SAN DIEGO (Reuters Health) - Bottle-fed babies may need higher doses of certain medications than those who are breast-fed because formula causes their bodies to metabolize the drugs more quickly, Canadian researchers report.

Doctors have observed that formula-fed premature infants who are given oral caffeine to treat a condition known as neonatal apnea have consistently lower blood concentrations of the drug than breast-fed babies. Apnea causes the infants to stop breathing for short periods; caffeine, a stimulant, helps regulate respiration.

In search of an explanation for the difference, the researchers compared the effects of breast milk and two brands of canned liquid baby formula derived from cow's milk on human liver cells in the lab. The liver metabolizes medicines.

At the annual Experimental Biology meeting here Saturday, they reported that formula prompted the liver cells to pump out substantially higher levels of an enzyme known as cytochrome P450 1A (CYP1A), which helps metabolize caffeine and various other compounds.

"In human liver cell lines, breast milk has no effect in terms of this particular enzyme," said study author Dr. Shinya Ito, head of clinical pharmacology and toxicology at the Hospital for Sick Children in Toronto, Ontario. "But with formula, the enzyme levels increased about five- to six-fold."

As a result, he told Reuters Health, formula-fed babies may need higher doses of caffeine, fever-reducers and other drugs metabolized by this enzyme.

Ito and colleagues also found that formula activated a protein called the aryl hydrocarbon receptor on the surface of cells, which in turn triggers the release of CYP1A.

This receptor is known to be stimulated by a variety of environmental pollutants, including cigarette smoke, because the body wants to get rid of the contaminants as quickly as possible, Ito explained.

So the findings hint at the possibility that there may be some component of infant formula, perhaps some byproduct from when it is heated during processing to kill off bacteria, that the body is trying to rapidly remove, he suggested.

But much more research is needed to uncover precisely why formula has this effect. "We're trying to identify the compounds in formula that are doing this," Ito said.

Meanwhile, doctors should take note of the findings, he said. "Clinicians may need to think about the potential differences between formula-fed versus breast-fed infants in terms of capabilities of metabolizing drugs," he added.