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 casein—three 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 Glahn.

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 iron—and release it at specific receptor cells in the human intestine—actually 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 inhibitor.

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 Issue—Measuring 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.