Hiking Human Zinc Absorption
Plant physiologist Ross Welch (left) and animal physiologist
William House examine a soybean plant used in research to improve the
methionine content of soy protein.
Boosting plants' internal levels of an amino acid called methionine could
increase the availability of dietary zinc in those plants.
This would be a valuable benefit in developing countries where meat, the
most common source of dietary zinc, may be scarce.
"When animals or people don't have enough zinc in their diet, problems
such as depressed growth of the young, slower wound healing, and birthing
difficulties have been observed," notes Agricultural Research Service
animal physiologist William A. House. He is at the U.S. Plant, Soil, and
Nutrition Laboratory at Ithaca, New York.
"Zinc is a major constituent of many enzymes, so many of the body's
metabolic processes depend on zinc," he says.
The Recommended Dietary Allowance of zinc for adults is 12 to 15 milligrams.
Although most non-vegetarian Americans gel the bulk of their dietary zinc from
meat, about 13 percent comes from cereal grains.
Zinc levels in a plant can be boosted by adding zinc to the soil it grows
in. And the added presence of methionine makes it possible for humans and
animals to use more of the plant's zinc, though it does not alter its zinc
In experiments at the Ithaca lab, House and ARS plant physiologist Ross M.
Welch fed rats a diet laced with traceable zinc. Some of the rats also ate
either extra methionine, extra lysineanother amino acidor a
combination of the two.
The rats that received the supplemental lysine were able to absorb 69
percent of the zinc, about the same as the 64 percent absorbed by rats with no
supplemental amino acids.
But the rats that ate supplemental methionine absorbed 82 percent of the
zinc, and those treated to lysine plus methionine absorbed 89 percent. In a
second experiment, rats were able to absorb about 20 percent more dietary zinc
when also fed supplemental methionine or cysteine, a third amino acid.
"This tells us that if you're marginally deficient in methionine in
your diet, you probably have impaired zinc absorption," House points out.
But he warns that simply loading up on methionine isn't the entire answer to
meeting dietary zinc needs.
The Ithaca studies revealed an oddity: Rats absorbed less zinc from corn
that had grown multiple layers of special cells where minerals are normally
stored, compared with rats eating corn with the typical number of those cells.
"One difference was that the normal corn contained less of a particular
form of phosphorus called phytate," explains House. "It's thought
that zinc forms an insoluble compound with phytate in the intestinal tract and
can't be absorbed because of that."
To further confuse the issue, fiber--the same fiber encouraged in virtually
every dietalso ties up zinc. By binding it, much as resins in water
softeners bind calcium and magnesium, fiber reduces availability of zinc for
absorption, House adds.
"Zinc deficiency is a bigger problem in developing countries than in
the United States," he notes.
"In some areas, people might depend heavily on foods grown right in the
neighborhood. If their soil is low in zinc, that increases their likelihood of
a deficiency. Plants with increased levels of methionine might increase the
availability of the zinc that's in those plants." By Sandy
Miller Hays, ARS.
M. Welch is at the USDA-ARS U.S. Plant, Soil and Nutrition Research Unit,
Tower Road, Ithaca, NY 14853; phone (607) 255-2454.
"Hiking Human Zinc Absorption" was published
in the June 1995
issue of Agricultural Research magazine.