Tests involving chickens have raised questions about the impact on health from engineered nano-particles, the ultra-fine grains commonly used in drugs and processed foods, the AFP reported on Sunday.

Chickens exposed to high oral doses of polystyrene particles 50 nanometers (50 billionths of a meter) across absorbed less iron in their diet, the agency reported, citing a study by scientists.

At the same time, birds that were chronically exposed to these doses had a "remodelling" of their intestinal villi, the microscopic finger-like projections that play an important role in absorbing nutrients.

The changes meant that the villi increased the surface area available for taking in iron.

Intestinal uptake of calcium, copper, zinc and vitamins A, D, E and K may also be affected by high exposure to nanoparticles, although further research is needed to investigate this, according to the scientists.

The team, led by Michael Shuler of Cornell University in New York, tested the particles on chickens as a substitute for the human intestine and also used lab-dish cells from the lining of the human gut.

The chickens were given roughly the same dose, weight for weight, as an adult human in a developed country.

"The intestinal epithelial layer represents the initial gate that ingested nanoparticles must pass to reach the body," says the paper, which appears in the specialist journal Nature Nanotechnology.

"The polystyrene particles used in these experiments are generally considered non-toxic, but their interaction with a normal physiological process suggests a potential mechanism for a chronic, harmful, but subtle response."