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Department of Nutritional Sciences, University of California, Berkeley, CA 94720
Plasma zinc concentration has been denigrated 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 homeostasis 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 reduced when the dietary zinc supply is low. However, since hepatic metallothionein concentrations rise in response to stress, plasma metallothionein concentrations are increased in this state when the plasma zinc concentrations are also likely to be low. Thus, measurement of both plasma metallothionein and plasma zinc concentrations could differentiate a low plasma zinc due to dietary zinc deficiency from a low concentration due to stress, infection, or other metabolic conditions.
KEY WORDS: plasma zinc zinc status metallothionein zinc homeostasis zinc deficiency
1 Presented as part of a conference, "Nutrition Monitoring and Nutrition Status Assessment", at the first fall meeting of the American Institute of Nutrition, Charleston, South Carolina, December 810, 1989. The conference was supported in part by cooperative agreement HPU880004-02-1 with the DHHS Office of Disease Prevention and Health Promotion, the USDA Human Nutrition Information 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. Helen A. Guthrie, Roy J. Martin, Linda D. Meyers, James A. Olson, Catherine E. Woteki, and Richard G. Allison (ex officio). The symposium 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 Sciences, University of California, Berkeley, CA 94720.
Manuscript received 25 March 1990. Revision accepted 11 July 1990.
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