Breast-feeding eases a baby's discomfort during a painful needle stick procedure and might work as a potent painkiller during potentially traumatizing experiences such as circumcision, researchers said today. Infants who were held and breast-fed while undergoing a painful heel lance, a routine hospital procedure used to obtain a blood sample, cried and grimaced less and their heartbeats remained calmer than infants who were not breast-fed, a University of Chicago study said. "Breast-feeding is a potent analgesic intervention in newborns during a standard blood collection," study author Larry Gray of the University of Chicago wrote in this month's issue of the journal Pediatrics.

Previous animal studies have shown that tastes and flavors in milk can block pain signals in the spinal cord, and suckling can have a calming effect. The researchers said infants' physical contact with their mothers likely also kept them calmer.

There is a debate about whether babies retain memories of single, intensely painful experiences such as circumcision, but some circumcised infants do manifest exaggerated reactions to a needle stick months later.

"The claim can no longer be made that newborn pain is for the moment only," Gray wrote.

In another study in the same journal on the subject of breast-feeding, researchers at the University of New Hampshire in Durham examined the hypothesis that a mother who exercises might produce less appetizing breast milk. A previous study found lactic acid levels in breast milk expressed 30 minutes after a strenuous treadmill test were high enough to deter some babies from drinking it.