06/19/2012 04:59 pm ET Updated Aug 19, 2012

Breastfeeding Mothers Who Exercise Don't Hinder Baby's Growth, Study Finds

By Amy Norton

NEW YORK, June 19 (Reuters Health) - A new study appears to debunk the notion that new mothers who engage in exercise may change the composition of their breast milk in ways that could hinder their babies.

Researchers pulled together the few clinical trials that have measured growth among breastfed babies whose mothers exercise, and found no evidence that the exercise slowed infants' weight gain.

"Based on what we know at the moment, babies of mums who exercise do not gain less weight than babies of mothers who do not exercise," lead researcher Amanda Daley of the University of Birmingham in Britain, whose findings were published in the journal Pediatrics on Monday, said in an email.

The findings seem to put to rest concerns over the effects that exercise could have on the makeup of a woman's breast milk. One previous study, for instance, found that when women exercised intensely, their breast milk showed short-term dips in immune-boosting proteins that are passed on to their babies when they breastfeed.

It's also possible that heavy exercise could raise lactic acid levels in breast milk, making it less palatable for babies. Some studies have suggested that's true, but others have not.

The new research found that on average babies of exercising mothers gained slightly more than breastfed babies whose mothers were not physically active. That difference was likely due to chance, the researchers said.

A scientist involved in earlier research on the subject, nutrition professor Cheryl Lovelady of the University of North Carolina Greensboro, said her findings showed that women can safely exercise while breastfeeding.

For women who were inactive during pregnancy, it's best to take it slowly after giving birth, according to Lovelady. She recommended that those women take a couple weeks to establish breastfeeding, then gradually become active.

"Start with brisk walking, 15 minutes per day, then add two minutes every day until (you) are walking 45 minutes per day," Lovelady said by email.

The findings from Daley's team were based on four clinical trials done between 1994 and 2009. In each, researchers recruited sedentary breastfeeding mothers, then randomly assigned them to either start a moderate exercise routine or remain inactive. In two studies, the women also cut calories to help reduce some post-pregnancy pounds.

Daley noted that the small number of trials, which studied a total of 170 mothers, means that the evidence showing exercise during breastfeeding to be safe is limited. The follow-up period was also limited: three trials ran for 10 to 16 weeks, and the fourth for only 11 days.

In general, experts recommend that women try to breastfeed exclusively for the first six months after a baby is born, then add solid foods to the child's diet. Ideally, babies should keep getting breast milk for at least their first year, they say.

Experts also recommend that adults, including new moms, get regular moderate exercise.

The full study is available at (Editing by Ivan Oransky and Will Dunham)