Got Milk? How About Iron?
In research that may one day affect food crops, ARS
and Cornell University researchers in Ithaca, New York, are studying
the mechanisms by which humans absorb iron from milk.
"Although human milk is not high in iron content,
there appear to be some compounds in it that promote iron uptake,"
says animal physiologist Ray Glahn, of ARS's U.S. Plant, Soil, and Nutrition
Research Laboratory. "By identifying these compounds and understanding
how they work, we may be able to make other foods, such as staple food
crops, more nutritious."
In collaboration with Paz Etcheverry, a Cornell graduate
conducting postdoctoral studies, and Dennis D. Miller, a professor in
the Ithaca-based university's Department of Food Science, Glahn used
a combination of human cell culture and separation techniques to reveal
clues about how iron's bioavailability is affected by fat, whey, and
caseinthree major components of both human milk and cow's milk.
"Using ultracentrifugation, we divided the milk into
fractions and then identified the iron uptake characteristics of each
fraction with cultures of human intestinal epithelial cells," says
To no one's surprise, the study found that there seems
to be a factor in the whey of human milk that enhances iron uptake.
Noteworthy, however, was that this factor wasn't lactoferrin. For years,
scientists have debated whether that protein's ability to bind and transport
ironand release it at specific receptor cells in the human intestineactually
enhances iron absorption.
Instead, the study found that it's the low-molecular-weight
fractions of whey that promote uptake. "Low molecular weight corresponds
directly to the size of molecules," says Glahn. "In this study,
we used filters to separate compounds in whey by their molecular size.
When these substances were exposed to our human intestinal cell cultures
in the presence of iron, iron uptake was higher with the low-molecular-weight
fractions. The high-molecular-weight fractions of whey, including lactoferrin,
did not enhance iron uptake."
Overall, the study showed that removal of whey from human
milk resulted in less iron uptake. Removal of the fat fraction increased
iron uptake, indicating that this component hinders iron bioavailability.
Removal of the casein component had no effect.
In cow's milk, the story is different: Removal of whey
and fat had no effect on iron uptake, whereas removal of the casein
fraction increased uptake, indicating that casein is an iron-uptake
The simulated digestion and cell culture techniques have
been an integral part of Glahn's research since the late 1990s. These
methods mimic human food digestion and uptake to the point where nutrients
are actually absorbed by a line of human intestinal cells. (See "A
Gut IssueMeasuring Iron Bioavailability," Agricultural
Research, August 1999, p. 4.)By Luis
Pons, Agricultural Research Service Information Staff.
Raymond P. Glahn
is with the USDA-ARS U.S.
Plant, Soil, and Nutrition Research Laboratory, Tower Rd., Ithaca,
NY 14853-2901; phone (607) 255-2452, fax (607) 255-1132.
"Got Milk? How About Iron?" was published in the July 2004 issue of Agricultural Research magazine.