The findings are important for the creation of future iron-rich foods, say the scientists from the US Agricultural Research Service and Cornell University.
<>"Although human milk is not high in iron content, there appear to be some compounds in it that promote iron uptake," said animal physiologist Ray Glahn from ARS's US 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."
The team used a combination of human cell culture and separation techniques to reveal clues about the bioavailability of iron in both human and cow's mil, and how it was affected by fat, whey and casein, major components of both types of milk.
Unsurprisingly, they found that some compounds in human milk promote iron uptake. But lactoferrin was not one of the facilitating factors.
Scientists have long debated whether the protein's ability to bind and transport iron - and release it at specific receptor cells in the human intestine - actually enhances iron absorption. But their study found no evidence of this.
Instead, it was the low-molecular-weight fractions of whey that promoted 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.
But 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.