Natural stress hormone found in breast milk

A hormone that is released when someone is under physical or emotional stress has been found in breast milk and appears to affect babies differently depending on their sex, according to a study done on laboratory monkeys.

Scientists found that levels of the stress hormone cortisol in breast milk can vary between mothers and that it affects sons and daughters in a different way. The researchers believe the same is likely to be true for human breast milk, the Independent reported.

Female babies fed on breast milk with relatively high concentrations of cortisol showed behavioural changes, such as irritability, fear, anger and discomfort, which were not shown in sons fed on breast milk with similar concentrations of the hormone, Katie Hinde, an evolutionary biologist at Harvard University in Cambridge, Massachusetts, said.

The findings lend further support to the idea that "breast is best" and that formula milk does not provide the same nutritional benefits as breast milk.

However, when formula milk is unavoidable, the results also suggest that its make-up might be altered depending on whether the baby is a boy or a girl, Dr Hinde said.

She told the American Association for the Advancement of Science that these studies are showing that mother's milk affects behaviour and there seems to be differences in that effect for sons and daughters.