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Investigating Minor Nutrients of Major Importance
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In a study of magnesium absorption,
chemist Joseph Domek measures
magnesium content in water with
an atomic absorption spectrophotometer.
(K9286-1)
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American favorites like chocolate
bars and some kinds of beer are—perhaps surprisingly—rich in an
essential nutrient, copper. So, too, are the foods that Mom may have told you
to eat, like liver and other organ meats; peas, beans, and other legumes; and
whole-grain breads or other grain products.
Even though copper has been known for a century to play a key role in our
health, much more remains to be discovered about this vital mineral.
Investigations led by Judith R. Turnlund at the
ARS Western Human Nutrition Research
Center in Davis, California, are helping to fill in missing pieces of this
puzzle. Her studies are revealing new clues about what our bodies do with the
copper that we get from everyday foods. |
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Chemist Judith Turnlund and
physical scientist William
Keyes use thermal ionization
mass spectrometry to measure
trackable forms of copper,
called stable isotopes, in
blood plasma.
(K9287-1)
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"We already know that we need copper for strong bones, a well-functioning
nervous system, and a healthy heart," says Turnlund. "But we'd like
to know more, like where exactly does it go after we eat? How fast does it get
there? How much do we store? How much do we lose?"
To help answer these questions, Turnlund's group uses rare, trackable forms of
copper, called stable isotopes, to follow copper's fate in the body. Turnlund
pioneered the use of isotopes to study essential minerals in humans. The
information that her team uncovers can be used—along with findings from
labs elsewhere—in setting the national nutrition guidelines, or
recommended dietary intakes, which help ensure that Americans of all ages get
enough copper to stay healthy. The recommended intakes for some nutrients show
up in the familiar lists printed on regular products, like a box of breakfast
cereal, can of fruit, or bottle of vitamin/mineral tablets.
Among Turnlund's other research targets are molybdenum and magnesium.
Molybdenum is an essential component of three enzymes needed to chemically
process—or metabolize—certain amino acids that we get from protein.
Magnesium, like copper, is important for our bones, nervous system, and
heart. |
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Chemist Joseph Domek prepares ion
exchange chromatography columns for
separation of copper, iron, and
zinc in samples from participants
in a study of high dietary copper.
(K9285-1)
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Turnlund's work with these trace
elements, plus zinc and iron, has made her an international authority on
mineral nutrition. Her copper studies include unique investigations in which
she has scrutinized the effects of very low and very high intakes to find out
what's safe for us—and what likely isn't.
"In our tests of healthy young men," she reports, "we found that
0.38 milligrams of copper a day is too low." Levels of a copper-containing
enzyme called lysyl oxidase—needed in the skin, bones, and
heart—decreased in most of the volunteers during the low-copper regimen.
What's more, a trio of other indicators of copper nutrition—serum copper,
ceruloplasmin, and superoxide dismutase—also declined.
Turnlund and collaborators in the United Sates and abroad are currently
analyzing the data from their recent high-copper study. "We want to
know," she says, "what might happen to people who take two or three
vitamin/mineral supplements a day, meaning that they'd be getting several extra
milligrams of copper. We think it's important to establish whether that
amount—if taken over a long period of time—is too high."
Model Mimics Copper's Cycle
Discoveries from each of the copper experiments help Turnlund refine a major
product of her research—a handy, computer-driven model for filling in gaps
about how our bodies use copper. Says Turnlund, "We started with a very
sophisticated, highly versatile modeling program that scientists at the
National Institutes of Health and the University of Washington in Seattle
developed for simulating many different kinds of biochemical processes. We use
the program in desktop computers to process—with mathematical
equations—our data as well as data from other nutrition studies of animals
and humans.
"The result, in our case, is a best estimate of how copper makes its way
through the body. For example, the model predicts how copper might move from
our digestive tract to the plasma in our blood, then to our liver—where it
is incorporated into enzymes—and, later, to the cells of other tissues and
out of our systems.
"Each of these points along the pathway," she notes, "is called
a compartment. Some, like plasma or urine, can be easily sampled to get a
better idea of how much copper goes into each and for how long, and how much
remains."
But what about compartments like the liver, where following copper's progress
would be difficult unless one sampled a little chunk of liver from time to
time? "For those compartments," explains Turnlund, "we start
with animal data. The mathematical equations can help us extrapolate it to
match—to the greatest extent possible—what may happen in humans.
"Our model," she says, "is only a prediction, or estimate, of
course. But it allows us to apply the computer's enormous speed and power to
crunch mountains of data into possible scenarios. Looking at the scenarios
helps reveal new possibilities. And it can help us decide what needs to be
studied next."
Turnlund's copper model, first published in 1994, was based on studies of
healthy volunteers. It was the first-ever computerized model of how copper is
metabolized in healthy people. The model has provided new support for what some
scientists had suspected earlier, namely that copper most likely progresses
though our bodies primarily in one-way paths, with little recycling. Notes
Turnlund, "That's unlike molybdenum, zinc, or a number of other nutrients
that tend to move back and forth from one compartment to another."
Monitoring Molybdenum and Magnesium
In their studies of molybdenum, Turnlund and coinvestigators have produced
another computerized model similar to the copper one. Data for the model came
from studies in which Turnlund's team fed volunteers various amounts of
molybdenum, "ranging from as low as we could get to as high as might be
conceivable," she says.
"We found that it's unlikely that healthy people would develop a
molybdenum deficiency at any of these levels," she reports. "We
estimated that the minimum requirement for adults is about 25 micrograms a day.
People usually eat considerably more than that in a day's worth of ordinary
meals." The investigation was the first to provide an estimated minimum
level for molybdenum in healthy volunteers.
Seeds, such as those from sunflower or pumpkin; legumes like peas, beans, and
peanuts; and grain-based products, such as breakfast cereals or whole-grain
breads, are often good sources of molybdenum. The level varies, however,
according to the quantity of molybdenum in the soil where the crop was grown.
In her magnesium research, Turnlund is now tackling the daunting task of
determining an easy, reliable way to measure how much of this mineral we absorb
from our food. Good sources of magnesium include green, leafy vegetables; whole
grains; and nuts.
In this venture, she's collaborating with the producers of Perrier, a premium
bottled water. "Perrier researchers," says Turnlund, "want to
know if bottled water is a good source of magnesium. To find out, they need a
good way to measure magnesium absorption. We need that for our research, too.
So we're working together to find a test that is fast, easy, accurate,
reliable, reproducible, and, of course, affordable."
So far, they've found that a urine test appears to be as accurate as fecal
analyses—and is faster and easier. For the urine test, researchers provide
volunteers with a food or beverage that's spiked with a traceable form of
magnesium. They also inject a tiny quantity of the tracer magnesium into the
volunteers' blood, then collect urine specimens about 2 days later. To
determine the amount of tracer magnesium in the sample, the researchers use a
high-tech instrument known as an inductively coupled plasma mass spectrometer.
From this they can calculate how much magnesium each volunteer absorbed and
used.
Turnlund's team will also use data from the magnesium study—hundreds of
specimens in all—to create a new model of how our bodies cycle this
important mineral.—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.
Judith R. Turnlund is with the
USDA-ARS Western Human Nutrition Research
Center, One Shields Ave., Davis, CA 95616; phone (530) 752-5249, fax (530)
752-5271. |
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"Investigating Minor Nutrients of Major
Importance" was published in the
March 2001
issue of Agricultural Research magazine.
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