...From the pages of Agricultural Research magazine
Zooming In on Zinc
Zinc is an essential mineral
that has many functions in
the human body. You can get
your zinc from many foods,
such as oysters (6 medium),
76.28 mg; beef, top sirloin,
broiled (3 oz.), 4.97 mg;
and peanuts, dry roasted (1 oz.),
0.94 mg. (Source: USDA Nutrient
Database for Standard Reference,
Release 14.) ARS scientists are
studying how zinc gets
transported throughout the
body and how iron supplements
might affect zinc absorption.
Zinc likely affects every general function in our bodies. Discoveries
from scientists at the ARS Western
Human Nutrition Research Center at Davis, California, are unlocking
new and important information about this essential mineral.
Some of the findings may lead to a new test that would help predict
an individual's ability to use zinc. Other results may modify today's
use of iron supplements during pregnancy and breast-feeding. The method
most commonly used to check body levels of zinca test of serum,
from bloodisn't sensitive enough to detect a mild deficiency.
"Mild zinc deficiency may exist in the United States among otherwise
healthy infants, toddlers, preschool children, pregnant and lactating
women, and seniors," says ARS research geneticist Liping Huang.
Zinc is a critical part of the mechanism that turns genes on and off. "Genes that are turned on prompt the body to make proteins, enzymes, or hormones; genes that are turned off do not," she says. "In a healthy body, at any given time, some genes are turned on and some are turned off. Without zinc, some genes won't turn on and off properly. Zinc is also a very important part of many enzymes needed for healthy skin and properly functioning immune, nervous, and digestive systems."
Foods and spices that contain
the essential mineral zinc.
Transporter Proteins Move Zinc
To get zinc from foods that we eat into the cells that need it, the
body relies on proteins known as zinc transporters. These specialized
proteins also take excess zinc out of cells to prevent toxic buildup.
Zinc is absorbed into the blood from food in the small intestine. Next,
zinc circulating in the blood needs to be transported into cells.
Huang is investigating a family of zinc transporters called ZnTs. "We discovered and characterizedin micethe function of ZnT4, a transporter that deposits zinc into milk in the mammary glands of mice. In humans, zinc is already known to be essential for proper growth during infancy. So it makes sense that ZnT4 would be found in abundance in the cells of the mammary gland," says Huang.
"Our work suggests two other roles for this transporter protein.
First, ZnT4 may move zinc out of the small intestine into the bloodstream.
Second, ZnT4 may help store zinc in the cells of the prostate gland,"
she notes. "The prostate finding is of special interest. Other
scientists' studies indicated that healthy men have more zinc in their
prostate than anywhere else in their bodies," says Huang. These
results suggest that zinc supplements might protect men against prostate
cancer." In the United States, prostate cancer is the second leading
cause of cancer deaths among men.
In a study of how zinc levels
between cells are regulated,
geneticist Liping Huang studies
ZnT transporter proteins in
normal rat kidney cells.
Genes: An Ideal Basis for a Test?
Now Huang is hot on the trail of what she thinks are three additional
ZnTs. The genetic material the body uses to create transporter proteins
might prove ideal as the basis for a new and improved test of zinc in
"The amount of a particular zinc transporter protein that an individual
produces is partially controlled by genes," Huang explains. "An
indirect way to measure the protein is to measure levels of genetic
material called messenger RNA, or mRNA. Zinc transporter genes have
the instructions the body needs in order to create zinc transporter
proteins. The mRNA carries out those instructions." Levels of mRNA
may thus be a good indicator of an individual's ability to process zinc.
The goal is a sensitive, accurate test that is also inexpensive, rapid, reliable, and reproducible. Says Huang, "An ideal test could be administered in your doctor's office, with the results ready before you leave. Today, no test of zinc levels in the body meets all of those criteria.
This woman is one of 32
volunteers participating in
a nutrition study to evaluate
the effect of iron supplements
on zinc during pregnancy and
"Zinc is unusual in that it seems to have a large array of transporter
proteins to shuttle it in and out of cells," she says. That may
be why there are very few diseases or disorders related to zinc imbalances.
In contrast, copper and iron, for example, apparently have fewer transporter
proteins than zinc. "Not surprisingly," notes Huang, "there
are several diseases caused by imbalances of copper and iron."
In her new studies, Huang says she "wants to help develop new national guidelines for zinc intake in foodor in supplementsfor different age groups." She will also study whether zinc transporter proteins could prevent the proliferation of prostate cancer cells.
Meanwhile, a puzzling interaction of iron and zinc is the focus of research by Janet C. King, director of the Western Human Nutrition Research Center. "In a study we did some years ago on the zinc requirements of pregnant women," says King, "we were very surprised to find that iron supplements taken with a meal inhibit zinc uptake."
Registered nurse Earl Laih
prepares to inject a pregnant
volunteer with a trackable
form of zinc. This method lets
nutrition scientist Janet C.
King (center) measure how
much zinc the body uses.
This finding was important for two reasons. First, iron supplementation
is common for women: Iron supplements are prescribed routinely worldwide
for fetal growth and also for milk production during breast-feeding.
Second, these iron supplements are apparently interfering with zinc
during these critical times.
King conducted the study with 13 pregnant women, aged 22-40. "We
followed their zinc uptake and use from the beginning of pregnancy through
the first 3 months of breast-feeding," she says. Four of these
women were taking prenatal iron supplements of at least 100 milligrams
a day under the orders of their doctors. That's in contrast to the Recommended
Dietary Allowance of 18 milligrams for women of childbearing age.
At regular intervals throughout the study, volunteers were given an easily detected form of zinc in a beverage and another as an injection. These trackable kinds of zinc known as stable isotopes occur naturally. The amounts of the stable isotopes were measured in blood and urine samples collected a few hours later. From these measurements, scientists determined the amount of the zinc that each volunteer absorbed and used. Of all the volunteers, only the four who took the prescribed iron supplements didn't show any increase in zinc absorption during the early months of breast-feeding.
King and her colleagues reported their findings in the American Journal of Clinical Nutrition and other scientific publications.
King is now working on a follow-up study with pregnant volunteers aged
22-40 years. Each volunteer receives 15 milligrams of zinc from food
and supplements every day. Some volunteers receive prenatal iron supplements
of 65 milligrams a day.
King notes, "Some take their iron supplement in the morning, with
their breakfast, and others at bedtime, without a meal. We want to determine
whether zinc uptake is affected by taking the iron supplement at a particular
time of day and with or without a meal."
Concepcion Mendoza, a postdoctoral fellow in King's laboratory, is
leading this study. Fernando Viteri, a faculty member at the University
of California at Berkeley, is also a collaborator.
Studies in Brazil and South Korea are providing more information about
the interplay of zinc and iron or zinc and phytatea form of phosphorus.
Center scientists collaborated in these investigations.
"More than half of the people in the world don't get enough zinc,"
King points out. "In countries where grains or legumes such as
beans make up a significant component of the day's meals, zinc deficiency
is a special concern. That's because the zinc in grains and legumes
is less available to the body than the zinc in beef, pork, and dark-meat
The Brazilian experiment looked at the zinc used by 15 pregnant women
in Rio de Janeiro. The volunteers, aged 21-34, were given daily iron
supplements of 50 or 100 milligrams for most of the last half of pregnancy.
Volunteers ate their normal foods, including meats. Findings showed
that the 100-milligram iron supplements interfered with the uptake of
zinc but that the 50-milligram iron supplements did not.
Carmina L. Vargas Zapata and Carmen M. DonAngelo of the Federal University
of Rio de Janeiro conducted the research.
Zinc and Phytate Link Probed
A study in South Korea is exploring the interaction of phytate and
zinc in the body. Phytate is an essential nutrient. "The phytate
in grains such as rice and corn is thought to interfere with the uptake
and use of zinc," says Huang, a collaborator in the study.
Scientists are working with groups of female volunteersages 18-24
and 60-70. In the three-phase experiment, the researchers are altering
the relative amount of phytate to zinc the volunteers consume. In the
first phase, the women eat familiar fare, predominantly rice and vegetables.
This diet provides about 20 parts phytate to 1 part zinc.
In the second phase, the volunteers eat foods with less phytate, resulting
in a ratio of about 8 parts phytate to 1 part zinc. That's about the
ratio of phytate to zinc that Americans consume. The scientists lowered
the phytate consumption by providing white rice in place of higher phytate
For the third phase of the study, the volunteers eat their familiar
foods, including high-phytate rice. But they also take a 25-milligram
zinc supplement each day. This regimen again gives them a ratio of about
8 parts phytate to 1 part zinc.
Hee Young Paik of Seoul National University is directing the study and providing blood samples from the volunteers to Huang, at her Davis lab, for analysis of zinc transporters.
Is an mRNA-based Zinc Test Ahead?
Huang is looking at the levels of mRNAcreated from zinc transporter
genesin the samples. She wants to see which, if any, of the transporters
predict an individual's zinc uptake and use. If there is a correlation
between zinc use and the level of zinc transporter mRNA, the mRNA could
become the basis of a better test for the body's use of zinc.
She is also determining whether the levels of ZnT proteins vary with
age. "If there are significant differences in the way we process
zinc as we age," Huang says, "then the dietary allowance of
zinc may need to be adjusted."
Scientists have known for decades that zinc is essential for human
health. Foods that are good sources of zinc include beans, whole grains,
shellfish, red meat, and dark-meat poultry.By Marcia
Wood, Agricultural Research Service Information Staff.
This research is part of Human Nutrition, an ARS National Program
(#107) described on the World Wide Web at http://www.nps.ars.usda.gov.
Liping Huang and Janet
C. King are with the USDA-ARS Western
Human Nutrition Research Center, One Shields Ave., Davis, CA 95616;
phone (530) 754-5756, fax (530) 754-6015 [Huang], phone (530) 752-5268,
fax (530) 752-5271 [King].
|"Zooming In on Zinc" was published in the March 2002 issue of Agricultural Research magazine.|