In a laboratory study, the researchers found that human intestinal cells with excess iron were more susceptible to attack by bacteria that cause infection of the small intestine.
The study suggests that enriching breakfast cereals and other foods with high doses of iron - a nutritional strategy that has been widely adopted to eliminate iron deficiency in the population - could be causing other health problems.
"Instead of fortifying everyone's diet with excess iron, we should diagnose iron deficiency and then provide supplemental iron only to those who need it," said Mark Failla, professor of nutrition at Ohio State University and the senior author of the study. Failla and his colleagues reported the study in a recent issue of the Journal of Nutrition.
Failla's group arrived at these conclusions after laboratory experiments with human cells that are virtually identical to cells lining the small intestine. These epithelial cells of the small intestine transfer iron from digested foods to the bloodstream for transport to the body's various organs. Iron is vital for a variety of metabolic processes, including the binding and release of oxygen by red blood cells.
In their experiments, Failla and his colleagues exposed cells to a common form of iron that is present in iron supplements. Iron-laden cells were then exposed to Salmonella enteritidis, a bacterium that causes intestinal infections in humans. Infection from Salmonella enteritidis often results from eating undercooked eggs, and is accompanied by fever, abdominal cramps and diarrhea.
The scientists found that cells containing high levels of iron were more easily invaded by the bacteria. Moreover, greater numbers of bacteria survived inside cells with high iron than cells with normal amounts of iron.
Cells with elevated levels of iron also synthesized higher amounts of cytokines and chemokines - classes of defense proteins -- in response to bacterial infection. The secretion of many of these proteins is associated with the development of inflammation.
"The finding suggests that excess iron in the absorptive cells in the gut may increase both the risk of infection and damage other tissues in the intestine in response to the greater degree of inflammation," Failla said.
To prevent iron overload, the researchers suggest a revision of the standard policy of iron fortification of the general population. They recommend periodic evaluation of the iron status of individuals, particularly women in the reproductive period of their life cycle. If an individual exhibits iron deficiency, therapeutic iron can be administered by injection or by judicious use of dietary supplements as prescribed by a physician or registered dietitian.
"Iron is a double-edged sword," Failla said. "Although it is an absolutely essential nutrient, excess tissue iron can cause pathology."
Failla and his colleagues are planning further experiments to explain why intestinal cells with high iron levels are more readily infected. The studies would also help understand the mechanism behind the heightened expression of defense proteins in these cells in response to bacterial invasion.
The research was funded by the U.S. Department of Agriculture and the North Carolina Institute of Nutrition.
Written by Yudhijit Bhattacharjee, (614)292-8456; bhattacharjee.5@osu.edu
Journal
Journal of Nutrition