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Chemist Betty Burri and biologist
Terry Neidlinger compare models
of tracer beta-carotene and natural
beta-carotene. The tracer model
(held by Burri) contains deuterium
atoms (red) in place of some
hydrogen atoms.
(K9298-1)
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The rich yellow of a mango or deep
orange of a carrot are the work of nutrients called carotenes. Our bodies can
convert some carotenes—namely, alpha-carotene, beta-carotene, and
beta-cryptoxanthin—into vitamin A, a nutrient essential for proper growth
and reproduction as well as for good eyesight. What's more, new evidence
further supports the value of carotenes as antioxidants that may reduce our
risk of cancer, stroke, arteriosclerosis, and cataracts.
Dozens of familiar, brightly colored, yellow, orange, or dark-green vegetables
and fruits provide carotenes. Perhaps most studied to date is beta-carotene.
Scientists have long suspected that individuals differ in their ability to
absorb beta-carotene and convert it to vitamin A. Early beta-carotene studies
with humans gave researchers a glimpse of this variability. But a series of
investigations over the past 5 years, led by ARS chemist Betty J. Burri, offers new,
more detailed proof of this diversity.
These findings are important for people who are cutting back on the amount of
meat and dairy products they eat. "Meat, eggs, cheese, and whole milk are
rich in vitamin A," says Burri, "so people who eat little if any of
these foods need to be sure they are getting an adequate supply of this
nutrient from other sources."
Burri is with the ARS Western Human Nutrition Research Center in Davis,
California. She did the work with Terry J. Neidlinger, also at the center;
Andrew J. Clifford, Stephen R. Dueker, Sabrina J. Hickenbottom, and Yumei Lin
of the University of California, Davis, Department of Nutrition; and Jin-Young
K. Park, formerly with ARS and now with the Food and Drug Administration.
Special Compounds Used As Trackers
The researchers studied 45 male and female volunteers, aged 18 to 42. For some
of the studies, volunteers were fed supplements containing special forms of
vitamin A and of beta-carotene. These forms can be traced, or detected, because
they weigh more than naturally occurring vitamin A and beta-carotene. The
sophisticated laboratory instruments that the researchers used—a
gas-chromatograph mass spectrometer and a high-performance liquid
chromatograph—can differentiate the tracer compounds from the naturally
occurring forms.
Research done elsewhere has tracked the fate of one or another of the compounds
in human volunteers. But the California studies were apparently the first to
evaluate uptake and use of both tracer beta-carotene and tracer vitamin A
concurrently. That gave Burri's team what is probably the best-ever look at the
interaction of these nutrients in healthy humans.
Surprising Variability
"We found new extremes in the amount of time it takes for beta-carotene to
be absorbed and converted—and in the amount that is converted," Burri
reports. "But most unexpected was the statistically significant difference
in beta-carotene uptake and conversion by physically similar volunteers,
including one pair who were so alike that they could well have been twins.
"Both were females of nearly identical age, height, and weight. They had a
similar amount of body fat and about the same amount of vitamin A in their
blood at the start of the study. Their uptake of our tracer vitamin A was
similar. That isn't unusual, because we already know that most well-fed people
absorb vitamin A in nearly the same way. But the first volunteer used about 30
percent of the tracer beta-carotene within only 12 hours of taking it. Of that
amount, she converted about 30 percent to vitamin A.
"The second volunteer took up only about 15 percent of the tracer
beta-carotene and took about 3 days to do it. Then, she converted only about 8
percent to vitamin A.
"Essentially," Burri summarized, "the first volunteer used up
about twice as much beta-carotene and converted it to about 8 times more
vitamin A. We hadn't expected individuals who were so similar in so many key
variables to be so different in their processing of beta-carotene."
With the exception of a volunteer who was very low in vitamin A at the outset
of one of the studies, most volunteers handled vitamin A similarly, as had been
shown in previous research in the United States and abroad. But about half of
all Burri's volunteers—male and female—didn't take up much
beta-carotene at all. Uptake amounts ranged from undetectable to about 50
percent. About half of the volunteers didn't form much vitamin A from the
beta-carotene they did absorb.
Basic Chemistry Doesn't Apply
Notes Burri, "None of our volunteers metabolized 100 percent of the
beta-carotene, but that's what we expected to happen. Even though
beta-carotene—of all the carotenoids—is the easiest for us to convert
into vitamin A, we don't do it as efficiently as the basic chemistry of
beta-carotene might suggest.
"Beta-carotene is a large molecule. Its chemical structure looks like two
molecules of vitamin A joined end to end but facing opposite directions. It
would seem—on paper, at least—that one molecule of beta-carotene
should, logically, yield two molecules of vitamin A. But the body isn't a
perfect chemical factory. We don't form two molecules of vitamin A for every
one molecule of beta-carotene that we consume."
Burri says the findings may help explain why giving beta-carotene supplements
to people who are deficient in vitamin A may not be sufficient to prevent the
blindness and death that lack of vitamin A causes today in Southeast Asia,
sub-Saharan Africa, or South America, for instance. The procedure that her team
used for tracking vitamin A and beta-carotene simultaneously could be adapted
to screen individuals in these regions for their ability to process
beta-carotene. That could save vision and lives by identifying—earlier
on—those who likely won't respond to beta-carotene supplementation.
Vitamin A deficiency isn't prevalent in the United States. Nevertheless, the
procedure could be used here to help healthcare professionals identify
individuals at risk of developing a shortage of this nutrient. An example:
people who don't process fats efficiently. Fats, like those in whole milk, help
our bodies absorb and digest vitamin A.
Genes Likely Control Beta-Carotene Processing
"The variation in the way our bodies respond to beta-carotene is likely
gene-based," Burri points out. "Some genes that govern our use of
this compound have already been identified, and more will likely be pinpointed
as a result of the human genome project. That might lead to new strategies for
fighting vitamin A deficiency. And it may reveal useful clues about how other
genes control processing of other compounds and nutrients.
"Ideally," adds Burri, "it may also help us produce customized
dietary guidelines that take into account an individual's ability to convert
carotenes from fruits and vegetables into vitamin A."
Burri and co-researchers published their findings in the American Journal of
Clinical Nutrition and in Mathematical Modeling in Experimental
Nutrition.—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.
Betty J. Burri is with the
USDA-ARS Western Human Nutrition Research
Center, One Shields Ave., Davis, CA 95616; phone and fax (530) 752-4748.
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