The finding comes from researchers Karen Wosje, PhD, and Heidi Kalkwarf, PhD, RD, of Cincinnati Children's Hospital Medical Center.
Previous studies on the topic have been contradictory, leaving breastfeeding's effects on weight and body fat unclear.
Studying New Moms
First, Wosje and Kalkwarf focused on brand-new moms during a six-month study that looked at body changes after delivery.
With new moms, body composition can change quickly after delivery. In the first six months after giving birth, the study's 81 nonbreastfeeding mothers lost fat from their whole body, arms, and legs faster than the 87 breastfeeding moms.
In addition, the lactating women gained fat in their arms.
A change in body composition was determined by imaging the whole body and determining fat and muscle mass.
However both groups lost weight at similar rates and decreases in body weight was not influenced by breastfeeding. All mothers lost some fat in their trunk (chest, stomach, and pelvic region), but it was the rate of fat loss that differed.
The breastfeeding moms may have also consumed more calories. In breastfeeding mothers the hormone prolactin stimulates appetite, as well as prompts milk production, which could account for the extra calories consumed by these women.
In addition, the nonbreastfeeding women reported more intense physical activity than the breastfeeders.
Next, the researchers examined data from women who had had babies six to 12 months earlier.
By then, any differences in body composition between lactating and nonlactating moms had disappeared.
"There was no influence of lactation ... on fat mass losses in the women in the weaning study," write the researchers.
"The rates of decrease in body weight and whole body percentage fat were not significantly influenced by lactation."
On average, all the women in the weaning study lost fat mass at all body sites. Body composition keeps changing until at least one year after giving birth, say the researchers.
No Help From Calcium
It didn't have an effect in this study.
Calcium supplements of 1 gram per day (1 g/d) made no difference in weight or fat loss in any of the moms.
"We observed no beneficial influence of calcium supplementation on changes in weight or fat mass," write the researchers in Aug. 1 issue of The American Journal of Clinical Nutrition.
The researchers did not know if any of the women were intentionally trying to lose weight during the study.
Making the Choice
Deciding whether to breastfeed is an intimate decision.
The researchers do not recommend making weight and fat loss a priority in considering whether breastfeeding is best for mothers and their babies.