Changes in regulation
of human
zinc metabolism
with age
M. E. WASTNEY, S. AHMED, AND R. I. HENKIN Departments of Pediatrics and Family and Community Medicine, Georgetown University Medical Center, Washington, DC 20007; and Taste and Smell Clinic, Washington, DC 20016 Wastney, M. E., S. Ahmed, and R. I. Henkin. Changes in regulation of human zinc metabolism with age. Am. J. Physiol. 263 (Regulatory Integrative Comp. Physiol. 32): R13.62R1168, 1992.-To assess changes in zinc metabolism with age, kinetic studies were performed in healthy adults (26 men, 21 women) aged 20-84 yr after a single oral or intravenous bolus of 65Zn Studies covered two consecutive 9-mo periods while subjects’were on a basal dietary intake of - 10 mg Zn/day and while taking an additional 100 mg Zn/day orally. Zinc metabolism was analyzed by compartmental analysis using data from plasma, red blood cells, urine, feces, liver, thigh, and whole body [M. E. Wastney, R. L. Aamodt, W. F. Rumble, and R. I. Henkin. Am. J. Physiol. 251 (Regulatory Integrative Comp. Physiol. 20): R398R408, 19861. Changes in observed and model calculated values of zinc metabolism were assessed on age by regression. During basal state, zinc release from red blood cells decreased with age. During zinc loading, response (defined as change from basal state) of plasma zinc concentration, urinary zinc excretion, and liver zinc increased with age, while response of fraction of zinc taken up by red blood cells decreased with age. In men, response of amount of zinc absorbed increased with age and in women response of fraction of endogenous zinc excreted decreased with age. Four responses that changed with age (urinary excretion, red blood cell exchange, absorption, and endogenous excretion) occurred at previously defined sites of regulation of zinc metabolism. Results show that regulation of zinc metabolism changes with age. zinc kinetics; modeling
aging; trace elements;
compartmental
analysis;
ZINC METABOLISM of humans may be compromised with aging for two reasons: 1) zinc intake of older subjects is often low (IS), and 2) zinc absorption decreases with age (2, 48). Zinc is considered important with respect to aging because zinc restores function of some older tissues. Bone mineralization in aging rats is stimulated by zinc (52), and in humans macular degeneration is reported to be retarded by zinc administration (38). Immune function of aged spleen cells is restored by zinc (51), and in one study immune function of some elderly subjects was improved by zinc administration (15), although this was not found in another study (11). Age-related changes in zinc metabolism occur in various tissues. Zinc uptake by adipocytes from older rats is less than that of younger animals (43). Zinc uptake by human fetal fibroblasts declines with successive generations (42) and zinc concentration of human lens tissue decreases with age (44). Based on these and other observations, intracellular zinc deficiency has been proposed as a factor in the aging process (22). Although zinc appears to be important in aging, specific changes in human zinc metabolism with age have not been defined. To assesschanges during aging it is necessary to describe zinc metabolism in subjects over a wide age range. Human zinc metabolism has been described previously based on kinetic studies and compartmental modeling (5, 50). By comparing data from basal R1162
0363-6119/92
$2.00 Copyright
0
and zinc loading states these and other studies have shown that long-term zinc metabolism is regulated at five sites: absorption from gut, excretion into urine, secretion into gut (5, 30), exchange of zinc with red blood cells, and release of zinc from muscle (50). In the present study the kinetics of zinc metabolism in humans, described over a wide age range (20-84 yr) (50), are compared to determine sites of zinc metabolism that change with age. METHODS Subjects. Forty-seven subjects (26 men, 21 women) were studied; data from 32 of these subjects were described previously (50). Subjects were healthy, did not have either acute or chronic disease, and were not taking any medication. Age distribution by year and gender is shown in Fig. 1 and subject descriptions are given in Table 1. Lean body mass was derived from measurements of whole body 40K (20). Measurements were made in the 2-pi scintillation counter of the National Institutes of Health, Bethesda, MD. Total body 40K was measured in a copper-steel shielded scintillator into which the whole subject was placed. Counts were obtained for 500 s before initiation of the study and for 200 s before every measurement of total body zinc. The isotopes 65Zn and 40K were distinguished by energy differences using a multichannel analyzer. Dietary zinc intake was determined from 3-day dietary records that were analyzed by a registered dietitian using USDA Handbook 8 and additional published sources of dietary data (21). Diets were reviewed in personal interviews with each subject to ensure completeness and accuracy using food models as necessary to determine food amounts. Experimental protocol. The protocol has been described previously in detail (50). Subjects were studied during two consecutive 9-mo periods while maintaining their normal lifestyle; for the first period subjects consumed their regular diet containing -10 mg Zn/day, and for the second period they took an additional 100 mg Zn/day orally. At the start of the first period (n = 7) or orally (n subjects were given 6”Zn either intravenously = 40) after an overnight fast. Data (65Zn activity) were obtained by sampling plasma, red blood cells, urine, and feces and by external counting over liver, thigh, and whole body during the first and second (feces excepted) periods. Chemical analyses. Activity of 65Zn was determined in whole body, over liver and thigh regions, and in plasma, red blood cell, urine, and fecal samples by gamma-ray spectroscopy (1). Zinc concentration in plasma, red blood cells, and urine was determined by atomic absorption spectrophotometry as previously described (50). Kinetic analyses. Kinetic analyses were undertaken using the SAAM/CONSAM software (6, 7). For each subject tracer data from all tissues (i.e., plasma, red blood cells, liver, thigh, whole body, urine, and feces) were fitted simultaneously by a previously developed compartmental model for zinc metabolism (Fig. 2) (50). The model consists of 11 compartments that represent zinc in various forms or tissues. Compartment 23 represents a site of rapid zinc absorption high in gut and accounts for firstpass uptake of absorbed zinc by liver. Compartment 24 represents the main site of zinc absorption lower in gut. Compartment 25 represents a delay before zinc is excreted in feces. Compartment 15 represents tissue zinc that turns over in -2
1992 the American
Physiological
Society
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (138.026.031.003) on November 20, 2018. Copyright © 1992 the American Physiological Society. All rights reserved.
ZINC li’~‘l’
IIll
III1
III1
III1
III1
METABOLISM
III1
WOMEN
q
moo
cJn
00
0
0
q
0
q
0
unm
MEN 0
0
m3mo
IllI
IO
00
0
IllI
zo
m
000
1111
30
0
0
1’11
UIDO
‘I”
50
40
0
0
“‘1
60
0
1’11
70
1111
80
90
AGE (YEARS)
Fig. 1. Age distribution
of women
(n = 21) and men
(n = 26).
Table 1. Subject description Subjects Parameter All Age,
yr
Body weight, kg Lean body mass, kg Basal Zn intake, mg/day Plasma Zn, mg/l Basal Zn loading Urine Zn, mg/day Basal Zn loading
Men
Women
45.5k19.6 69.7H3.2 52.7Hl.8 9.0t4.5
42.1t18.5 76.0tl2.3* 63.lt7.6* ll.lt5.2*
49.6t20.6 62.0&9.9-t 43.9+6.4-j6.6kl.5.f
0.86t0.10 1.59t0.29
0.90t0.10* 1.64kO.26
0.81+0.07”f 1.54t0.31
0.40t0.18 1.90t0.98
0.48t0.18* 2.12tl.14
0.3OkO.14”r 1.67kO.76
Values are means t SD. Subjects (26 men, 21 women) consumed their normal diet (-10 mg Zn/day) for 9 mo in basal state; of these subjects, 17 men and 16 women took an additional 100 mg Zn/day orally for a further 9 mo during Zn loading. Values with different superscripts differ significantly between men and women [P < 0.05, based on t test or t test for unequal variances if variances between groups differed significantly, (P < 0.05, F test)].
days and, based on detailed analyses (50), includes plasma, 20% of zinc in red blood cells, and 80% of zinc in liver. Compartment 6 represents 80% of zinc in red blood cells and turns over in -9 days. Compartments 11 and 22 represent zinc in other tissues that turns over in 2 and 48 days, respectively. Compartment 3 represents muscle and compartment 7 represents bone. Zinc loss is represented by excretion into urine (compartment 12) and excretion into feces (compartment 4). Because liver and thigh data were obtained by external counting, liver data were fitted by adding fractions of compartments 3 (8%), 6 (21%), 15 (92%),
(.013i.O08)
AND
AGING
R1163
and 22 (9%) and thigh data were fitted by adding fractions of compartments 3 (17%), 7 (21%), 11 (13%), 15 (1.4%), and 22 (7%) (50). Kinetic data obtained during zinc loading were fitted by adjusting values of five parameters representing five sites of zinc regulation (50). They were absorption [L(15,23) and L(15,24)] secretion [L(24,15)], urine excretion [L(12,15)], red blood cell uptake [L(6,15)], and release from muscle [L(15,3)] (Fig. 2). Calculation of parameters of zinc metabolism. All kinetic data of each subject were fitted by the model to calculate fractional rate constants, L(i,j) and transport rates, R(ij) (terms and equations are defined in Table 2). Compartment masses of zinc, M(i), and zinc entry into the body, U(23), were calculated using plasma zinc content and rate constants of the model. Absorption (fraction/day) was calculated by adding fractions absorbed from the two sites in gut. Rate of zinc absorption (mg/day) was calculated by multiplying fractional zinc absorption by zinc entry into compartment 23 (Table 2). Fractional endogenous excretion was calculated by multiplying secretion into gut by fraction excreted into feces (1 - cy2) (Table 2). Rate of endogenous zinc excretion was calculated by multiplying fractional endogenous excretion by mass of zinc in the compartment containing plasma, compartment 15 (Table 2). StatisticaL analyses. Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS, Chicago, IL). Zinc metabolism was assessed on age by regression analysis. To identify which parameters changed with age, values of each parameter, from all subjects, were fitted against age by linear regression. Change with age was significant if slope of the regression line differed from zero (P < 0.05, F test). Some parameters changed value during zinc loading and, because loading and basal values were not independent, the % change from basal (defined as “response” to zinc loading) was assessed on age by regression analysis. To determine degree of change with age, regression equations were solved using upper and lower limits of ages studied (20 and 80 yr, defined as a “lifetime”). The percent change in parameter value in basal state and difference in response to zinc loading were determined for 20 vs. 80-yr-old subjects. RESULTS
Results of regression analyses of zinc metabolism on age are given in Table 3. For parameters that change significantly with age, regression equations and the square of the correlation coefficients are given in Tables
Fig. 2. A compartmental model of zinc metabolism in humans (50). Terms are defined in Table 2. Large numbers in the circles are compartment numbers, small numbers are mass of Zn (mg) on basal Zn intake of -10 mg/day. Values in parentheses were calculated during Zn loading when Zn intake was lo-fold higher. Values next to arrows are fractional rates of Zn movement between compartments (means & SD, fraction/day). Values of parameters that changed during Zn loading are given in parentheses. *Site of tracer entry into the gut; double arrow represents calculated Zn intake.
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Rl164
ZINC
Table 2. Kinetic
METABOLISM
terms and equations
General terms General equation G(i) L (i, j) Fraction of compartment j transferred into compartment i per unit time Mass of compartment i M(i) R(i, j) Transport of mass from compartment j into compartment i per unit time, [R (i, j) = L( i, j) X M(j)] Steady-state input into compartment i per unit time U(i) Specific terms and equations for model in Fig. 2 G(81) Fraction of compartment 15 in liver G(91) Fraction of compartment 15 in thigh U(23) Oral zinc intake (mg/day) into compartment 23 Absorption (fraction/day) from the first site in the gut
a1 = L(15, 23)/[L(15, from
the second
AGING
Table 3. Probabilities of regression of parameter of zinc metabolism with age
Observed Body weight Lean body mass Diet Zn Plasma Zn Urine Zn Calculated a1
23) + L(24, 23)]
site in the gut
= L(15, 24)/[L(15,
AND
24) + L(25, 24)]
fro:
both sites A = a1 + (1 - q) x ct!2 Secretion from plasma into gut (fraction/day) = L(24, 15) Fractional endogenous excretion from plasma into feces (fraction/day) = secretion X (1 - a2) Zinc absorbed (mg/day) = absorption X U(23) Endogenous zinc excretion (mg/day) = fractional endogenous excretion x M(15)
4 and 5. Changes in zinc metabolism between 20 and 80 yr are summarized in Table 6. Changes with age during basal zinc intake. No significant changes with age were determined for body weight, lean body mass, dietary zinc intake, plasma zinc concentration, urinary zinc excretion, fractional zinc absorption, or fractional endogenous excretion (Table 3). A significant change with age was determined for release of zinc from red blood cells [L(l5,6)], which decreased by 35% over a lifetime (Fig. 3, Table 6). The square of the correlation coefficient indicates that age accounted for 19% of the variability of the parameter between subjects (Table 4). The fraction of compartment 15 measured in liver [G(8l)] increased significantly by 13% over a lifetime, while the fraction of compartment 15 measured in thigh [G(91)] decreased significantly by 75% over a lifetime. Rates of zinc uptake from gut [R(l5,24)] and zinc secretion into gut [R(24,15)] d ecreased significantly with age, by 28% and 24%, respectively, over a lifetime (Table 6). In men, L(l5,6) and G(91) decreased and G(81) increased significantly with age (Tables 3 and 4). Turnover of tissue compartment 11 [L( 15,11)] decreased significantly by 34% over a lifetime, and mass of zinc in red blood cell compartment 6 [M(6)] increased significantly, by 74% over a lifetime (Table 6). No significant changes in zinc metabolism with age were determined in women when they were considered alone. Changes with age in response to zinc loading. Regression analyses of responses (percent change from basal during zinc loading) on age are shown in Tables 3, 5, and 6. Plasma zinc concentration increased over basal during zinc loading (Table 1) and the response was significantly related to age; on the same zinc load (110 mg/day), plasma zinc response was higher in older than in younger subjects (115% response in 80-yr-old subjects vs. 55% response in ZO-yr-old subjects, Fig. 4A and Tables 5 and 6). Responses of amount and fraction [L(l2,15), Fig. 4B]
Abzrption FEE
L(3, 15) L(6, 15) L(l1, 15) L(12, 15) L(l5,3) L(l5, 6) L(15, 11) L(15, 22) L(22, 15) L(24, 15)
mw GW) Zn Absorbed EE Zn
R(l5, R(l5, R(l5, R(15, R(24, M(3)
3) 6) 7) 24) 15)
Mu3 M(l5) RBC Zn Liver Zn Whole body
U23)
Zn
0.4 0.6
0.9 0.6
0.9 0.6
0.1 0.8 0.4
0.3 0.8 0.4
0.9 0.6 0.2
0.7 0.1 0.2 0.2 0.6 0.9 0.9 0.6 0.9 0.01* 0.9 0.1 0.4 0.1 0.01'" 0.01* 0.6 0.7 0.2 0.7 0.3 0.05* 0.03* 0.2 0.07 0.3 0.1 0.3 0.1 0.9
0.7 0.4 0.4 0.7 0.2 0.4 0.2 0.3 0.3 0.01* 0.01* 0.4 0.9 0.4 0.01* 0.01" 0.8 0.6 0.9 0.6 0.6 0.3 0.2 0.6 0.05* 0.4 0.06 0.8 0.5 0.9
0.5 0.07 0.07 0.2 0.06 0.5 0.4 0.2 0.6 0.1 0.5 0.3 0.3 0.07 0.2 0.2 0.4 0.3 0.3 0.7 0.7 0.2 0.2 0.5 0.2 0.7 0.2 0.4 0.5 0.7
O.Ol* 0.08 0.01* 0.05*
0.04* 0.03*
0.4 0.9 0.2 0.5
0.3 0.8 0.1 0.04*
0.7 0.7 0.6 0.2
0.02* 0.1
0.1
O.Ol* 0.06 0.3 0.4
0.04* 0.5
0.7
0.6
0.9
0.2 0.6 0.01* 0.3 0.01* 0.04* 0.05* 0.9 0.2 0.01* 0.4 0.01* 0.4
0.01* 0.06 0.1 0.5 0.1 0.09
0.7 0.2 0.05* 0.3 0.06 0.2
0.1
0.2
0.7 0.5 0.1 0.6 0.04* 0.5
0.8 0.3 0.06 0.5 0.2 0.6
Parameters are defined in Tables 1 and 2. Regressions of parameter value (basal state) or response of parameter value (loading state) were calculated with age. Probabilities are not given for parameters that remained at basal value or were changed in a defined way [ U(23)] during Zn loading. Parameters M(7), M(ll), and M(22) did not change with age in basal state and during zinc loading changed in the same way as M(15); R(l5, 11) and R(15,22) did not change with age in basal state and during zinc loading changed in the same way as R (15, 7). Parameters R(3, l5), R(7, 15), and R(6, 15) equal R(15,3), R(15,7), and R(l5, 6), respectively. FEE, fractional endogenous excretion; EE Zn, endogenous excretion of Zn; RBC, red blood cell. * P < 0.05.
of zinc excreted in urine were significantly related to age (Table 3). Older subjects, on the same zinc load, excreted over five-fold more zinc than younger subjects (681% increase over basal in 80-yr-old subjects vs. 150% increase in ZO-yr-old subjects, Table 5). Response of fractional uptake of zinc by red blood cells [L(6,15)] decreased with age (Table 6). Compartment 15 contains zinc that exchanges rapidly (~2 days) with plasma. Responses of mass of zinc in compartment 15 [M(l5)] and transport rates of zinc from plasma to muscle and bone [R(l5,3) and R(l5,7) respectively] increased significantly with age by ~60%, in the sameway as plasma (Table 6). Transport of zinc from gut to plasma [R(l5,24)], and secretion from plasma into gut
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ZINC
METABOLISM
Table 4. Regression equations of zinc metabolism on age during basal zinc intake (10 mg Zn/day) All Subjects LO% 6) (/day) = 0.155 - 0.0008 X Age G(81) (fraction) = 0.807 + 0.0018 X Age G(91) (fraction) = 0.030 - 0.0003 x Age R(15, 24) (mg/day) = 6.41 - 0.027 X Age R(24, 15) (mg/day) = 6.65 - 0.025 X Age Men L(15, 6) (/day) = 0.163 - 0.0009 x Age L(15, 11) (/day) = 1.02 - 0.0052 x Age G(81) (fraction) = 0.717 + 0.0034 x Age G(N) (fraction) = 0.037 - 0.0004 x Age M(6) (mg) = 23.4 + 0.386 X Age Only significant differences (P < 0.05) from differences occurred for women).
Table
Regression Equation for Response
Change in Parameter in Basal State, %
r2 0.19 0.20 0.29 0.09 0.09 0.23 0.21 0.36 0.56 0.14 3 are included
Predicted
(no
response
%
r2 20 yr old
80 yr old
0.21 0.23 0.15 0.20 0.18 0.17 0.15 0.11 0.17 0.17
55 150 -43 59 59 61 -6 -3 62 32
115 681 -72 257 119 119 84 47 119 77
% = -45.3 + 7.40x Age % = -125.8 + 5.41 X Age % = 7.1 + 0.96 x Age
0.22 0.38 0.24
103 -18 26
547 307 84
% % % % %
0.25 0.28 0.33 0.25 0.24
5 214 109 93 60
115 810 1 316 119
All Plasma Zn Urine Zn L(6, 15) L(12, 15) R(l5, 3) R(l5, 7) R(15, 24) R(24, 15) M(l5) Liver Zn Men Urine Zn Zn absorbed Liver Zn Women Plasma Zn Urine Zn Fr. end. excr. L(12, 15) R(l5, 3)
% = % = %= % = % = % = % = % = % = % =
= = = = =
35.0 -27.5 -33.3 -6.58 39.4 42.0 -35.7 -19.0 42.6 16.5
38.1 15.0 145.0 18.3 40.7
+ 1.00 x Age + 8.86 X Age - 0.48 x Age + 3.29 X Age + 1.00 x Age + 0.96 X Age + 1.50 x Age + 0.82 X Age + 0.95 X Age + 0.75 x Age
+ 0.96 X Age + 9.94 x Age - 1.80 X Age + 3.72 X Age + 0.98 x Age
Only significant differences (P < 0.05) from Table 3 are included. Predicted response was calculated by solving the regression equation with age = 20 and 80. %, percent change from basal.
[R(24w1,
R1165
AGING
Table 6. Changes in zinc metabolism with age*
Table 5. Regression equations for response of zinc metabolism on age during zinc loading (110 mg Zn/day) Subjects
AND
responded positively with age to zinc loading (Tables 5 and 6). Regression equations for these parameters predict that younger subjects have small negative responses to zinc loading but that responses become increasingly positive with age (Table 5). Response of liver zinc to zinc loading increased significantly with age by 45% over a lifetime (Tables 3 and 6). In men, responses of urinary zinc and liver zinc increased significantly with age and, in addition, amount of zinc absorbed on zinc loading increased significantly with age (Table 3). During zinc loading the amount of zinc absorbed by younger (20 yr) subjects decreased 18% compared with basal, while the amount absorbed in older (80 yr) subjects increased three-fold compared with basal (Table 5). In women responses of plasma zinc, urinary zinc, fractional urinary excretion [L( 12,15)], and release of zinc from muscle [R( 15,3)] increased significantly with age.
Fractional release of red blood cell Zn Fraction of compartment 15 in liver Fraction of compartment 15 in thigh Zn uptake from gut Zn secretion into gut Fractional release from compartment 11 Zn in red blood cell compartment 6 Plasma Zn Urine Zn Fractional urine Zn excretion Fractional RBC Zn uptake Zn exchange with muscle Zn exchange with bone Zn uptake from gut Zn secretion into gut Zn in compartment 15 Liver Zn Zn absorbed Fractional endogenous Zn excretion
Change in Response to Zinc Loading, %
-35 +13 -75 -28 -24 -34,
Men
+74,
Men +60 +531 +198 -29 +60 +58 +90 +50 +57 +45 +325, -108,
Men Women
* Change in parameter in basal state (%) between ages 20 and 80 yr, calculated by solving equations in Table 4 with age = 20 (y) and age = 80 (z) as 100 X (x - y)/y; and change in response to zinc loading between ages 20 and 80 yr, calculated as [ (% response for age = 80) - (% response at age = 20)], from Table 5. Values are only given for parameters that changed significantly.
In
0 0
5
0.05
-Y
= 0.15 - 0.0008
X AGE 0
_ 0.00
0
r 2=- 0.19 (P
of human
zinc metabolism
with age
M. E. WASTNEY, S. AHMED, AND R. I. HENKIN Departments of Pediatrics and Family and Community Medicine, Georgetown University Medical Center, Washington, DC 20007; and Taste and Smell Clinic, Washington, DC 20016 Wastney, M. E., S. Ahmed, and R. I. Henkin. Changes in regulation of human zinc metabolism with age. Am. J. Physiol. 263 (Regulatory Integrative Comp. Physiol. 32): R13.62R1168, 1992.-To assess changes in zinc metabolism with age, kinetic studies were performed in healthy adults (26 men, 21 women) aged 20-84 yr after a single oral or intravenous bolus of 65Zn Studies covered two consecutive 9-mo periods while subjects’were on a basal dietary intake of - 10 mg Zn/day and while taking an additional 100 mg Zn/day orally. Zinc metabolism was analyzed by compartmental analysis using data from plasma, red blood cells, urine, feces, liver, thigh, and whole body [M. E. Wastney, R. L. Aamodt, W. F. Rumble, and R. I. Henkin. Am. J. Physiol. 251 (Regulatory Integrative Comp. Physiol. 20): R398R408, 19861. Changes in observed and model calculated values of zinc metabolism were assessed on age by regression. During basal state, zinc release from red blood cells decreased with age. During zinc loading, response (defined as change from basal state) of plasma zinc concentration, urinary zinc excretion, and liver zinc increased with age, while response of fraction of zinc taken up by red blood cells decreased with age. In men, response of amount of zinc absorbed increased with age and in women response of fraction of endogenous zinc excreted decreased with age. Four responses that changed with age (urinary excretion, red blood cell exchange, absorption, and endogenous excretion) occurred at previously defined sites of regulation of zinc metabolism. Results show that regulation of zinc metabolism changes with age. zinc kinetics; modeling
aging; trace elements;
compartmental
analysis;
ZINC METABOLISM of humans may be compromised with aging for two reasons: 1) zinc intake of older subjects is often low (IS), and 2) zinc absorption decreases with age (2, 48). Zinc is considered important with respect to aging because zinc restores function of some older tissues. Bone mineralization in aging rats is stimulated by zinc (52), and in humans macular degeneration is reported to be retarded by zinc administration (38). Immune function of aged spleen cells is restored by zinc (51), and in one study immune function of some elderly subjects was improved by zinc administration (15), although this was not found in another study (11). Age-related changes in zinc metabolism occur in various tissues. Zinc uptake by adipocytes from older rats is less than that of younger animals (43). Zinc uptake by human fetal fibroblasts declines with successive generations (42) and zinc concentration of human lens tissue decreases with age (44). Based on these and other observations, intracellular zinc deficiency has been proposed as a factor in the aging process (22). Although zinc appears to be important in aging, specific changes in human zinc metabolism with age have not been defined. To assesschanges during aging it is necessary to describe zinc metabolism in subjects over a wide age range. Human zinc metabolism has been described previously based on kinetic studies and compartmental modeling (5, 50). By comparing data from basal R1162
0363-6119/92
$2.00 Copyright
0
and zinc loading states these and other studies have shown that long-term zinc metabolism is regulated at five sites: absorption from gut, excretion into urine, secretion into gut (5, 30), exchange of zinc with red blood cells, and release of zinc from muscle (50). In the present study the kinetics of zinc metabolism in humans, described over a wide age range (20-84 yr) (50), are compared to determine sites of zinc metabolism that change with age. METHODS Subjects. Forty-seven subjects (26 men, 21 women) were studied; data from 32 of these subjects were described previously (50). Subjects were healthy, did not have either acute or chronic disease, and were not taking any medication. Age distribution by year and gender is shown in Fig. 1 and subject descriptions are given in Table 1. Lean body mass was derived from measurements of whole body 40K (20). Measurements were made in the 2-pi scintillation counter of the National Institutes of Health, Bethesda, MD. Total body 40K was measured in a copper-steel shielded scintillator into which the whole subject was placed. Counts were obtained for 500 s before initiation of the study and for 200 s before every measurement of total body zinc. The isotopes 65Zn and 40K were distinguished by energy differences using a multichannel analyzer. Dietary zinc intake was determined from 3-day dietary records that were analyzed by a registered dietitian using USDA Handbook 8 and additional published sources of dietary data (21). Diets were reviewed in personal interviews with each subject to ensure completeness and accuracy using food models as necessary to determine food amounts. Experimental protocol. The protocol has been described previously in detail (50). Subjects were studied during two consecutive 9-mo periods while maintaining their normal lifestyle; for the first period subjects consumed their regular diet containing -10 mg Zn/day, and for the second period they took an additional 100 mg Zn/day orally. At the start of the first period (n = 7) or orally (n subjects were given 6”Zn either intravenously = 40) after an overnight fast. Data (65Zn activity) were obtained by sampling plasma, red blood cells, urine, and feces and by external counting over liver, thigh, and whole body during the first and second (feces excepted) periods. Chemical analyses. Activity of 65Zn was determined in whole body, over liver and thigh regions, and in plasma, red blood cell, urine, and fecal samples by gamma-ray spectroscopy (1). Zinc concentration in plasma, red blood cells, and urine was determined by atomic absorption spectrophotometry as previously described (50). Kinetic analyses. Kinetic analyses were undertaken using the SAAM/CONSAM software (6, 7). For each subject tracer data from all tissues (i.e., plasma, red blood cells, liver, thigh, whole body, urine, and feces) were fitted simultaneously by a previously developed compartmental model for zinc metabolism (Fig. 2) (50). The model consists of 11 compartments that represent zinc in various forms or tissues. Compartment 23 represents a site of rapid zinc absorption high in gut and accounts for firstpass uptake of absorbed zinc by liver. Compartment 24 represents the main site of zinc absorption lower in gut. Compartment 25 represents a delay before zinc is excreted in feces. Compartment 15 represents tissue zinc that turns over in -2
1992 the American
Physiological
Society
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (138.026.031.003) on November 20, 2018. Copyright © 1992 the American Physiological Society. All rights reserved.
ZINC li’~‘l’
IIll
III1
III1
III1
III1
METABOLISM
III1
WOMEN
q
moo
cJn
00
0
0
q
0
q
0
unm
MEN 0
0
m3mo
IllI
IO
00
0
IllI
zo
m
000
1111
30
0
0
1’11
UIDO
‘I”
50
40
0
0
“‘1
60
0
1’11
70
1111
80
90
AGE (YEARS)
Fig. 1. Age distribution
of women
(n = 21) and men
(n = 26).
Table 1. Subject description Subjects Parameter All Age,
yr
Body weight, kg Lean body mass, kg Basal Zn intake, mg/day Plasma Zn, mg/l Basal Zn loading Urine Zn, mg/day Basal Zn loading
Men
Women
45.5k19.6 69.7H3.2 52.7Hl.8 9.0t4.5
42.1t18.5 76.0tl2.3* 63.lt7.6* ll.lt5.2*
49.6t20.6 62.0&9.9-t 43.9+6.4-j6.6kl.5.f
0.86t0.10 1.59t0.29
0.90t0.10* 1.64kO.26
0.81+0.07”f 1.54t0.31
0.40t0.18 1.90t0.98
0.48t0.18* 2.12tl.14
0.3OkO.14”r 1.67kO.76
Values are means t SD. Subjects (26 men, 21 women) consumed their normal diet (-10 mg Zn/day) for 9 mo in basal state; of these subjects, 17 men and 16 women took an additional 100 mg Zn/day orally for a further 9 mo during Zn loading. Values with different superscripts differ significantly between men and women [P < 0.05, based on t test or t test for unequal variances if variances between groups differed significantly, (P < 0.05, F test)].
days and, based on detailed analyses (50), includes plasma, 20% of zinc in red blood cells, and 80% of zinc in liver. Compartment 6 represents 80% of zinc in red blood cells and turns over in -9 days. Compartments 11 and 22 represent zinc in other tissues that turns over in 2 and 48 days, respectively. Compartment 3 represents muscle and compartment 7 represents bone. Zinc loss is represented by excretion into urine (compartment 12) and excretion into feces (compartment 4). Because liver and thigh data were obtained by external counting, liver data were fitted by adding fractions of compartments 3 (8%), 6 (21%), 15 (92%),
(.013i.O08)
AND
AGING
R1163
and 22 (9%) and thigh data were fitted by adding fractions of compartments 3 (17%), 7 (21%), 11 (13%), 15 (1.4%), and 22 (7%) (50). Kinetic data obtained during zinc loading were fitted by adjusting values of five parameters representing five sites of zinc regulation (50). They were absorption [L(15,23) and L(15,24)] secretion [L(24,15)], urine excretion [L(12,15)], red blood cell uptake [L(6,15)], and release from muscle [L(15,3)] (Fig. 2). Calculation of parameters of zinc metabolism. All kinetic data of each subject were fitted by the model to calculate fractional rate constants, L(i,j) and transport rates, R(ij) (terms and equations are defined in Table 2). Compartment masses of zinc, M(i), and zinc entry into the body, U(23), were calculated using plasma zinc content and rate constants of the model. Absorption (fraction/day) was calculated by adding fractions absorbed from the two sites in gut. Rate of zinc absorption (mg/day) was calculated by multiplying fractional zinc absorption by zinc entry into compartment 23 (Table 2). Fractional endogenous excretion was calculated by multiplying secretion into gut by fraction excreted into feces (1 - cy2) (Table 2). Rate of endogenous zinc excretion was calculated by multiplying fractional endogenous excretion by mass of zinc in the compartment containing plasma, compartment 15 (Table 2). StatisticaL analyses. Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS, Chicago, IL). Zinc metabolism was assessed on age by regression analysis. To identify which parameters changed with age, values of each parameter, from all subjects, were fitted against age by linear regression. Change with age was significant if slope of the regression line differed from zero (P < 0.05, F test). Some parameters changed value during zinc loading and, because loading and basal values were not independent, the % change from basal (defined as “response” to zinc loading) was assessed on age by regression analysis. To determine degree of change with age, regression equations were solved using upper and lower limits of ages studied (20 and 80 yr, defined as a “lifetime”). The percent change in parameter value in basal state and difference in response to zinc loading were determined for 20 vs. 80-yr-old subjects. RESULTS
Results of regression analyses of zinc metabolism on age are given in Table 3. For parameters that change significantly with age, regression equations and the square of the correlation coefficients are given in Tables
Fig. 2. A compartmental model of zinc metabolism in humans (50). Terms are defined in Table 2. Large numbers in the circles are compartment numbers, small numbers are mass of Zn (mg) on basal Zn intake of -10 mg/day. Values in parentheses were calculated during Zn loading when Zn intake was lo-fold higher. Values next to arrows are fractional rates of Zn movement between compartments (means & SD, fraction/day). Values of parameters that changed during Zn loading are given in parentheses. *Site of tracer entry into the gut; double arrow represents calculated Zn intake.
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Rl164
ZINC
Table 2. Kinetic
METABOLISM
terms and equations
General terms General equation G(i) L (i, j) Fraction of compartment j transferred into compartment i per unit time Mass of compartment i M(i) R(i, j) Transport of mass from compartment j into compartment i per unit time, [R (i, j) = L( i, j) X M(j)] Steady-state input into compartment i per unit time U(i) Specific terms and equations for model in Fig. 2 G(81) Fraction of compartment 15 in liver G(91) Fraction of compartment 15 in thigh U(23) Oral zinc intake (mg/day) into compartment 23 Absorption (fraction/day) from the first site in the gut
a1 = L(15, 23)/[L(15, from
the second
AGING
Table 3. Probabilities of regression of parameter of zinc metabolism with age
Observed Body weight Lean body mass Diet Zn Plasma Zn Urine Zn Calculated a1
23) + L(24, 23)]
site in the gut
= L(15, 24)/[L(15,
AND
24) + L(25, 24)]
fro:
both sites A = a1 + (1 - q) x ct!2 Secretion from plasma into gut (fraction/day) = L(24, 15) Fractional endogenous excretion from plasma into feces (fraction/day) = secretion X (1 - a2) Zinc absorbed (mg/day) = absorption X U(23) Endogenous zinc excretion (mg/day) = fractional endogenous excretion x M(15)
4 and 5. Changes in zinc metabolism between 20 and 80 yr are summarized in Table 6. Changes with age during basal zinc intake. No significant changes with age were determined for body weight, lean body mass, dietary zinc intake, plasma zinc concentration, urinary zinc excretion, fractional zinc absorption, or fractional endogenous excretion (Table 3). A significant change with age was determined for release of zinc from red blood cells [L(l5,6)], which decreased by 35% over a lifetime (Fig. 3, Table 6). The square of the correlation coefficient indicates that age accounted for 19% of the variability of the parameter between subjects (Table 4). The fraction of compartment 15 measured in liver [G(8l)] increased significantly by 13% over a lifetime, while the fraction of compartment 15 measured in thigh [G(91)] decreased significantly by 75% over a lifetime. Rates of zinc uptake from gut [R(l5,24)] and zinc secretion into gut [R(24,15)] d ecreased significantly with age, by 28% and 24%, respectively, over a lifetime (Table 6). In men, L(l5,6) and G(91) decreased and G(81) increased significantly with age (Tables 3 and 4). Turnover of tissue compartment 11 [L( 15,11)] decreased significantly by 34% over a lifetime, and mass of zinc in red blood cell compartment 6 [M(6)] increased significantly, by 74% over a lifetime (Table 6). No significant changes in zinc metabolism with age were determined in women when they were considered alone. Changes with age in response to zinc loading. Regression analyses of responses (percent change from basal during zinc loading) on age are shown in Tables 3, 5, and 6. Plasma zinc concentration increased over basal during zinc loading (Table 1) and the response was significantly related to age; on the same zinc load (110 mg/day), plasma zinc response was higher in older than in younger subjects (115% response in 80-yr-old subjects vs. 55% response in ZO-yr-old subjects, Fig. 4A and Tables 5 and 6). Responses of amount and fraction [L(l2,15), Fig. 4B]
Abzrption FEE
L(3, 15) L(6, 15) L(l1, 15) L(12, 15) L(l5,3) L(l5, 6) L(15, 11) L(15, 22) L(22, 15) L(24, 15)
mw GW) Zn Absorbed EE Zn
R(l5, R(l5, R(l5, R(15, R(24, M(3)
3) 6) 7) 24) 15)
Mu3 M(l5) RBC Zn Liver Zn Whole body
U23)
Zn
0.4 0.6
0.9 0.6
0.9 0.6
0.1 0.8 0.4
0.3 0.8 0.4
0.9 0.6 0.2
0.7 0.1 0.2 0.2 0.6 0.9 0.9 0.6 0.9 0.01* 0.9 0.1 0.4 0.1 0.01'" 0.01* 0.6 0.7 0.2 0.7 0.3 0.05* 0.03* 0.2 0.07 0.3 0.1 0.3 0.1 0.9
0.7 0.4 0.4 0.7 0.2 0.4 0.2 0.3 0.3 0.01* 0.01* 0.4 0.9 0.4 0.01* 0.01" 0.8 0.6 0.9 0.6 0.6 0.3 0.2 0.6 0.05* 0.4 0.06 0.8 0.5 0.9
0.5 0.07 0.07 0.2 0.06 0.5 0.4 0.2 0.6 0.1 0.5 0.3 0.3 0.07 0.2 0.2 0.4 0.3 0.3 0.7 0.7 0.2 0.2 0.5 0.2 0.7 0.2 0.4 0.5 0.7
O.Ol* 0.08 0.01* 0.05*
0.04* 0.03*
0.4 0.9 0.2 0.5
0.3 0.8 0.1 0.04*
0.7 0.7 0.6 0.2
0.02* 0.1
0.1
O.Ol* 0.06 0.3 0.4
0.04* 0.5
0.7
0.6
0.9
0.2 0.6 0.01* 0.3 0.01* 0.04* 0.05* 0.9 0.2 0.01* 0.4 0.01* 0.4
0.01* 0.06 0.1 0.5 0.1 0.09
0.7 0.2 0.05* 0.3 0.06 0.2
0.1
0.2
0.7 0.5 0.1 0.6 0.04* 0.5
0.8 0.3 0.06 0.5 0.2 0.6
Parameters are defined in Tables 1 and 2. Regressions of parameter value (basal state) or response of parameter value (loading state) were calculated with age. Probabilities are not given for parameters that remained at basal value or were changed in a defined way [ U(23)] during Zn loading. Parameters M(7), M(ll), and M(22) did not change with age in basal state and during zinc loading changed in the same way as M(15); R(l5, 11) and R(15,22) did not change with age in basal state and during zinc loading changed in the same way as R (15, 7). Parameters R(3, l5), R(7, 15), and R(6, 15) equal R(15,3), R(15,7), and R(l5, 6), respectively. FEE, fractional endogenous excretion; EE Zn, endogenous excretion of Zn; RBC, red blood cell. * P < 0.05.
of zinc excreted in urine were significantly related to age (Table 3). Older subjects, on the same zinc load, excreted over five-fold more zinc than younger subjects (681% increase over basal in 80-yr-old subjects vs. 150% increase in ZO-yr-old subjects, Table 5). Response of fractional uptake of zinc by red blood cells [L(6,15)] decreased with age (Table 6). Compartment 15 contains zinc that exchanges rapidly (~2 days) with plasma. Responses of mass of zinc in compartment 15 [M(l5)] and transport rates of zinc from plasma to muscle and bone [R(l5,3) and R(l5,7) respectively] increased significantly with age by ~60%, in the sameway as plasma (Table 6). Transport of zinc from gut to plasma [R(l5,24)], and secretion from plasma into gut
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ZINC
METABOLISM
Table 4. Regression equations of zinc metabolism on age during basal zinc intake (10 mg Zn/day) All Subjects LO% 6) (/day) = 0.155 - 0.0008 X Age G(81) (fraction) = 0.807 + 0.0018 X Age G(91) (fraction) = 0.030 - 0.0003 x Age R(15, 24) (mg/day) = 6.41 - 0.027 X Age R(24, 15) (mg/day) = 6.65 - 0.025 X Age Men L(15, 6) (/day) = 0.163 - 0.0009 x Age L(15, 11) (/day) = 1.02 - 0.0052 x Age G(81) (fraction) = 0.717 + 0.0034 x Age G(N) (fraction) = 0.037 - 0.0004 x Age M(6) (mg) = 23.4 + 0.386 X Age Only significant differences (P < 0.05) from differences occurred for women).
Table
Regression Equation for Response
Change in Parameter in Basal State, %
r2 0.19 0.20 0.29 0.09 0.09 0.23 0.21 0.36 0.56 0.14 3 are included
Predicted
(no
response
%
r2 20 yr old
80 yr old
0.21 0.23 0.15 0.20 0.18 0.17 0.15 0.11 0.17 0.17
55 150 -43 59 59 61 -6 -3 62 32
115 681 -72 257 119 119 84 47 119 77
% = -45.3 + 7.40x Age % = -125.8 + 5.41 X Age % = 7.1 + 0.96 x Age
0.22 0.38 0.24
103 -18 26
547 307 84
% % % % %
0.25 0.28 0.33 0.25 0.24
5 214 109 93 60
115 810 1 316 119
All Plasma Zn Urine Zn L(6, 15) L(12, 15) R(l5, 3) R(l5, 7) R(15, 24) R(24, 15) M(l5) Liver Zn Men Urine Zn Zn absorbed Liver Zn Women Plasma Zn Urine Zn Fr. end. excr. L(12, 15) R(l5, 3)
% = % = %= % = % = % = % = % = % = % =
= = = = =
35.0 -27.5 -33.3 -6.58 39.4 42.0 -35.7 -19.0 42.6 16.5
38.1 15.0 145.0 18.3 40.7
+ 1.00 x Age + 8.86 X Age - 0.48 x Age + 3.29 X Age + 1.00 x Age + 0.96 X Age + 1.50 x Age + 0.82 X Age + 0.95 X Age + 0.75 x Age
+ 0.96 X Age + 9.94 x Age - 1.80 X Age + 3.72 X Age + 0.98 x Age
Only significant differences (P < 0.05) from Table 3 are included. Predicted response was calculated by solving the regression equation with age = 20 and 80. %, percent change from basal.
[R(24w1,
R1165
AGING
Table 6. Changes in zinc metabolism with age*
Table 5. Regression equations for response of zinc metabolism on age during zinc loading (110 mg Zn/day) Subjects
AND
responded positively with age to zinc loading (Tables 5 and 6). Regression equations for these parameters predict that younger subjects have small negative responses to zinc loading but that responses become increasingly positive with age (Table 5). Response of liver zinc to zinc loading increased significantly with age by 45% over a lifetime (Tables 3 and 6). In men, responses of urinary zinc and liver zinc increased significantly with age and, in addition, amount of zinc absorbed on zinc loading increased significantly with age (Table 3). During zinc loading the amount of zinc absorbed by younger (20 yr) subjects decreased 18% compared with basal, while the amount absorbed in older (80 yr) subjects increased three-fold compared with basal (Table 5). In women responses of plasma zinc, urinary zinc, fractional urinary excretion [L( 12,15)], and release of zinc from muscle [R( 15,3)] increased significantly with age.
Fractional release of red blood cell Zn Fraction of compartment 15 in liver Fraction of compartment 15 in thigh Zn uptake from gut Zn secretion into gut Fractional release from compartment 11 Zn in red blood cell compartment 6 Plasma Zn Urine Zn Fractional urine Zn excretion Fractional RBC Zn uptake Zn exchange with muscle Zn exchange with bone Zn uptake from gut Zn secretion into gut Zn in compartment 15 Liver Zn Zn absorbed Fractional endogenous Zn excretion
Change in Response to Zinc Loading, %
-35 +13 -75 -28 -24 -34,
Men
+74,
Men +60 +531 +198 -29 +60 +58 +90 +50 +57 +45 +325, -108,
Men Women
* Change in parameter in basal state (%) between ages 20 and 80 yr, calculated by solving equations in Table 4 with age = 20 (y) and age = 80 (z) as 100 X (x - y)/y; and change in response to zinc loading between ages 20 and 80 yr, calculated as [ (% response for age = 80) - (% response at age = 20)], from Table 5. Values are only given for parameters that changed significantly.
In
0 0
5
0.05
-Y
= 0.15 - 0.0008
X AGE 0
_ 0.00
0
r 2=- 0.19 (P
