Metabolism
Zinc metabolism in animals and man has been the subject of extensive reviews (e.g.
Chesters, 1997). Zinc is absorbed mainly by the small intestine, although the major section(s) from which absorption takes place have not been identified. During absorption, the rate of intramucosal Zn transport may be controlled by an inter- action between cysteine-rich intestinal protein (CRIP), serving as an intracellular carrier, and metallothionein, which appears to inhibit intracellular transport (Chesters, 1997). As a result of desquama- tion of mucosal cells, more Zn enters the mucosa than is transported to plasma. This mechanism appears to be important in the regulation of Zn absorption. Zinc absorp- tion determined by the isotope dilution technique (Weigand and Kirchgessner, 1976) does not include the fraction of Zn which enters, but does not cross, the mucosal lining to the bloodstream. Once in the plasma, Zn is transported predominantly in association with albumin and to a lesser extent with a high-molecular weight protein fraction. Transfer of Zn to liver from plasma is 5–6 times faster than transfer to other major tissues (House and Wastney, 1997). Although the liver is very active in Zn metabolism, it represents <5%
of whole-body Zn, while bone and muscle
are normally the largest Zn pools. In small animals, however, the integument becomes quantitatively significant. In the rat, the Zn pool in the pelt (skin plus hair) may exceed that in the bone or muscle. Sources of endogenous Zn entering the gastrointestinal tract include saliva, gastric secretions, pan- creatic secretions, bile and intestinal secre- tions. Of these, pancreatic secretions may be the most quantitatively significant. In normal pigs, more Zn was secreted in pan- creatic fluid than in bile, but this order was reversed in Zn-deficient pigs since pan- creatic secretion was reduced to a greater extent than biliary secretion, indicating that regulation of both pancreatic and biliary secretion of Zn is important in Zn homeo- stasis (Sullivan et al., 1981). The quantity of Zn secreted into the intestine from all sources may be as much as dietary Zn intake. Although biliary Cu is known to be absorbed with much lower efficiency than dietary Cu, there has been insufficient work to determine if Zn responds in a similar manner. A lower rate of resorption of biliary and/or pancreatic Zn compared with dietary Zn could be a factor in maintaining homeostasis.
Growth depression is characteristic of Zn deficiency in the young of all species studied. Zinc deficiency normally results in the loss of Zn from bone and liver, but not from skeletal muscle. Integument losses also are a significant fraction of whole-body loss during Zn deficiency in rats and presumably other small animals.
Loss of bone Zn has been found as a result of Zn deficiency in rats, calves, chickens and quail, but not in monkeys or cows.
Loss of liver Zn has been reported for rats, calves, chickens, quail and monkeys, but not cows. Except for the stability of Zn in skeletal muscle, variable responses to Zn deficiency have been found in other tissues.
Above the nutritional requirement, whole-body Zn and Zn content of tissues appear to be regulated over a range of Zn intakes, although homeostatic control varies with species and physiological state.
A small increase in digestive tract tissue Zn was observed in rats fed 104 compared
with 24 µg Zn g1diet, while there was no effect on Zn content of liver, kidneys, spleen, brain, lungs, reproductive tissues, muscle, skeleton or pelt (Windisch and Kirchgessner, 1993). These and other studies have indicated that rats are able to maintain relatively constant tissue Zn concentration up to 600 µg Zn g1 diet;
however, higher intakes lead to substantial increases in Zn content of most internal organs and bone. Based on limited reports, the ability to maintain tissue Zn homeo- stasis with increasing dietary Zn concen- tration varies considerably among species, with quail and chickens having little ability to maintain homeostasis, calves and rats having moderate ability and mice having the greatest dietary range of species reported. Although the congruence is less than desirable among studies, species and physiological states with respect to tissues affected by excess Zn and the dietary range over which homeostasis can be main- tained, it is possible to conclude that a homeostatic plateau exists for some range of Zn intakes above the nutritional require- ment for most species.
Tissue Zn turnover varies among tissues and is a factor in mobilization of Zn and in its excretion from the body. Rapidly turning over tissues can respond more rapidly to a reduction in Zn intake.
Disappearance curve analysis of individual tissues from sequentially sacrificed rams following intravenous 65Zn administration showed that liver, heart, pancreas, salivary glands, kidney cortex and spleen had rapid Zn turnover ≥18% day1, while skeletal muscle and bone had slow turnover, <2%
day1 and other tissues had intermediate values (McKenney et al., 1962). Comparison of Zn kinetics in mice, rats, dogs and humans shows that the biological half-life of Zn increases as the size of the species increases.
Simulation of zinc dynamics – model development
Model simulation provides a means of com- piling existing knowledge of metabolism
from numerous studies to yield a more com- prehensive view than can be provided by individual investigations. Based on the results of published studies, a model of Zn metabolism (Fig. 8.1) was constructed in order to simulate Zn dynamics. The simula- tion was prepared with the aid of a compart- mental modelling program, SAAM II (SAAM Institute, 1994). The simulation has been constructed for male, growing rats consum- ing diets varying in Zn concentration from deficient to adequate. Although constructed for rats, similarities with other species are apparent in the results of the simulation.
Williams and Mills (1970) thoroughly documented feed intake, body weight changes and whole-body Zn concentrations over time with growing rats fed varying Zn concentrations from deficient (3 µg g1) to adequate (12 µg g1). Much of these data have been incorporated into the model, pro- viding basic Zn data for growing rats.
Dietary Zn intake varies with time (body size) and dietary Zn concentration (Fig.
8.2). Several other studies have shown that feed intake and growth rate no longer vary with dietary Zn concentration for a considerable range above the nutritional requirement. Thus, feed intakes for 15 and 18 µg Zn g1 diet were included in the model equal to the intake at 12 µg Zn g1 diet (Fig. 8.2). Whole-body Zn of young rats increased with age and dietary Zn concen- tration up to the nutritional requirement (Williams and Mills, 1970) (Fig. 8.3). Since Williams and Mills (1970) and others have shown repeatedly that whole-body Zn or tissue Zn does not increase with increases in dietary Zn above the minimum require- ment up to relatively high intakes, dietary concentrations of 15 and 18 µg Zn g1 diet were included in the simulation with no increase in whole-body Zn compared with rats fed 12 µg Zn g1diet (Fig. 8.3).
Johnson et al. (1988) investigated the effect of dietary Zn concentration on the coefficient of absorption of Zn using grow- ing male rats similar in age to those studied by Williams and Mills (1970). Their measurements of Zn absorption included the dietary Zn range of 3.6–50 µg g1. A substantial reduction in coefficient of
absorption occurred with increasing dietary Zn concentration. Their data (Johnson et al., 1988) were subjected to non-linear regres- sion, to yield the following relationship which was incorporated into the model:
Abs = 0.986 0.0233 Zn p.p.m.
+ 0.0002 Zn p.p.m.2 (8.1) where Abs = coefficient of absorption and Zn p.p.m. = dietary Zn concentration, µg g1(Fig. 8.4).
Plasma Zn was held in steady-state in the simulation because it is only about 0.5% of whole-body Zn in the rat, and variations in the plasma Zn pool size, which are observed during deficiency, are negligible with respect to changes in the major tissue Zn pools. It was assumed that
the relative distribution of plasma Zn to tissues in the body of the growing rat would be similar to that found for mature rats in a recent compartmental modelling study (House and Wastney, 1997). Thus, values for the fractional transfer coefficients to tissues from plasma (determined by House and Wastney, 1997) are incorporated into the current simulation. The values utilized were 7.1 day1 for transfer of plasma Zn to muscle (k(5,2) in Fig. 8.1), 5.62 day1 for transfer to skin and hair (k(6,2)), 35.7 day1 for transfer to internal organs (k(3,2)) and 7.22 day1 for transfer to bone (k(4,2)). Thus, Zn transfer from plasma to internal organs, mainly liver, in the simulation is relatively rapid, which is consistent with other observations on the 5
Muscle 6 Pelt
4 Bone
7 Urine
3 Internal
organs
9 Faeces 1
Gut 2 Plasma
Dietary Zn
k(5,2)
k(2,5)
k(2,1)
k(6,2) k(2,6)
k(2,4) k(4,2)
k(2,3)
k(3,2) k(7,2)
k(1,3)
k(9,1)
Fig. 8.1. Kinetic model of Zn metabolism in rats. Circles represent body Zn compartments, which are numbered arbitrarily. Arrows are fractional transfer coefficients (day1), k(i,j), indicating transfer to compartment i from compartment j; for example, k(2,1) is the fraction of compartment 1 (gut Zn) which is transferred to compartment 2 (plasma Zn) per day.
Fig. 8.2. Dietary Zn intake in young growing male rats fed a semi-synthetic diet with varying Zn concentrations. Adapted from Williams and Mills (1970).
Fig. 8.3. Whole-body Zn accumulation in young growing male rats fed a semi-synthetic diet with varying Zn concentrations. Adapted from Williams and Mills (1970).
rate of transfer of plasma 65Zn to liver in humans, rats and cattle.
Tracer studies with rats, cows and calves show enhanced retention and/or increased specific activity of tracer in tissues of Zn-deficient animals compared with Zn-adequate animals, which indicates that Zn transfer from tissue to plasma is decreased in Zn deficiency (e.g. Windisch and Kirchgessner, 1994). On the other hand, excessive Zn intake leads to a reduc- tion in tissue tracer specific activity or retention. These results indicate that tissue Zn turnover is decreased in Zn deficiency and increased when Zn supply is exces- sive. Thus, variations in tissue Zn mass in the simulation were accomplished through changes to tissue Zn egress transfer coeffi- cients. The value of each coefficient was controlled by a function designed to adjust the compartment size, expressed as a percentage of whole-body Zn, to match tissue Zn contents obtained from several studies (Giugliano and Millward, 1984;
Windisch and Kirchgessner, 1994; House and Wastney, 1997). The tissue egress
coefficients decreased with decreasing Zn intake in the simulation, as would be expected from published observations of changes in tissue Zn turnover. Percentages of whole-body Zn in muscle and bone of Zn- adequate growing rats used in the simula- tion were 26.7 and 27.9%, while percentages for Zn-deficient growing rats were 40.0 and 13.4%, respectively (from Giugliano and Millward, 1984, after correcting for pelt Zn). The percentage of whole-body Zn in the internal organs was fixed at 15.1 (from Giugliano and Millward, 1984, after correcting for pelt Zn). The percentage of Zn in the pelt was fixed at 30.5 (House and Wastney, 1997). Percentages of whole-body Zn in internal organs and in the pelt were fixed in the simulation because they were unaffected by variations in dietary Zn (Giugliano and Millward, 1984;
Windisch and Kirchgessner, 1994).
Measurements of urinary Zn excretion for growing rats at various dietary Zn intakes from the study of Johnson et al.
(1988) were subjected to non-linear regres- sion to yield the following relationship:
Fig. 8.4. Absorption of Zn by young growing male rats fed a semi-synthetic diet with varying Zn concentrations. Adapted from Johnson et al.(1988).
Urinary Zn = 7.4
(1 e0.0074 Zn intake) (8.2) where units of urinary Zn and Zn intake are µg day1(Fig. 8.5). This expression was utilized in the simulation to describe the loss of Zn via urine.
All aspects of the model were derived from published results except for the rate of endogenous faecal excretion, which was determined for various nutritional states by solving the model. The model was solved for endogenous faecal excretion rather than using available data from the literature because endogenous faecal excretion cannot be varied independently from changes in whole-body Zn mass, and changes in whole-body mass were entered into the model from the data of Williams and Mills (1970) as discussed above.
Simulation of zinc dynamics – model output The model was solved for dietary Zn intake by young growing male rats consuming
diets varying from 3 µg g1 (deficient) to 18 µg g1under conditions in which 12 µg g1 provided adequate Zn. Urinary and faecal Zn excretion are shown in Figs 8.6 and 8.7. As has been well documented in animals, the simulation shows that faeces are the main route of excretion of dietary Zn. However, at lower dietary Zn intakes, urinary Zn excretion became a significant percentage of total Zn excreted (Fig. 8.8).
Similar results have been found in humans and calves.
As might be expected, Zn balance varied considerably in response to dietary intake according to the simulation (Fig.
8.9). Except for the lowest level of dietary Zn (3 µg g1), Zn balance, expressed as µg day1, varied with time in the growing rats, and was highest for 9–18 µg Zn g1diet at 28 days. The simulation shows that measurement of balance alone may not be sufficient to distinguish an adequate from a deficient diet. At 28 days, balance for rats fed 9 µg Zn g1 diet was as great as for 12–18 µg Zn g1diet, even though 9 µg Zn
Fig. 8.5. Urinary excretion of Zn by young growing male rats fed a semi-synthetic diet with varying Zn concentrations. Adapted from Johnson et al. (1988).
Fig. 8.6. Urinary Zn excretion by young growing male rats. The figure is a simulation based on a kinetic model of Zn metabolism (Fig. 8.1) and published data for rats fed a semi-synthetic diet with varying Zn concentrations as described in the text.
Fig. 8.7. Faecal Zn excretion by young growing male rats. The figure is a simulation as described in Fig. 8.6.
Fig. 8.8. Percentage of total Zn excretion (faeces + urine) excreted in the urine of young growing male rats.
The figure is a simulation as described in Fig. 8.6.
Fig. 8.9. Zinc balance of young growing male rats. The figure is a simulation as described in Fig. 8.6.
g1 diet was unable to support as rapid growth as 12–18 µg Zn g1diet during the full 28-day period. During the last days of the simulation, whole-body accumulation of Zn at 9 µg Zn g1 diet was nearly parallel to that at 12 µg Zn g1 diet (Fig.
8.3), leading to nearly equivalent balance values. Thus, due to observed differences in feed intake, percentage absorption and percentage excretion, the simulation reveals that 9 µg Zn g1diet should be able to meet the nutritional requirement for Zn at the end of the 28-day period but not at the beginning.
Control of endogenous faecal Zn excretion is recognized as an important mechanism of Zn homeostasis. In the current simulation, endogenous faecal excretion was almost negligible at 3, 6 and 9 µg Zn g1 diet in the growing rat (Fig.
8.10). The whole-body retention of Zn found by Williams and Mills (1970) was achieved by the nearly complete inhibition of endogenous faecal excretion. It shows that the rat has considerable capacity for
regulating loss of Zn from the body, since there was very little loss at intakes less than the nutritional requirement. The ability of organisms to minimize endo- genous faecal excretion of Zn in response to dietary deficiency has been reported in studies with rats, calves, goats and human infants.
Although the coefficient of absorption decreases as dietary Zn concentration increases (Fig. 8.4), quantitative true absorption (µg day1) increases because the elevated Zn intake is not fully matched by the reduction in the coefficient of absorp- tion. This relationship was observed by Weigand and Kirchgessner (1978) and is shown by solving the simulation for dietary concentrations up to 48 µg g1(Fig.
8.11). Since there was no further increase in whole-body Zn concentration above the nutritional requirement, the additional absorbed Zn was excreted in the faeces.
Apparent absorption remained relatively constant above the nutritional requirement (Fig. 8.11), while both true absorption (µg
Fig. 8.10. Endogenous faecal excretion of Zn by young growing male rats. The figure is a simulation as described in Fig. 8.6.
day1) and endogenous faecal excretion continued to rise, which is the pattern observed by Weigand and Kirchgessner (1978). As dietary Zn intake was increased to 141 µg Zn g1 diet in the study of Weigand and Kirchgessner (1978), the quantity of endogenous faecal Zn excretion also increased until it was 90% of the value of apparent Zn absorption. Thus, regulation of endogenous excretion plays a critical role in conservation of body Zn during Zn deficiency and, also, in the elimination of Zn when excess Zn is absorbed.
Although regulation of absorption alone was not sufficient to maintain homeostasis in response to elevated dietary intake (Fig. 8.11), the threefold reduction in coefficient of absorption over the range of dietary intakes investigated is substan- tial (Fig. 8.4). A reduction in the coefficient of absorption with increasing dietary consumption of Zn has been observed repeatedly in rats, and also in calves, sheep, human infants and human adults.
Changes in absorption may be of greater
significance at higher dietary Zn concentra- tions in ruminants than in monogastrics.
Studies with ewes and calves indicate that homeostasis was maintained predomin- antly by changes in absorption, with very little if any change in endogenous faecal excretion when dietary Zn ranged from adequate to excessive.
Redistribution of Zn within the major Zn pools of the body has been observed several times in response to Zn deficiency in growing rats (e.g. Giugliano and Millward, 1984) as well as in mature rats and chicks. Redistribution involves the relatively greater loss of Zn from bone com- pared with other tissues during deficiency.
Loss of bone Zn has also been demon- strated in Zn-deficient young pigs, although changes in other tissues have not been measured. Redistribution appears to be a mechanism of maintaining Zn concen- tration in preferred tissues at the expense of others and, as such, may be an important metabolic strategy influencing the health of deficient animals. Changes in skin and hair Zn as well as bone, muscle and internal Fig. 8.11. Consumption, absorption and excretion of Zn in young growing male rats. The figure is a simulation as described in Fig. 8.6.
organ Zn were simulated for growing rats receiving adequate (Fig. 8.12) or deficient (Fig. 8.13) dietary Zn. Rats fed a deficient Zn diet accumulated less Zn overall in tissues, but muscle accumulated relatively more zinc than the other tissues, mainly at the expense of bone. The tissue distribu- tion of Zn shown at day 28 in Figs 8.12 and 8.13 is from Giugliano and Millward (1984). The redistribution of Zn was simulated using modelling techniques for regulating Zn tissue mass as described above. The rate of redistribution was assumed to take about one-half of the 28- day period simulated, which is consistent with the time course of 65Zn redistribution observed in another study with marginally deficient rats.