Microwaving can lower breast milk benefits.

Women who work outside the home can express and store breast milk for feedings when they're away. But parents and caregivers should be careful how they rewarm this milk. A new study shows that microwaving human milk -- even at a low setting -- can destroy some of its important disease-fighting capabilities.

Breast milk can be refrigerated safely for a few days or frozen for up to a month; however, studies have shown that heating the milk well above body temperature -- 37[degrees] C -- can break down not only its antibodies to infectious agents, but also its lysozymes, or bacteria-digesting enzymes. So when pediatrician John A. Kerner Jr. witnessed neonatal nurses routinely thawing or reheating breast milk with the microwave oven in their lounge, he became concerned.

In the April Pediatrics (Part 1), he and his Stanford University co-workers report finding that compared to unheated breast milk, mocrowaved milk lost lysozyme activity, lost antibodies and fostered the growth of more potentially pathogenic bacteria. Milk heated at a high setting (72[degrees] to 98[degrees] C) lost 96 percent of its lysozyme activity and 98 percent of its immunoglobulin-A antibodies, agents that fend off invading microbes.

What really surprised him, Kerner says, was finding some loss of anti-infective properties in the milk microwaved at a low setting -- and to a mean of just 33.5[degrees] C. Adverse changes at such low temperatures suggest "microwaving itself may in fact cause some injury to the [milk] above and beyond the heating," he says.

But Randall M. Goldblum of the University of Texas Medical Branch in Galveston disagrees, saying, "I don't see any compelling evidence that the microwaves did any harm. It was the heating." Lysozyme and antibody degradation in the coolest samples may simply reflect the development of small hot spots -- potentially 60[degrees] C or above -- during microwaving, notes Madeleine Sigman-Grant, of Pennsylvania State University in University Park. And that's to be expected, she says, because microwave heating is inherently uneven -- and quite unpredictable when volumes less than 4 milliliters are involved, as they were in Kerner's study.

Goldblum considers use of a microwave to thaw milk an especially bad idea, since it is likely to boil some of the milk before all has even liquefied. Stanford University Medical Center no longer microwaves any breast milk, Kerner notes. And that's appropriate, Sigman-Grant believes, because of the small volumes of milk that hospitals typically serve newborns -- especially premature infants.