A Close Look at Milk and Iron Uptake

In studies that may one day impact food crops, an Agricultural Research Service scientist and colleagues have examined the mechanisms by which iron in milk is absorbed by the human body.

Milk is not high in iron content. But animal physiologist Ray Glahn of the ARS U.S. Plant, Soil and Nutrition Research Laboratory in Ithaca, N.Y., and cooperators have used a combination of human cell culture and separation techniques to reveal clues about the bioavailability of the iron that is there. They especially wanted to know how that bioavailability was affected by fat, whey and casein, three major components of both human milk and cow's milk.

Glahn is working with Paz Etcheverry, now a postdoctoral associate at Baylor College of Medicine in Houston, Texas, and professor Dennis D. Miller of Cornell University's Department of Food Science.

To no one's surprise, the study found that there appear to be some compounds in human milk that promote iron uptake. According to Glahn, identifying these compounds and gaining understanding toward how they work may enable researchers to make other foods, such as staple food crops, more nutritious.

Noteworthy, however, was that one of the facilitating factors was not 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.

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 was different. Removal of whey and fat had no effect on iron uptake, while removal of the casein fraction increased uptake, indicating that casein is an iron-uptake inhibitor in cow's milk.

Read more about these findings in the July issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.