(HealthDay) -- Breast-feeding mothers sometimes worry that exercise may affect their breast milk -- and ultimately their baby's growth. Now, researchers who re-evaluated the few published research studies that exist say it does not appear that mom's workout will affect her infant's growth.
Studies are sparse, according to Amanda Daley, a researcher at the University of Birmingham in the United Kingdom, who led the analysis. She and her colleagues searched databases and found only four studies scientific enough to re-evaluate.
The analysis is published in the July issue of Pediatrics.
While exercise appears to have no effect on growth, research is so sparse that Daley said she can't make her conclusion any stronger. "Rather, we are saying, based on the evidence we have, exercise does not appear to negatively influence gain in weight," she said.
"The evidence is suggestive, not conclusive," Daley said.
Complete data from the studies were reported on 160 women, including 71 in the intervention groups and 89 in the comparison groups.
The studies looked at exercise interventions lasting at least a week. The comparison groups were assigned either to do less exercise than the other group or to do no exercise.
The women were exclusively or mainly breast-feeding. Information on the babies' weight and, in one study, length, were recorded.
Mothers' exercise did not appear to affect infant weight gain, Daley's team found. In the one study that looked at infant length, no differences were found in the exercising or non-exercising mothers.
The concern about exercise stems from previous research that suggested that a mother's workout may drain the body of fluid and in turn reduce milk volume, or that lactic-acid concentrations after exercise might affect breast milk's taste. But experts say the lactic-acid effect on taste would probably occur only with very vigorous exercise, much more strenuous than most new mothers might do.
In the studies reviewed, the women in the exercising groups did moderate to hard-intensity exercise three to five days a week or light to moderate exercise most days.
Daley said she can't address concerns people might have over more vigorous activity. For example, she said, "I can't comment on the issue of marathons, as none of the studies includes this type of exercise."
But, Daley concluded: "Based on limited evidence, we found that breast-fed infants whose mothers exercised did not gain less weight than infants of sedentary mothers."
Daley said a new study is needed that includes information on the baby's body length, the frequency and duration of breast-feeding, fussiness, milk volume and milk composition.
Dr. Richard Schanler, chair of the American Academy of Pediatrics' section on breast-feeding, reviewed the analysis.
"I basically have no concerns about maternal exercise during lactation," he said. "The manuscript suggests more data are needed because there is a paucity of research data in this area."
He said there might be a transient buildup of lactic acid in the milk during strenuous exercise. "That might change the taste of the milk. A sweaty mother might have more salt in their skin so the baby would taste it and it would differ from usual feeding."
Meanwhile, La Leche League, an organization whose stated mission is to promote breast-feeding worldwide, has suggested that because women feel better when they exercise, it is good for both mothers and babies.
However, on their website, La Leche League advises that breast-feeding moms wait until the baby is at least 6 weeks old to resume exercising, start gradually and consume plenty of fluids. Brisk walking, mild aerobic exercise and water exercise may be an ideal way to start, they suggest, but don't overdo it. Getting a doctor's approval before beginning any exercise routine is also advised by many experts.
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To learn more about breast-feeding, visit the U.S. Department of Health and Human Services.