Even so, stored breast milk -- even frozen breast milk -- retains more antioxidant activity than formula. The findings come from a study led by Thomas Hegyi, MD, director of neonatology and vice chairman of the department of pediatrics at UMDNJ-Robert Wood Johnson Medical School in New Brunswick, N.J.
"We don't have a connection -- yet -- between ingesting breast milk with high levels of antioxidants and better clinical outcome," Hegyi tells WebMD. "But clearly, fresh breast milk has a significantly higher concentration of antioxidants compared to stored breast milk or formula."
The findings appear in this month's fetal and neonatal edition of Archives of Disease in Childhood.
Breast Milk vs. Formula vs. Free Radicals
Hegyi notes that destructive molecules called free radicals can build up in an infant's tiny body. Antioxidants clear them away. Premature babies are particularly vulnerable to diseases caused by free radicals. And premature babies do better when fed breast milk than when fed formula. One reason may be that mothers' milk is rich in antioxidants.
"There is data to show that breast milk is better for premature infants than formula," Hegyi says. "Babies fed breast milk have a lower incidence of intestinal complications than those not fed breast milk. And it may be protective in a number of diseases. But that effect may be more than just breastfeeding itself."
It may have something to do with antioxidants, Hegyi and colleagues suspected. So they tested breast milk for antioxidants. The milk came from eight women with normal-term babies and eight women with premature babies. The milk was tested when fresh, after being refrigerated for two or seven days, and after being frozen for two or seven days. They also tested five brands of formula (Similac, Enfamil, and Isomil term formulas and PE20 and PE24 preterm formulas).
Regardless of whether they'd had a premature baby, the women's breast milk was good at neutralizing free radicals. This antioxidant activity declined after 48 hours of refrigeration. Freezing caused an even greater drop in antioxidant activity. But whether fresh, refrigerated for seven days, or frozen for seven days, breast milk still had higher antioxidant activity than formula.
For peak antioxidant action, Hegyi and colleagues conclude, breast milk should be used fresh or stored for no more than 48 hours.
Does this mean women shouldn't use breast milk refrigerated or frozen for more than two days? No, Hegyi says.
"Fresh is best," Hegyi says. "But if you can't get fresh, store it and give it to the baby as soon as you can. It is certainly better than formula."
Storing breast milk ensures a steady supply for infants, says certified lactation consultant Katie Lebbing, manager of the center for breastfeeding information at La Leche League. Lebbing says there several good reasons for storing breast milk for later use.
"About two to three weeks before returning to work after giving birth, a woman will start storing a little milk, about 2-3 ounces a day, and freeze it, and that gives her a little milk in the bank," Lebbing tells WebMD. "As the mother gets tired toward the end of the week, her milk supply will be less. So sometimes she needs to dip into that store on days when her stress level is higher."
Freezing is also important for milk banking, Lebbing says.
"Some women are blessed with too much milk. They have an oversupply problem and often donate to milk banks," she notes. "These women freeze their milk, and then it is shipped to milk banks, processed by heat treatment, and then they refreeze it and send it to different places. It's only available by a doctor's prescription. It is very superior to formula, although some nutrients are lessened. That is another big reason to freeze milk."
Hegyi is quick to note that the current findings don't mean there's anything wrong with formula. He and Lebbing agree that formula is an excellent source of nutrition when breast milk is not an option.
"We are not against formula. We are just against it being used inappropriately," La Leche's Lebbing says. "A woman without enough milk has to feed her baby."