006;20:A985 (abstr).[Free Full Text]
  25. Hunt JR, Matthys LA, Johnson LK. Zinc absorption, mineral balance, and blood lipids in women consuming controlled lactoovovegetarian and omnivorous diets for 8 wk. Am J Clin Nutr 1998;67:421–30.[Abstract]
  26. Hambidge KM, Abebe Y, Gibson RS, et al. Zinc absorption during late pregnancy in rural southern Ethiopia. Am J Clin Nutr 2006;84:1102–6.[Abstract/Free Full Text]
  27. Adams CL, Hambidge M, Raboy V, et al. Zinc absorption from a low-phytic acid maize. Am J Clin Nutr 2002;76:556–9.[Abstract/Free Full Text]
  28. US Department of Agriculture. MyPyramid.gov: steps to a healthier you. Sample menus for a 2000 calorie food pattern. Internet: http://www.mypyramid.gov/downloads/sample_menu.pdf (accessed 5 October 2007).
  29. Gibson RS, Hotz C. Dietary diversification/modification strategies to enhance micronutrient content and bioavailability of diets in developing countries. Br J Nutr 2001;85(suppl):S159–66.[Medline]
  30. Johnson PE, Hunt CD, Milne DB, Mullen LK. Homeostatic control of zinc metabolism in men: zinc excretion and balance in men fed diets low in zinc. Am J Clin Nutr 1993;57:557–65.[Abstract/Free Full Text]
  31. Andree KB, Kim J, Kirschke CP, et al. Investigation of lymphocyte gene expression for use as biomarkers for zinc status in humans. J Nutr 2004;134:1716–23.[Abstract/Free Full Text]
Received for publication October 5, 2007. Accepted for publication January 8, 2008.




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J. R Hunt and J. M Beiseigel
Dietary calcium does not exacerbate phytate inhibition of zinc absorption by women from conventional diets
Am. J. Clinical Nutrition, March 1, 2009; 89(3): 839 - 843.
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