(HealthDay) -- How many children a woman bears and whether or not she breast-feeds them may affect her weight decades later, according to new research.
Childbearing is linked with an increased risk of obesity, but breast-feeding cuts that risk by about 1 percent for every six months of nursing, the researchers found.
"Women who are in their childbearing years can take from our and other research that breast-feeding is good for them and their children and that the benefits for their own health may be extremely long-lasting," said researcher Kirsty Bobrow, a clinical researcher at the University of Oxford in England.
The study is published July 10 in the International Journal of Obesity.
While the reduction in body mass index (BMI) associated with breast-feeding is small for an individual woman, it is substantial in terms of obesity-related diseases across the population, Bobrow said. BMI is a measurement of body size based on height and weight.
For the study, Bobrow and her colleagues evaluated information on about 740,000 postmenopausal women participating in Britain's Million Women Study between 1996 and 2001. Their average age was 58.
All reported their height, weight, childbearing history and other data. They also answered questions about breast-feeding.
Average BMI inched upward with the number of children a woman had, the researchers found. (A normal BMI is 18.5 to 24.9. A BMI of 30 or more is considered obese.)
Among those who had no children, average BMI was 25.6 -- slightly overweight. For women with four or more deliveries, average BMI was 27.2.
Among the women who had given birth, 70 percent had breast-fed and did so for an average of 7.7 months. The researchers found that for every six months of breast-feeding, average BMI was about 1 percent lower. They called this a "relatively small, but important, persistent reduction."
The study doesn't prove that breast-feeding will make you slimmer; it merely shows an association between the two. However, the link held even when the researchers took into account smoking, physical activity and other factors that can affect weight.
Researchers have several hypotheses about why breast-feeding helps control weight over the long term. One is the so-called "reset" hypothesis, Bobrow said. It says that breast-feeding ''may be involved in re-setting various metabolic control centers in the brain after childbirth," Bobrow explained.
Erica Gunderson, a research scientist with the Kaiser Permanente Northern California division of research in Oakland, Calif., said the research is unique partly because of the many women studied. "This type of cross-sectional study has never been accomplished with this sample size," she said.
That's a definite strength of the research, she noted. But it would have been helpful to know the women's BMI earlier in life, she added.
For women who have not yet given birth, the study suggests that it's important to consider breast-feeding and to ask your doctor how much weight you should gain.
Getting advice about nutrition is a good idea, too, Gunderson said. "When you get pregnant, get very good advice about what to eat," she suggested.
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To learn more about breast-feeding, visit the U.S. Department of Health and Human Services.