Symposium
Assessment of Zinc Status12 JANET
C. KING3
Department of Nutritional Sciences, University of California, Berkeley, CA 94720 tions in plasma zinc due to poor status could be dif ferentiated from those due to a change in metabolic conditions. A thorough study of metabolic and homeostatic responses to insufficient zinc intakes in lab oratory animals and humans may identify new poten tial markers of zinc nutriture. In a recent review (1), Golden compared the devel opment of zinc deficiency with that of other nutrients. When the intake of most nutrients is insufficient, stores or functional reserves are mobilized. With fur ther depletion, tissue nutrient concentrations decline, followed by deterioration in one or more specific functions or metabolic pathways. A functional reserve or store of zinc does not seem to be available for use when zinc intakes are insufficient. Instead, tissue zinc is conserved by a reduction or cessation of growth in growing organisms or by a decrease in excretion in non-growing organisms. If the dietary deficiency is mild, zinc homeostasis may be reestablished by ad justing growth rates and excretion. Thereafter, no fur ther biochemical or clinical changes develop. In this case, adaptation to lower zinc intakes is accomplished by reducing the rate of growth or excretion. However, with a more severe deficiency, further metabolic changes develop within a short period of time. Zinc
ABSTRACT Plasma zinc concentration has been den igrated as a measure of zinc status because it responds to metabolic conditions unrelated to zinc status and because it is insensitive to changes in dietary zinc. The insensitivity of plasma zinc to reductions in dietary zinc reflects the tremendous capacity of the organism to conserve tissue zinc by reductions in zinc excretion and/or reductions in the rate of growth. Changes in plasma zinc concentrations do not seem to occur until the capacity to reestablish homeostasis by reducing excretion and/or growth has been exceeded. Reductions in dietary zinc beyond the capacity to maintain homeo stasis lead to utilization of zinc from a small, rapidly turning over pool. This pool is located, at least in part, in the bone, liver, and plasma. Loss of a small, critical amount of zinc from this pool leads to the rapid onset of both biochemical and clinical signs of zinc deficiency. Thus, plasma zinc is a valid, useful indicator of the size of this exchangeable pool of zinc. Plasma metallothionein concentrations may prove useful for identifying poor zinc status. Plasma metallothionine concentrations reflect hepatic concentrations and, therefore, are re duced when the dietary zinc supply is low. However, since hepatic metallothionein concentrations rise in re sponse to stress, plasma metallothionein concentra tions are increased in this state when the plasma zinc concentrations are also likely to be low. Thus, mea surement of both plasma metallothionein and plasma zinc concentrations could differentiate a low plasma zinc due to dietary zinc deficiency from a low concen tration due to stress, infection, or other metabolic conditions. J. Hutr. 120:1474-1479, 1990.
1 Presented as part of a conference, "Nutrition Monitoring and Nutrition Status Assessment," at the first fall meeting of the Amer ican Institute of Nutrition, Charleston, South Carolina, December 8-10, 1989. The conference was supported in part by cooperative agreement HPU880004-02-1 with the DHHS Office of Disease Pre vention and Health Promotion, the USDA Human Nutrition In formation Service, the DHHS National Center for Health Statistics, and the International Life Sciences Institute-Nutrition Foundation. 2 The Planning Committee for the meeting consisted of Drs. He
INDEXING KEY WORDS:
•plasma zinc •zinc status •metallothionein •zinc homeostasis •zinc deficiency
len A. Guthrie, Roy J. Martin, Linda D. Meyers, James A. Olson, Catherine E. Woteki, and Richard G. Allison (ex officio). The sym posium papers were edited by a committee consisting of Dr. James Allen Olson (coordinator), Dept. of Biochemistry & Biophysics, Iowa State University, Ames, IA; Dr. Cathy C. Campbell, Division of Nutritional Sciences, Cornell University, Ithaca, NY; Dr. Roy J. Martin, Dept. of Foods &. Nutrition, University of Georgia, Athens, GA; and Dr. Catherine E. Woteki, Food &.Nutrition Board, National Academy of Sciences, Washington, DC. 3 Address reprint requests to J. C. King, Dept. of Nutritional
Lack of a specific sensitive biochemical or functional test for zinc status is a barrier to the study of human zinc nutrition. Measurement of plasma zinc is used most frequently, but it is not ideal because metabolic conditions unrelated to zinc status cause it to decline. Attempts to identify another indicator specific for changes in zinc nutriture have not been successful. However, if an indicator of the metabolic conditions lowering plasma zinc could be identified, then reduc 0022-3166/90
S3.00 ©1990 American Institute of Nutrition.
Sciences, University
of California,
Berkeley, CA 94720.
Received 25 March 1990. Accepted 11 July 1990.
1474 Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
ZINC
1475
STATUS ASSESSMENT
balance becomes negative and there is a net loss of tissue zinc. The pool from which zinc is mobilized appears to be small and to have a rapid turnover rate, as shown by the prompt appearance of deficiency signs in laboratory animals and humans fed diets lacking zinc. The progression of the steps in development of both mild and severe deficiencies is displayed in Fig. 1. This model is based on our present understanding of the metabolic and homeostatic responses to zinc deficiency. A brief review of this information follows.
Steps in Dietary Zinc Deficiency Reduced Growth or Excretion
Avid Tissue Zinc Conservation
In mild deficiency homeostasis is ' zinc re-established.
In severe deficiency. Mobilization of Zinc
ZINC DEFICIENCY IN LABORATORY ANIMALS
In laboratory animals, a reduction or cessation of growth is an early response to zinc deficiency. Wean ling rats fed a zinc-deficient diet generally stop growing in 4-5 d, and this cessation of growth enables them to maintain a normal whole-body zinc concentration (2). Although the whole-body zinc concentration is unchanged, the response of specific tissues to zinc de ficiency is not uniform. Some tissues lose zinc in order to support other tissues. For example, the zinc con centration in muscle is conserved in zinc-deficient an imals, while zinc concentration in bone, liver, and plasma falls (3-5). This redistribution places an ap preciable demand on the animal since the muscle comprises ~50-60%of the whole-body zinc (6). Mus cle tissue in weanling zinc deficient animals increases a small amount, leading to an increase in total muscle zinc (5). The concentration of zinc in bone usually drops ~ 65%, plasma zinc ~ 45%, and liver zinc ~ 720% (4, 5). Testes zinc concentrations may also fall. A system of variable priorities seems to regulate tissue zinc content in severely deficient animals. Bone zinc and, to a lesser extent, plasma and liver zinc are lowpriority tissues. Zinc is mobilized from those tissues to support high-priority tissues, such as muscle and skin (5). Although ~30% of the whole-body zinc is found in bone, the metabolic function of zinc in bone is uncertain (6). The small, rapidly exchangeable zinc pool that is mobilized in zinc deficiency probably de rives in large part from bone, but also from the liver and plasma.
ZINC DEPLETION
IN HUMANS
Data from human studies also show that growth retardation is an early response to zinc deficiency. A growth response to zinc supplementation has been seen in growth-retarded infants and children in Denver and in adolescents in Iran (7-11). In the Iranian ado lescents, where the extent of growth retardation was severe, major gains in growth occurred in response to supplemental zinc. The infants and children in Denver Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
from Exchangeable Pool
General Tissue Dysfunction
FIGURE 1 A flow-chart depicting steps in the develop ment of mild and severe zinc deficiency.
were much less growth retarded, and the growth rates of the zinc-supplemented children were only ~10% greater than those of placebo-treated controls (12). Apparently, very little change in tissue zinc was re quired before the growth response could occur, since a rapid change in growth was seen within a month. Plasma zinc levels and serum alkaline phosphatase ac tivities of the growth-retarded Denver infants did not differ from the controls before or after zinc supple mentation (10). The rapid growth response and the absence of any change in plasma zinc suggest that little to no zinc had been mobilized from the exchangeable pool. Based on the criteria in Fig. 1, these growth-re tarded infants and children seem to have had a mild zinc deficiency. The etiology of mild zinc deficiency in these Denver children is uncertain. Reported zinc intakes were not lower than that of the controls, and there was no ev idence of health problems that may have precipitated zinc deficiency (10). One can only speculate that the ability to adjust to low zinc intakes is highly variable and that, as a consequence, those infants and children with a reduced capacity for adaptation are at a greater risk of mild zinc deficiency. Since mild zinc deficiency, as defined in this paper, is not associated with any biochemical or functional changes, the only way to diagnose it is to measure the growth response to a zinc supplement. Walravens and co-workers (10) recom mend that a daily dose of 5 mg zinc be used both as a diagnostic test and as a potential treatment of mild zinc deficiency in all infants and children with declin ing weight percentiles. ADAPTATION TO LOW INTAKES OF ZINC Full-grown animals and humans adjust to low zinc intakes by reducing their zinc excretion. It is well
1476
KING
known that zinc deficiency is difficult to induce in ma ture laboratory animals, which makes them a poor model for study of zinc depletion. In metabolic balance studies of adult men and women, zinc balance was achieved with zinc intakes ranging from 54-292 /¿mol/ d (3.5 to 19 mg/d) (13). Data from our own laboratory show that zinc balance was reestablished within 9 d after initiation of an 85-/miol (5.5 mg) zinc diet (14). During this period fecal zinc dropped from 215 to 60 ¿imol/d(14 to 4 mg/d), but urinary zinc remained un changed. The decline in fecal zinc probably was pri marily due to a decrease in unabsorbed dietary zinc, but the secretion of endogenous gastrointestinal zinc also may have dropped. Jackson and co-workers (5) proposed that rapid adjustments in gastrointestinal zinc secretion enabled individuals to adapt to fluctu ations in dietary zinc. The absolute amount of zinc absorbed changes little when dietary zinc is increased or reduced. For example, a reduction in dietary zinc from 255 to 85 /¿mol/d(16.5 to 5.5 mg/d) in a group of men (14) caused the fractional zinc absorption to double, whereas the absolute amount of zinc absorbed only decreased ~15 ^mol/d (l mg/d). As the zinc intake is progressively lowered toward a zinc-free diet, the time required to achieve zinc bal ance lengthens. Zinc balance was achieved in only 9 d when intake was reduced from 255 to 85 /¿mol/d (16.5 to 5.5 mg/d) (14) without a reduction in urinary or plasma zinc. When dietary zinc was reduced from 126 to 55 /miol/d (8.2 to 3.6 mg/d) (16), balance was also achieved, but the amount of time required was not reported. Plasma zinc was relatively stable for the 4-mo low-zinc period, but losses in sweat declined by as much as 65%. Thus, sweat losses may be an im portant adaptive mechanism for conservation of zinc during low intakes. In another study, balance was achieved in 14-21 d when intake was reduced from 185 to 25 Mtnol/d (12 to 1.7 mg/d) (H. Anderson and B. O'Dell, personal communication). This was accom plished with a 50% reduction in urinary zinc and a 60% reduction in fecal zinc; plasma zinc also declined ~22%. Feeding a virtually zinc-free diet (0.3 mg/d) did not allow zinc balance, but it is remarkable how close it came. After 21 d, total zinc losses in the urine and feces dropped from > 11 mg/d to ~ 1 mg/d, dem onstrating the tremendous capacity of the body to re duce zinc output and to conserve whole-body zinc when the dietary supply is extremely low (Fig. 2) (17). Plasma zinc levels, however, fell 44%. Although zinc balance could be achieved with in takes as low as 25 ¿imol/d(1-7 mg/d) (H. Anderson and B. O'Dell, personal communication), this was not done without a fall in plasma zinc. Thus, the attain ment of zinc balance does not imply that normal zinc concentrations will be maintained in all body fluids and tissues. Studies in laboratory animals, described above, show that zinc is mobilized from some tissues to maintain zinc levels in others. The same seems to Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
Urinary Zinc FtcalZInc
0,
u Ñ
1-3
4-6
7-9 10-1213-1516 Day« of Depletion
1819-21
FIGURE 2 Changes in urinary and fecal zinc losses in young men fed a 0.3-mg zinc diet for 21 d (17).
be true for humans. Jackson and co-workers (4) sam pled tissues at the post mortem examination of an 11yr-old thalassemic child who had been treated with diethylene triamine pentacetic acid (DTPA), a zinc chelator. Although oral zinc supplements were given, large quantities of zinc were excreted in the urine, and the child displayed clinical symptoms of zinc defi ciency prior to death. The concentration of zinc in the bone, liver, testes, and plasma were below the normal range, while muscle, skin, and hair zinc concentrations remained normal at the time of death. Thus, the tissues that lost zinc in this patient were the same as those that lost zinc in zinc-deficient rats.
INDICATORS OF ZINC STATUS Plasma zinc has been denigrated as a measure of zinc status because it does not reflect reductions in dietary zinc intake or changes in whole-body zinc. Both of these criticisms are valid. Since only minimal changes in whole-body zinc develop during zinc de pletion, changes in plasma zinc are not linked to whole-body zinc. Also, as discussed above, plasma zinc seems to fall only when the dietary intake is so low that homeostasis cannot be established without use of some zinc from the exchangeable pool, of which plasma zinc is a component. Thus, plasma zinc is a valid, useful indicator of the size of the exchangeable pool zinc; a reduction in plasma zinc reflects a loss of zinc from bone and liver and an increased risk for de velopment of metabolic and clinical signs of zinc de ficiency (Fig. 1). From the limited data available, it appears that the exchangeable pool of zinc is not used until the dietary zinc intake falls below 5-6 mg/d. Further studies are needed to delineate this threshold for utilization of exchangeable pool zinc more pre cisely. The extent to which the exchangeable pool of zinc is influenced by age, gender, body size, body com-
ZINC
STATUS ASSESSMENT
position, and long-term dietary zinc intake is not yet known. Both the rapid onset and reversal of biochemical and clinical signs of zinc deficiency in laboratory an imals and humans suggest that the exchangeable zinc pool is quite small. Acute and chronic human depletion studies can be used to estimate total zinc losses and to approximate the size of the exchangeable pool. When 0.3 mg zinc was fed to young men (18), bio chemical and clinical changes were seen as soon as 34 wk after depletion was started. By the end of the depletion period, i.e., when plasma zinc concentrations fell below 10.8 ^mol/L (70 Mg/100 mL), a number of significant changes in both biochemical and clinical indices were evident. Serum concentrations of albu min, retinol-binding protein, lactate dehydrogenase, delta amino-levulinic acid, and uric acid were signif icantly lower, and the ability to remove an oral glucose load was reduced. All of the final values for these in dices were still within the normal range, however. Of the clinical signs, skin lesions were the most prevalent. Severity of the lesions ranged from small patches of dry, rough skin to severe acne (18). Other clinical signs included diarrhea, sore throats, poor ap petite, and an increase in the amount of energy re quired to maintain body weight. Although body com position changes were not measured in this study, a reduction in nitrogen balance from the predepletion period suggests that lean tissue was lost. The increase in dietary energy required to maintain body weight probably allowed this lean tissue to be replaced by fat. By the end of the depletion period, which ranged from 4 to 9 wk among the six subjects, total zinc losses averaged ~1460 ¿tmol(95 mg) (17). This is equivalent to only 5% of the whole-body zinc, or ~12% of the bone zinc. Total losses of the six men varied almost fourfold, from 740 to 2540 ¿unol(48-165 mg). The man with the highest total loss of zinc was released from the depletion phase after the 4th wk because of the onset of severe acne symptoms and a rapid decline in his plasma zinc to 3.7 /¿mol/L(24 ¿ig/100mL). His fecal zinc losses remained high into the second week of depletion, whereas the other five men adapted by showing a marked drop in fecal zinc output. The man with the lowest total loss was on the depletion diet for 9 wk. He reported a low pre-study zinc intake, and his plasma zinc at the time of the pre-study physical examination was < 10.8 ¿¿rnol/L (70 /ig/100 mL). Con sequently, he was given a daily supplement of 154 /¿mol(10 mg) for 2 wk prior to entering the study. These two cases show that the ability to adjust to an acute reduction in dietary zinc is highly variable. Ad justment in gastrointestinal losses seems to be an im portant aspect of the adaptive process. Also, the past history of zinc intake may be a factor. In laboratory animals, low dietary zinc intakes in the past are as sociated with lower endogenous losses in subsequent studies (19). Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
1477
Prasad (20) studied chronic zinc depletion in four patients fed either 42 or 54 /¿mol(2.7 or 3.5 mg) zinc/ d for 24 wk. All of the patients lost weight, and two of them complained of mild roughening of skin and of lethargy. Plasma, erythrocyte, and leukocyte zinc concentrations decreased significantly. Plasma alkaline phosphatase, ribonuclease, lactic dehydrogenase, and ammonia levels also declined significantly, and thymidine kinase activity in the connective tissue of a sponge implanted subcutaneously was reduced. The biochemical and clinical changes seen in this study of chronic zinc depletion are similar to those reported for acute depletion. Total zinc losses in the two situ ations are quite different, however. Based on balance data, Prasad (20) estimated that his patients lost ~7700 timol (500 mg) zinc during the 24-wk depletion period. Thus, ~25% of the estimated whole body pool of 2 g, or about 65% of the bone zinc, was lost during that period. These losses seem unusually high, given the evidence that humans have a remarkable capacity to reduce zinc excretion when intakes are low. Zinc losses may have been overestimated in these earlier studies (20) since continuous balances were not done. Plasma zinc concentration, although reflecting changes in the size of the exchangeable zinc pool, also is responsive to other metabolic changes. For example, plasma zinc also changes in response to stress (21), infection (22), meals (23), short-term fasting (24), and the hormonal state (25). The fall in plasma zinc with all of these conditions except fasting may occur be cause zinc is redistributed to other tissues in response to a metabolic need. For example, administration of interleukin 1, a cytokine released in response to in fection, tissue injury, and inflammation, causes an up take of 65Znin the liver, bone marrow, and thymus of laboratory animals and reduces the uptake in bone, skin, and intestine (26). Circulating zinc is conse quently depressed for a short period of time. Thus, plasma zinc can only be useful as a specific indicator of zinc status if the effects of zinc status and of these other metabolic conditions can be differentiated.
METALLOTHIONEN AS AN ACCESSORY INDICATOR OF ZINC STATUS Golden (1) suggested that metallothionein may be the key to diagnosing tissue zinc redistribution. Me tallothionein is a low-molecular-weight protein with a high cysteine content (~30% of amino acid residues) and a high metal-binding capacity (~7 g-atoms/mol) (25). It is found in variable amounts in most tissues and particularly in liver, pancreas, kidney, and intes tinal mucosa. Under normal physiological conditions, metallothionein primarily binds zinc and/or copper. Tissue metallothionein concentrations are often pro portional to zinc status (25); e.g., they are reduced to nondetectable levels in zinc-deficient animals and are
1478
KING
increased after parenteral or dietary administration of zinc. Dietary and liver zinc content are closely related. As dietary zinc concentrations increase, metallothionein synthesis is induced; thus, most of the additional zinc in the liver is bound to metallothionein. Metallothionein can be detected in the plasma and erythrocytes by a radioimmunoassay (27). Both plasma and erythrocyte metallothionein levels are sensitive to dietary zinc intake. Plasma, but not erythrocyte metallothionein, reflects changes in hepatic metallo thionein concentrations. An increase in hepatic me tallothionein in response to stress, inflammation, or hormonal changes will cause comparable changes in plasma metallothionein levels. For example, liver me tallothionein levels were increased 14-fold after interleukin-1 administration (26). Thus, measurement of both zinc and metallothionein in the plasma will allow plasma zinc concentrations to be better inter preted. Low levels of both plasma zinc and metallo thionein would imply a reduction in the size of the exchangeable zinc pool in response to low zinc intakes (Fig. 3). A low level of plasma zinc with a high level of metallothionein would imply that tissue zinc is being redistributed in response to confounding con ditions, not that zinc nutriture is compromised. If acute zinc deficiency and stress are both present, both plasma metallothionein and zinc levels would be low because the hepatic synthesis of metallothionein is not stimulated by stress in zinc-deficient animals (27). Thus, stress in a zinc-deficient animal does not prevent the detection of zinc deficiency. Standardization of the amount of time since food was consumed is also im portant for all measurements, inasmuch as plasma zinc declines from 15% to 20% following a meal (23). A human metallothionein assay has been developed (28). Preliminary studies of the response of erythrocyte metallothionein to changes in dietary zinc are en couraging. After human volunteers were fed a very low zinc intake (~0.5 mg/d) for 8 d, erythrocyte me tallothionein declined 59% whereas fasting plasma zinc concentrations were reduced by only 7%. Con versely, supplementation with 50 mg zinc/d increased erythrocyte metallothionein sevenfold in 7 d. Eryth rocyte metallothionein, like plasma metallothionein, is sensitive to changes in dietary zinc. As discussed above, plasma zinc concentrations do not reflect recent changes in dietary zinc. Possibly, erythrocyte metal lothionein concentrations will also prove to be a useful indicator of changes in dietary zinc. Since erythrocyte metallothionein is not responsive to stress as is plasma metallothionein, the presence of stress will not inter fere with detection of low dietary zinc. SUMMARY
Much has been learned about zinc metabolism in the past several years. This information, summarized Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
Assessment of Zinc Status Reduced Pool Size:
Low Plasma Zinc Low Plasma Metallothionein
Tissue Redistribution:
Low Plasma Zinc High Plasma Metallothionein
FIGURE 3 Differentiation of declines in plasma zinc due to decreases in pool size from tissue redistribution.
below, provides a potential approach for assessing zinc status. 1. The initial response to low dietary zinc is con servation of tissue zinc. Little to no change in plasma zinc concentrations occurs during this initial phase. Preliminary studies suggest that erythrocyte metallothionein concentrations sen sitively reflect changes in dietary zinc. 2. Plasma zinc is a component of the exchangeable zinc pool, which is utilized when dietary zinc is so low that homeostasis cannot be reestablished by reducing the rate of growth or zinc excretion. 3. The rapid onset and reversal of the biochemical and clinical signs of zinc depletion imply that the exchangeable zinc pool is small; loss of as little as 770-1540 nmol (50-100 mg) in adults, or 2.5-5% of the whole body content of zinc, leads to the onset of zinc-deficiency signs. 4. Plasma zinc concentrations decline in response to stress, infection, or a variety of hormonal changes. These conditions also induce metallo thionein synthesis in the liver. Hepatic and plasma metallothionein are closely related. Thus, low plasma zinc concentrations in the presence of increased plasma metallothionein imply tissue zinc redistribution caused by confounding con ditions, not an impaired zinc status.
LITERATURE CITED 1. GOLDEN,M. H. N. (1989) The diagnosis of zinc deficiency. In: Zinc in Human Biology, pp. 323-333 (C. F. Mills, ed.), SpringerVerlag, London. 2. WILLIAMS,R. B. & MILLS, C. F. (1970) The experimental pro duction of zinc deficiency in the rat. Br. /. Nutr. 24: 989-1003. 3. O'LEARY,M.}., McLAiN, C. J. & HEGARTY,P. V. J. (1979) Effect of zinc deficiency on the weight, cellularity and zinc concentra tion of different skeletal muscles in the post-weanling rat. Br. /. Nutr. 42: 487-499. 4. JACKSON,M. J., JONES,D. A. & EDWARDS,R. H. T. (1982) Tissue zinc levels as an index of body zinc status. Clin. Physiol. 2: 333343. 5. GIUGLIANO,R. & MILLWARD,D. J. (1984) Growth and zinc ho meostasis in the severely Zn-deficient rat. Br. J. Nutr. 52: 545560.
ZINC STATUS ASSESSMENT 6. HAMBIDGE,K. M., CASEY, C. E. & KREBS,N. F. (1986) Zinc. In: Trace Elements in Human and Animal Nutrition, 5th edition, pp. 1-137 (W. Mertz, ed.), Academic Press, Orlando. 7. HAMBIDGE,K. M., HAMBIDGE,C., JACOBS,M., BAUM,J. D. (1972) Low levels of zinc in hair, anorexia, poor growth and hypoguesia in children. Pediat. Res. 6: 868-874. 8. WALRAVENS, P. A., KREBS,N. F. & HAMBIDGE,K. M. (1983) Linear growth of low income preschool children receiving a zinc sup plement. Am. /. Clin. Nutr. 38: 195-201. 9. WALRAVENS, P. A. & HAMBIDGE,K. M. (1976) Growth of infants fed a zinc supplemented formula. Am. ¡.Clin. Nutr. 29: 11141121. 10. WALRAVENS,P. A., HAMBIDGE,K. M. & KOEPFER,D. M. (1989) Zinc supplementation in infants with a nutritional pattern of failure to thrive: a double-blind, controlled study. Pediatrics 83: 532-538. 11. HALSTED,J. A., RONAGHY,H. A., ABADI,P., HAGHSHENASS,M., AMIRHAKIMI,G. H., BARAKAT,R. M., & REINHOLD,J. G. (1972) Zinc deficiency in man. Am. /. Med. 53: 277-284. 12. HAMBIDGE,K. M. (1989) Mild zinc deficiency in human subjects. In: Zinc in Human Biology, pp. 281-296 (C. F. Mills, ed.), Springer-Verlag, London. 13. KING, J. C. (1986) Assessment of techniques for determining human zinc requirements. /. Am. Diet. Assoc. 86: 1523-1527. 14. WADA, L., TURNLUND,J. R. a KING, J. C. (1985) Zinc utilization in young men fed adequate and low zinc intakes. /. Nutr. 115: 1345-1354. 15. JACKSON,M. J., JONES,D. A., EDWARDS,R. H. T., SWAINBANK, I. G. & COLEMAN,M. L. (1984) Zinc homeostasis in man: studies using a new stable isotope-dilution technique. Br. /. Nutr. 51: 199-208. lé.MILNE, D. B., CANFIELD,W. K., MAHALKO,J. R. & SANDSTEAD, H. H. (1983) Effect of dietary zinc on whole body surface loss of zinc: impact on estimation of zinc retention by balance method. Am. /. Clin. Nutr. 38: 181-186. 17. BAER,M. T. & KING, J. C. (1984) Tissue zinc levels and zinc excretion during experimental zinc depletion in young men. Am. /. Clin. Nutr. 39: 556-570.
Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
1479
18. BAER,M. T., KING, J. C., TAMURA,T., MARGEN,S., BRADFIELD, R. B., WESTON, W. L. & DAUGHERTY,N. A. (1985) Nitrogen utilization, enzyme activity, glucose intolerance and leukocyte chemotaxis in human experimental zinc depletion. Am. /. Clin. Nutr. 41: 1220-1235. 19. JOHNSON, P. E., HUNT, J. R. &. RALSTON, N. V. C. (1988) The effect of past and current dietary Zn intake on Zn absorption and endogenous excretion in the rat. /. Nutr. 118: 1205-1209. 20. PRASAD,A. S. (1982) Clinical and biochemical spectrum of zinc deficiency in human subjects. In: Clinical, Biochemical, and Nutritional Aspects of Trace Elements, pp. 3-62 (A. S. Prasad, ed.), Alan R. Liss, New York. 21. COUSINS,R. J. (1989) Systemic transport of zinc. In: Mills, C. F., ed. Zinc in Human Biology, pp. 79-93 (C. F. Mills, ed.), SpringerVerlag, London. 22. SOLOMONS,N. W., ELSON, C. O., PEKAREK,R. S., JACOB, R., SANDSTEAD,H. H. & ROSENBERG,I. H. (1978) Leukocytic en dogenous mediator (LEM) in Crohn's disease. Infect. Immunol. 22: 637-639. 23. JACOBS-GOODALL, M., HAMBIDGE, K. M., STALL, C. PRITTS, J. &
24. 25.
26.
27.
28.
NELSON,D. E. Daily variations in plasma zinc in normal adult women. In: Trace Elements in Man and Animals, vol. 6, pp. 491-492 (L. S. Hurley, C. L. Keen, B. Lönnerdal, R. B. Rucker, eds.), Plenum Press, New York. HENRY,R. W. & ELMES,M. E. (1975) Plasma zinc in acute star vation. Br. Med. /. 4: 625-626. COUSINS, R. J. (1985) Absorption, transport, and hepatic me tabolism of copper and zinc: special reference to metallothionein and ceruloplasmin. Physiol. Rev. 65: 238-309. COUSINS,R. J. & LEINART,A. S. (1988) Tissue-specific regulation of zinc metabolism and metallothionein genes by interleukin 1. FASEB /. 2: 2884-2890. BREMNER,I. &.MORRISON,J. N. (1986) Assessment of zinc, cop per and cadmium status in animals by assay of extracellular me tallothionein. Acta Pharmacol. Toxicol. 59: (suppl. 7): 502-509. GRIDER, A., BAILEY,L. B. Si COUSINS, R. J. (1990) Erythrocyte metallothionein as a index of zinc status in humans. Proc. Nati Acad. Sci. USA 87: 1259-1262.
Assessment of Zinc Status12 JANET
C. KING3
Department of Nutritional Sciences, University of California, Berkeley, CA 94720 tions in plasma zinc due to poor status could be dif ferentiated from those due to a change in metabolic conditions. A thorough study of metabolic and homeostatic responses to insufficient zinc intakes in lab oratory animals and humans may identify new poten tial markers of zinc nutriture. In a recent review (1), Golden compared the devel opment of zinc deficiency with that of other nutrients. When the intake of most nutrients is insufficient, stores or functional reserves are mobilized. With fur ther depletion, tissue nutrient concentrations decline, followed by deterioration in one or more specific functions or metabolic pathways. A functional reserve or store of zinc does not seem to be available for use when zinc intakes are insufficient. Instead, tissue zinc is conserved by a reduction or cessation of growth in growing organisms or by a decrease in excretion in non-growing organisms. If the dietary deficiency is mild, zinc homeostasis may be reestablished by ad justing growth rates and excretion. Thereafter, no fur ther biochemical or clinical changes develop. In this case, adaptation to lower zinc intakes is accomplished by reducing the rate of growth or excretion. However, with a more severe deficiency, further metabolic changes develop within a short period of time. Zinc
ABSTRACT Plasma zinc concentration has been den igrated as a measure of zinc status because it responds to metabolic conditions unrelated to zinc status and because it is insensitive to changes in dietary zinc. The insensitivity of plasma zinc to reductions in dietary zinc reflects the tremendous capacity of the organism to conserve tissue zinc by reductions in zinc excretion and/or reductions in the rate of growth. Changes in plasma zinc concentrations do not seem to occur until the capacity to reestablish homeostasis by reducing excretion and/or growth has been exceeded. Reductions in dietary zinc beyond the capacity to maintain homeo stasis lead to utilization of zinc from a small, rapidly turning over pool. This pool is located, at least in part, in the bone, liver, and plasma. Loss of a small, critical amount of zinc from this pool leads to the rapid onset of both biochemical and clinical signs of zinc deficiency. Thus, plasma zinc is a valid, useful indicator of the size of this exchangeable pool of zinc. Plasma metallothionein concentrations may prove useful for identifying poor zinc status. Plasma metallothionine concentrations reflect hepatic concentrations and, therefore, are re duced when the dietary zinc supply is low. However, since hepatic metallothionein concentrations rise in re sponse to stress, plasma metallothionein concentra tions are increased in this state when the plasma zinc concentrations are also likely to be low. Thus, mea surement of both plasma metallothionein and plasma zinc concentrations could differentiate a low plasma zinc due to dietary zinc deficiency from a low concen tration due to stress, infection, or other metabolic conditions. J. Hutr. 120:1474-1479, 1990.
1 Presented as part of a conference, "Nutrition Monitoring and Nutrition Status Assessment," at the first fall meeting of the Amer ican Institute of Nutrition, Charleston, South Carolina, December 8-10, 1989. The conference was supported in part by cooperative agreement HPU880004-02-1 with the DHHS Office of Disease Pre vention and Health Promotion, the USDA Human Nutrition In formation Service, the DHHS National Center for Health Statistics, and the International Life Sciences Institute-Nutrition Foundation. 2 The Planning Committee for the meeting consisted of Drs. He
INDEXING KEY WORDS:
•plasma zinc •zinc status •metallothionein •zinc homeostasis •zinc deficiency
len A. Guthrie, Roy J. Martin, Linda D. Meyers, James A. Olson, Catherine E. Woteki, and Richard G. Allison (ex officio). The sym posium papers were edited by a committee consisting of Dr. James Allen Olson (coordinator), Dept. of Biochemistry & Biophysics, Iowa State University, Ames, IA; Dr. Cathy C. Campbell, Division of Nutritional Sciences, Cornell University, Ithaca, NY; Dr. Roy J. Martin, Dept. of Foods &. Nutrition, University of Georgia, Athens, GA; and Dr. Catherine E. Woteki, Food &.Nutrition Board, National Academy of Sciences, Washington, DC. 3 Address reprint requests to J. C. King, Dept. of Nutritional
Lack of a specific sensitive biochemical or functional test for zinc status is a barrier to the study of human zinc nutrition. Measurement of plasma zinc is used most frequently, but it is not ideal because metabolic conditions unrelated to zinc status cause it to decline. Attempts to identify another indicator specific for changes in zinc nutriture have not been successful. However, if an indicator of the metabolic conditions lowering plasma zinc could be identified, then reduc 0022-3166/90
S3.00 ©1990 American Institute of Nutrition.
Sciences, University
of California,
Berkeley, CA 94720.
Received 25 March 1990. Accepted 11 July 1990.
1474 Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
ZINC
1475
STATUS ASSESSMENT
balance becomes negative and there is a net loss of tissue zinc. The pool from which zinc is mobilized appears to be small and to have a rapid turnover rate, as shown by the prompt appearance of deficiency signs in laboratory animals and humans fed diets lacking zinc. The progression of the steps in development of both mild and severe deficiencies is displayed in Fig. 1. This model is based on our present understanding of the metabolic and homeostatic responses to zinc deficiency. A brief review of this information follows.
Steps in Dietary Zinc Deficiency Reduced Growth or Excretion
Avid Tissue Zinc Conservation
In mild deficiency homeostasis is ' zinc re-established.
In severe deficiency. Mobilization of Zinc
ZINC DEFICIENCY IN LABORATORY ANIMALS
In laboratory animals, a reduction or cessation of growth is an early response to zinc deficiency. Wean ling rats fed a zinc-deficient diet generally stop growing in 4-5 d, and this cessation of growth enables them to maintain a normal whole-body zinc concentration (2). Although the whole-body zinc concentration is unchanged, the response of specific tissues to zinc de ficiency is not uniform. Some tissues lose zinc in order to support other tissues. For example, the zinc con centration in muscle is conserved in zinc-deficient an imals, while zinc concentration in bone, liver, and plasma falls (3-5). This redistribution places an ap preciable demand on the animal since the muscle comprises ~50-60%of the whole-body zinc (6). Mus cle tissue in weanling zinc deficient animals increases a small amount, leading to an increase in total muscle zinc (5). The concentration of zinc in bone usually drops ~ 65%, plasma zinc ~ 45%, and liver zinc ~ 720% (4, 5). Testes zinc concentrations may also fall. A system of variable priorities seems to regulate tissue zinc content in severely deficient animals. Bone zinc and, to a lesser extent, plasma and liver zinc are lowpriority tissues. Zinc is mobilized from those tissues to support high-priority tissues, such as muscle and skin (5). Although ~30% of the whole-body zinc is found in bone, the metabolic function of zinc in bone is uncertain (6). The small, rapidly exchangeable zinc pool that is mobilized in zinc deficiency probably de rives in large part from bone, but also from the liver and plasma.
ZINC DEPLETION
IN HUMANS
Data from human studies also show that growth retardation is an early response to zinc deficiency. A growth response to zinc supplementation has been seen in growth-retarded infants and children in Denver and in adolescents in Iran (7-11). In the Iranian ado lescents, where the extent of growth retardation was severe, major gains in growth occurred in response to supplemental zinc. The infants and children in Denver Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
from Exchangeable Pool
General Tissue Dysfunction
FIGURE 1 A flow-chart depicting steps in the develop ment of mild and severe zinc deficiency.
were much less growth retarded, and the growth rates of the zinc-supplemented children were only ~10% greater than those of placebo-treated controls (12). Apparently, very little change in tissue zinc was re quired before the growth response could occur, since a rapid change in growth was seen within a month. Plasma zinc levels and serum alkaline phosphatase ac tivities of the growth-retarded Denver infants did not differ from the controls before or after zinc supple mentation (10). The rapid growth response and the absence of any change in plasma zinc suggest that little to no zinc had been mobilized from the exchangeable pool. Based on the criteria in Fig. 1, these growth-re tarded infants and children seem to have had a mild zinc deficiency. The etiology of mild zinc deficiency in these Denver children is uncertain. Reported zinc intakes were not lower than that of the controls, and there was no ev idence of health problems that may have precipitated zinc deficiency (10). One can only speculate that the ability to adjust to low zinc intakes is highly variable and that, as a consequence, those infants and children with a reduced capacity for adaptation are at a greater risk of mild zinc deficiency. Since mild zinc deficiency, as defined in this paper, is not associated with any biochemical or functional changes, the only way to diagnose it is to measure the growth response to a zinc supplement. Walravens and co-workers (10) recom mend that a daily dose of 5 mg zinc be used both as a diagnostic test and as a potential treatment of mild zinc deficiency in all infants and children with declin ing weight percentiles. ADAPTATION TO LOW INTAKES OF ZINC Full-grown animals and humans adjust to low zinc intakes by reducing their zinc excretion. It is well
1476
KING
known that zinc deficiency is difficult to induce in ma ture laboratory animals, which makes them a poor model for study of zinc depletion. In metabolic balance studies of adult men and women, zinc balance was achieved with zinc intakes ranging from 54-292 /¿mol/ d (3.5 to 19 mg/d) (13). Data from our own laboratory show that zinc balance was reestablished within 9 d after initiation of an 85-/miol (5.5 mg) zinc diet (14). During this period fecal zinc dropped from 215 to 60 ¿imol/d(14 to 4 mg/d), but urinary zinc remained un changed. The decline in fecal zinc probably was pri marily due to a decrease in unabsorbed dietary zinc, but the secretion of endogenous gastrointestinal zinc also may have dropped. Jackson and co-workers (5) proposed that rapid adjustments in gastrointestinal zinc secretion enabled individuals to adapt to fluctu ations in dietary zinc. The absolute amount of zinc absorbed changes little when dietary zinc is increased or reduced. For example, a reduction in dietary zinc from 255 to 85 /¿mol/d(16.5 to 5.5 mg/d) in a group of men (14) caused the fractional zinc absorption to double, whereas the absolute amount of zinc absorbed only decreased ~15 ^mol/d (l mg/d). As the zinc intake is progressively lowered toward a zinc-free diet, the time required to achieve zinc bal ance lengthens. Zinc balance was achieved in only 9 d when intake was reduced from 255 to 85 /¿mol/d (16.5 to 5.5 mg/d) (14) without a reduction in urinary or plasma zinc. When dietary zinc was reduced from 126 to 55 /miol/d (8.2 to 3.6 mg/d) (16), balance was also achieved, but the amount of time required was not reported. Plasma zinc was relatively stable for the 4-mo low-zinc period, but losses in sweat declined by as much as 65%. Thus, sweat losses may be an im portant adaptive mechanism for conservation of zinc during low intakes. In another study, balance was achieved in 14-21 d when intake was reduced from 185 to 25 Mtnol/d (12 to 1.7 mg/d) (H. Anderson and B. O'Dell, personal communication). This was accom plished with a 50% reduction in urinary zinc and a 60% reduction in fecal zinc; plasma zinc also declined ~22%. Feeding a virtually zinc-free diet (0.3 mg/d) did not allow zinc balance, but it is remarkable how close it came. After 21 d, total zinc losses in the urine and feces dropped from > 11 mg/d to ~ 1 mg/d, dem onstrating the tremendous capacity of the body to re duce zinc output and to conserve whole-body zinc when the dietary supply is extremely low (Fig. 2) (17). Plasma zinc levels, however, fell 44%. Although zinc balance could be achieved with in takes as low as 25 ¿imol/d(1-7 mg/d) (H. Anderson and B. O'Dell, personal communication), this was not done without a fall in plasma zinc. Thus, the attain ment of zinc balance does not imply that normal zinc concentrations will be maintained in all body fluids and tissues. Studies in laboratory animals, described above, show that zinc is mobilized from some tissues to maintain zinc levels in others. The same seems to Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
Urinary Zinc FtcalZInc
0,
u Ñ
1-3
4-6
7-9 10-1213-1516 Day« of Depletion
1819-21
FIGURE 2 Changes in urinary and fecal zinc losses in young men fed a 0.3-mg zinc diet for 21 d (17).
be true for humans. Jackson and co-workers (4) sam pled tissues at the post mortem examination of an 11yr-old thalassemic child who had been treated with diethylene triamine pentacetic acid (DTPA), a zinc chelator. Although oral zinc supplements were given, large quantities of zinc were excreted in the urine, and the child displayed clinical symptoms of zinc defi ciency prior to death. The concentration of zinc in the bone, liver, testes, and plasma were below the normal range, while muscle, skin, and hair zinc concentrations remained normal at the time of death. Thus, the tissues that lost zinc in this patient were the same as those that lost zinc in zinc-deficient rats.
INDICATORS OF ZINC STATUS Plasma zinc has been denigrated as a measure of zinc status because it does not reflect reductions in dietary zinc intake or changes in whole-body zinc. Both of these criticisms are valid. Since only minimal changes in whole-body zinc develop during zinc de pletion, changes in plasma zinc are not linked to whole-body zinc. Also, as discussed above, plasma zinc seems to fall only when the dietary intake is so low that homeostasis cannot be established without use of some zinc from the exchangeable pool, of which plasma zinc is a component. Thus, plasma zinc is a valid, useful indicator of the size of the exchangeable pool zinc; a reduction in plasma zinc reflects a loss of zinc from bone and liver and an increased risk for de velopment of metabolic and clinical signs of zinc de ficiency (Fig. 1). From the limited data available, it appears that the exchangeable pool of zinc is not used until the dietary zinc intake falls below 5-6 mg/d. Further studies are needed to delineate this threshold for utilization of exchangeable pool zinc more pre cisely. The extent to which the exchangeable pool of zinc is influenced by age, gender, body size, body com-
ZINC
STATUS ASSESSMENT
position, and long-term dietary zinc intake is not yet known. Both the rapid onset and reversal of biochemical and clinical signs of zinc deficiency in laboratory an imals and humans suggest that the exchangeable zinc pool is quite small. Acute and chronic human depletion studies can be used to estimate total zinc losses and to approximate the size of the exchangeable pool. When 0.3 mg zinc was fed to young men (18), bio chemical and clinical changes were seen as soon as 34 wk after depletion was started. By the end of the depletion period, i.e., when plasma zinc concentrations fell below 10.8 ^mol/L (70 Mg/100 mL), a number of significant changes in both biochemical and clinical indices were evident. Serum concentrations of albu min, retinol-binding protein, lactate dehydrogenase, delta amino-levulinic acid, and uric acid were signif icantly lower, and the ability to remove an oral glucose load was reduced. All of the final values for these in dices were still within the normal range, however. Of the clinical signs, skin lesions were the most prevalent. Severity of the lesions ranged from small patches of dry, rough skin to severe acne (18). Other clinical signs included diarrhea, sore throats, poor ap petite, and an increase in the amount of energy re quired to maintain body weight. Although body com position changes were not measured in this study, a reduction in nitrogen balance from the predepletion period suggests that lean tissue was lost. The increase in dietary energy required to maintain body weight probably allowed this lean tissue to be replaced by fat. By the end of the depletion period, which ranged from 4 to 9 wk among the six subjects, total zinc losses averaged ~1460 ¿tmol(95 mg) (17). This is equivalent to only 5% of the whole-body zinc, or ~12% of the bone zinc. Total losses of the six men varied almost fourfold, from 740 to 2540 ¿unol(48-165 mg). The man with the highest total loss of zinc was released from the depletion phase after the 4th wk because of the onset of severe acne symptoms and a rapid decline in his plasma zinc to 3.7 /¿mol/L(24 ¿ig/100mL). His fecal zinc losses remained high into the second week of depletion, whereas the other five men adapted by showing a marked drop in fecal zinc output. The man with the lowest total loss was on the depletion diet for 9 wk. He reported a low pre-study zinc intake, and his plasma zinc at the time of the pre-study physical examination was < 10.8 ¿¿rnol/L (70 /ig/100 mL). Con sequently, he was given a daily supplement of 154 /¿mol(10 mg) for 2 wk prior to entering the study. These two cases show that the ability to adjust to an acute reduction in dietary zinc is highly variable. Ad justment in gastrointestinal losses seems to be an im portant aspect of the adaptive process. Also, the past history of zinc intake may be a factor. In laboratory animals, low dietary zinc intakes in the past are as sociated with lower endogenous losses in subsequent studies (19). Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
1477
Prasad (20) studied chronic zinc depletion in four patients fed either 42 or 54 /¿mol(2.7 or 3.5 mg) zinc/ d for 24 wk. All of the patients lost weight, and two of them complained of mild roughening of skin and of lethargy. Plasma, erythrocyte, and leukocyte zinc concentrations decreased significantly. Plasma alkaline phosphatase, ribonuclease, lactic dehydrogenase, and ammonia levels also declined significantly, and thymidine kinase activity in the connective tissue of a sponge implanted subcutaneously was reduced. The biochemical and clinical changes seen in this study of chronic zinc depletion are similar to those reported for acute depletion. Total zinc losses in the two situ ations are quite different, however. Based on balance data, Prasad (20) estimated that his patients lost ~7700 timol (500 mg) zinc during the 24-wk depletion period. Thus, ~25% of the estimated whole body pool of 2 g, or about 65% of the bone zinc, was lost during that period. These losses seem unusually high, given the evidence that humans have a remarkable capacity to reduce zinc excretion when intakes are low. Zinc losses may have been overestimated in these earlier studies (20) since continuous balances were not done. Plasma zinc concentration, although reflecting changes in the size of the exchangeable zinc pool, also is responsive to other metabolic changes. For example, plasma zinc also changes in response to stress (21), infection (22), meals (23), short-term fasting (24), and the hormonal state (25). The fall in plasma zinc with all of these conditions except fasting may occur be cause zinc is redistributed to other tissues in response to a metabolic need. For example, administration of interleukin 1, a cytokine released in response to in fection, tissue injury, and inflammation, causes an up take of 65Znin the liver, bone marrow, and thymus of laboratory animals and reduces the uptake in bone, skin, and intestine (26). Circulating zinc is conse quently depressed for a short period of time. Thus, plasma zinc can only be useful as a specific indicator of zinc status if the effects of zinc status and of these other metabolic conditions can be differentiated.
METALLOTHIONEN AS AN ACCESSORY INDICATOR OF ZINC STATUS Golden (1) suggested that metallothionein may be the key to diagnosing tissue zinc redistribution. Me tallothionein is a low-molecular-weight protein with a high cysteine content (~30% of amino acid residues) and a high metal-binding capacity (~7 g-atoms/mol) (25). It is found in variable amounts in most tissues and particularly in liver, pancreas, kidney, and intes tinal mucosa. Under normal physiological conditions, metallothionein primarily binds zinc and/or copper. Tissue metallothionein concentrations are often pro portional to zinc status (25); e.g., they are reduced to nondetectable levels in zinc-deficient animals and are
1478
KING
increased after parenteral or dietary administration of zinc. Dietary and liver zinc content are closely related. As dietary zinc concentrations increase, metallothionein synthesis is induced; thus, most of the additional zinc in the liver is bound to metallothionein. Metallothionein can be detected in the plasma and erythrocytes by a radioimmunoassay (27). Both plasma and erythrocyte metallothionein levels are sensitive to dietary zinc intake. Plasma, but not erythrocyte metallothionein, reflects changes in hepatic metallo thionein concentrations. An increase in hepatic me tallothionein in response to stress, inflammation, or hormonal changes will cause comparable changes in plasma metallothionein levels. For example, liver me tallothionein levels were increased 14-fold after interleukin-1 administration (26). Thus, measurement of both zinc and metallothionein in the plasma will allow plasma zinc concentrations to be better inter preted. Low levels of both plasma zinc and metallo thionein would imply a reduction in the size of the exchangeable zinc pool in response to low zinc intakes (Fig. 3). A low level of plasma zinc with a high level of metallothionein would imply that tissue zinc is being redistributed in response to confounding con ditions, not that zinc nutriture is compromised. If acute zinc deficiency and stress are both present, both plasma metallothionein and zinc levels would be low because the hepatic synthesis of metallothionein is not stimulated by stress in zinc-deficient animals (27). Thus, stress in a zinc-deficient animal does not prevent the detection of zinc deficiency. Standardization of the amount of time since food was consumed is also im portant for all measurements, inasmuch as plasma zinc declines from 15% to 20% following a meal (23). A human metallothionein assay has been developed (28). Preliminary studies of the response of erythrocyte metallothionein to changes in dietary zinc are en couraging. After human volunteers were fed a very low zinc intake (~0.5 mg/d) for 8 d, erythrocyte me tallothionein declined 59% whereas fasting plasma zinc concentrations were reduced by only 7%. Con versely, supplementation with 50 mg zinc/d increased erythrocyte metallothionein sevenfold in 7 d. Eryth rocyte metallothionein, like plasma metallothionein, is sensitive to changes in dietary zinc. As discussed above, plasma zinc concentrations do not reflect recent changes in dietary zinc. Possibly, erythrocyte metal lothionein concentrations will also prove to be a useful indicator of changes in dietary zinc. Since erythrocyte metallothionein is not responsive to stress as is plasma metallothionein, the presence of stress will not inter fere with detection of low dietary zinc. SUMMARY
Much has been learned about zinc metabolism in the past several years. This information, summarized Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
Assessment of Zinc Status Reduced Pool Size:
Low Plasma Zinc Low Plasma Metallothionein
Tissue Redistribution:
Low Plasma Zinc High Plasma Metallothionein
FIGURE 3 Differentiation of declines in plasma zinc due to decreases in pool size from tissue redistribution.
below, provides a potential approach for assessing zinc status. 1. The initial response to low dietary zinc is con servation of tissue zinc. Little to no change in plasma zinc concentrations occurs during this initial phase. Preliminary studies suggest that erythrocyte metallothionein concentrations sen sitively reflect changes in dietary zinc. 2. Plasma zinc is a component of the exchangeable zinc pool, which is utilized when dietary zinc is so low that homeostasis cannot be reestablished by reducing the rate of growth or zinc excretion. 3. The rapid onset and reversal of the biochemical and clinical signs of zinc depletion imply that the exchangeable zinc pool is small; loss of as little as 770-1540 nmol (50-100 mg) in adults, or 2.5-5% of the whole body content of zinc, leads to the onset of zinc-deficiency signs. 4. Plasma zinc concentrations decline in response to stress, infection, or a variety of hormonal changes. These conditions also induce metallo thionein synthesis in the liver. Hepatic and plasma metallothionein are closely related. Thus, low plasma zinc concentrations in the presence of increased plasma metallothionein imply tissue zinc redistribution caused by confounding con ditions, not an impaired zinc status.
LITERATURE CITED 1. GOLDEN,M. H. N. (1989) The diagnosis of zinc deficiency. In: Zinc in Human Biology, pp. 323-333 (C. F. Mills, ed.), SpringerVerlag, London. 2. WILLIAMS,R. B. & MILLS, C. F. (1970) The experimental pro duction of zinc deficiency in the rat. Br. /. Nutr. 24: 989-1003. 3. O'LEARY,M.}., McLAiN, C. J. & HEGARTY,P. V. J. (1979) Effect of zinc deficiency on the weight, cellularity and zinc concentra tion of different skeletal muscles in the post-weanling rat. Br. /. Nutr. 42: 487-499. 4. JACKSON,M. J., JONES,D. A. & EDWARDS,R. H. T. (1982) Tissue zinc levels as an index of body zinc status. Clin. Physiol. 2: 333343. 5. GIUGLIANO,R. & MILLWARD,D. J. (1984) Growth and zinc ho meostasis in the severely Zn-deficient rat. Br. J. Nutr. 52: 545560.
ZINC STATUS ASSESSMENT 6. HAMBIDGE,K. M., CASEY, C. E. & KREBS,N. F. (1986) Zinc. In: Trace Elements in Human and Animal Nutrition, 5th edition, pp. 1-137 (W. Mertz, ed.), Academic Press, Orlando. 7. HAMBIDGE,K. M., HAMBIDGE,C., JACOBS,M., BAUM,J. D. (1972) Low levels of zinc in hair, anorexia, poor growth and hypoguesia in children. Pediat. Res. 6: 868-874. 8. WALRAVENS, P. A., KREBS,N. F. & HAMBIDGE,K. M. (1983) Linear growth of low income preschool children receiving a zinc sup plement. Am. /. Clin. Nutr. 38: 195-201. 9. WALRAVENS, P. A. & HAMBIDGE,K. M. (1976) Growth of infants fed a zinc supplemented formula. Am. ¡.Clin. Nutr. 29: 11141121. 10. WALRAVENS,P. A., HAMBIDGE,K. M. & KOEPFER,D. M. (1989) Zinc supplementation in infants with a nutritional pattern of failure to thrive: a double-blind, controlled study. Pediatrics 83: 532-538. 11. HALSTED,J. A., RONAGHY,H. A., ABADI,P., HAGHSHENASS,M., AMIRHAKIMI,G. H., BARAKAT,R. M., & REINHOLD,J. G. (1972) Zinc deficiency in man. Am. /. Med. 53: 277-284. 12. HAMBIDGE,K. M. (1989) Mild zinc deficiency in human subjects. In: Zinc in Human Biology, pp. 281-296 (C. F. Mills, ed.), Springer-Verlag, London. 13. KING, J. C. (1986) Assessment of techniques for determining human zinc requirements. /. Am. Diet. Assoc. 86: 1523-1527. 14. WADA, L., TURNLUND,J. R. a KING, J. C. (1985) Zinc utilization in young men fed adequate and low zinc intakes. /. Nutr. 115: 1345-1354. 15. JACKSON,M. J., JONES,D. A., EDWARDS,R. H. T., SWAINBANK, I. G. & COLEMAN,M. L. (1984) Zinc homeostasis in man: studies using a new stable isotope-dilution technique. Br. /. Nutr. 51: 199-208. lé.MILNE, D. B., CANFIELD,W. K., MAHALKO,J. R. & SANDSTEAD, H. H. (1983) Effect of dietary zinc on whole body surface loss of zinc: impact on estimation of zinc retention by balance method. Am. /. Clin. Nutr. 38: 181-186. 17. BAER,M. T. & KING, J. C. (1984) Tissue zinc levels and zinc excretion during experimental zinc depletion in young men. Am. /. Clin. Nutr. 39: 556-570.
Downloaded from https://academic.oup.com/jn/article-abstract/120/suppl_11/1474/4738626 by University of Minnesota Libraries - Twin Cities user on 12 May 2018
1479
18. BAER,M. T., KING, J. C., TAMURA,T., MARGEN,S., BRADFIELD, R. B., WESTON, W. L. & DAUGHERTY,N. A. (1985) Nitrogen utilization, enzyme activity, glucose intolerance and leukocyte chemotaxis in human experimental zinc depletion. Am. /. Clin. Nutr. 41: 1220-1235. 19. JOHNSON, P. E., HUNT, J. R. &. RALSTON, N. V. C. (1988) The effect of past and current dietary Zn intake on Zn absorption and endogenous excretion in the rat. /. Nutr. 118: 1205-1209. 20. PRASAD,A. S. (1982) Clinical and biochemical spectrum of zinc deficiency in human subjects. In: Clinical, Biochemical, and Nutritional Aspects of Trace Elements, pp. 3-62 (A. S. Prasad, ed.), Alan R. Liss, New York. 21. COUSINS,R. J. (1989) Systemic transport of zinc. In: Mills, C. F., ed. Zinc in Human Biology, pp. 79-93 (C. F. Mills, ed.), SpringerVerlag, London. 22. SOLOMONS,N. W., ELSON, C. O., PEKAREK,R. S., JACOB, R., SANDSTEAD,H. H. & ROSENBERG,I. H. (1978) Leukocytic en dogenous mediator (LEM) in Crohn's disease. Infect. Immunol. 22: 637-639. 23. JACOBS-GOODALL, M., HAMBIDGE, K. M., STALL, C. PRITTS, J. &
24. 25.
26.
27.
28.
NELSON,D. E. Daily variations in plasma zinc in normal adult women. In: Trace Elements in Man and Animals, vol. 6, pp. 491-492 (L. S. Hurley, C. L. Keen, B. Lönnerdal, R. B. Rucker, eds.), Plenum Press, New York. HENRY,R. W. & ELMES,M. E. (1975) Plasma zinc in acute star vation. Br. Med. /. 4: 625-626. COUSINS, R. J. (1985) Absorption, transport, and hepatic me tabolism of copper and zinc: special reference to metallothionein and ceruloplasmin. Physiol. Rev. 65: 238-309. COUSINS,R. J. & LEINART,A. S. (1988) Tissue-specific regulation of zinc metabolism and metallothionein genes by interleukin 1. FASEB /. 2: 2884-2890. BREMNER,I. &.MORRISON,J. N. (1986) Assessment of zinc, cop per and cadmium status in animals by assay of extracellular me tallothionein. Acta Pharmacol. Toxicol. 59: (suppl. 7): 502-509. GRIDER, A., BAILEY,L. B. Si COUSINS, R. J. (1990) Erythrocyte metallothionein as a index of zinc status in humans. Proc. Nati Acad. Sci. USA 87: 1259-1262.
