The reduction in risk increased with the amount of weight lost and was specific to women not using postmenopausal hormones, according to the study published in the JNCI: Journal of the National Cancer Institute.
"Even a modest amount of sustained weight loss is associated with lower breast cancer risk for women over 50," said the study's lead author Lauren Teras from Harvard University in the US.
In the USA, more than two in three adult women are overweight or obese.
While high the body mass index (BMI) is an established risk factor for postmenopausal breast cancer, there has not been adequate evidence to determine if that risk is reversible by losing excess weight.
To learn more, the research team used the Pooling Project of Prospective Studies of Diet and Cancer to estimate the association of sustained weight loss in middle or later adulthood on subsequent breast cancer risk.
The analysis included more than 180,000 women aged 50 and older from 10 prospective studies. It's the first with a large enough sample size to examine the important question of whether sustained weight loss can impact breast cancer risk with statistical precision.
Weight was assessed three times over approximately 10 years: at study enrolment; after about five years; then again about four years later.
The results showed women with sustained weight loss had a lower risk of breast cancer than women whose weight remained stable, and the larger the amount of sustained weight loss, the lower was the risk of breast cancer.
Women who lost 2-4.5 kg (about 4.4-10 lbs) had 13 per cent lower risk than women with stable weight. Women who lost 4.5-9 kg (10-20 lbs) had a 16 per cent lower risk. Women who lost 9 kg or more (20+ lbs) had a 26 per cent lower risk, the study said.
In addition, women who lost 9 kg or more and gained some of the weight back had a lower risk of breast cancer compared with those whose weight remained stable.
"The findings may be a strong motivator for the two-thirds of overweight American women to lose some of that weight. Even if you gain weight after 50, it's not too late to lower your risk of breast cancer," Teras said.
( With inputs from IANS )