Breast milk that is refrigerated or frozen for three days might not be safe for preterm infants at risk for necrotising enterocolitis, new research suggests.
The breast milk may become cytotoxic by the action of its own lipase. When stored, the milk forms unbound free fatty acids that could increase intestinal damage in at-risk infants, researchers said last week at the American Society for Parenteral and Enteral Nutrition's annual conference in Orlando, Florida.
"We don't know just how bad the cell death could be in an actual child with a permeable intestine, but I think it is safe to say that unpasteurised breast milk should not be used if it was stored in the refrigerator for even three days," Dr Alexander Penn, a bioengineer who led the study at the University of California, San Diego, said Reuters Health.
Likewise, infants with a permeable intestine should not drink unpasteurised breast milk frozen at - 20ºC for more than a week, Dr Penn says.
"That said, we have unpublished findings (currently under review) suggesting that even after digestion, the week-old freezer-stored breast milk was considerably less cytotoxic than digested infant formula. So I'd say if my only choice was stored breast milk or formula, I'd go with the breast milk," Dr. Penn said.
Current guidelines for preterm infants suggest up to one day of refrigerator storage or several months in the freezer, Dr Penn said. (He added, parenthetically, "The former is fine, but I find the latter alarming.")
Safe refrigeration period
For healthy full-term infants, the US Centers for Disease Control and Prevention recommends no more than five days of refrigeration at 4 degrees C and 6-12 months at - 20ºC.
Dr Penn and his team used lipase, protease, a mixture of the two, and a control saline solution to digest batches of milk that had been either frozen or refrigerated.
Stored for three days at four degrees C, the lipase-digested milk showed 43.1% cell death (p<0.0002). At minus 20 degrees C for three days, the milk had 18.5% cell death (p<0.002).
The concentration of free fatty acids had increased significantly in milk stored for seven days at 4 degrees C and correlated with cell death (correlation coefficient=0.71). Adding a lipase inhibitor before storage or removing unbound free fatty acids reduced cell death.
Protease digestion of the milk stored in any way studied decreased cytotoxicity (p<0.01), the researchers found. In a related study that hasn't been published yet, Dr Penn and his team found that lipase-digested formula is cytotoxic also. But fresh breast milk is not.
"As a food, fresh breast milk seems amazingly well-designed to deal with this very issue," Dr Penn says.
(Reuters Health, Rob Goodier, January 2012)
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