"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 J. Daley, of the University of Birmingham in the UK, said in an email.

Some questions have been raised about the effects exercise could have on the makeup of a woman's breast milk. One 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 -- and make it less palatable for babies. Some studies have suggested that's true, but others have not.

The ultimate question is whether exercise during breastfeeding affects babies' growth and development.

So for the new study, researchers pulled together the few clinical trials that have measured growth among breastfed babies whose moms exercise.

When they combined the results of those trials, they found no evidence that exercise slowed infants' weight gain.

On average, babies of exercising moms gained slightly more than breastfed babies whose mothers were inactive -- 18.6 grams, or about 0.6 ounces, more. That difference was likely due to chance, the researchers report in the journal Pediatrics.

Cheryl Lovelady, who led two of the studies in the review, said she thinks her research shows women can safely exercise while breastfeeding.

If you were inactive during pregnancy, it's best to take it slow after giving birth, according to Lovelady, a professor of nutrition at the University of North Carolina Greensboro.

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 suggested by email.

EVIDENCE STILL LIMITED

Daley said there are caveats, however. Her team was only able to find four clinical trials that have looked at the question of how exercise during breastfeeding might affect babies' growth. And the trials included only 170 mothers in all.

So the evidence that exercise during breastfeeding is safe is limited, Daley and her colleagues say.

On the other hand, the evidence that it could be harmful is also quite limited, according to Daley.

The findings are based on four clinical trials done between 1994 and 2009. In each, researchers recruited sedentary breastfeeding moms, then randomly assigned them to either start a moderate exercise routine or remain inactive.

In two studies, the women also cut calories to help cut some post-pregnancy pounds.

When Daley's team combined the trials' results, they found no significant difference in infants' weight gain, whether moms exercised or not.

Still, the results are based on a small number of women, the researchers point out. And the follow-up time was limited: three trials ran for 10 to 16 weeks, and the fourth for only 11 days.

"So maternal exercise does not appear to have any negative effects on weight gain," Daley said, "but really we need more studies before we can make a conclusive statement."

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

At the same time, experts also recommend that adults, including new moms, get regular moderate exercise.

Gaining too much weight during pregnancy, and keeping those extra pounds, may boost a woman's risk of obesity and the health problems associated with it. And post-pregnancy exercise could help prevent that, Daley's team notes.

SOURCE: bit.ly/Mlw5tK Pediatrics, online June 18, 2012.

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