Gaining weight at any time can increase the risk of breast cancer by up to 50 per cent, warn researchers.
But women can counter this by losing the excess pounds and returning to a "healthy" size. This then reduces the risk back to that of those who kept their weight stable.
There was no difference in risk according to how overweight the women were. The key factor was that any amount of weight gain was important in relation to breast cancer risk.
It is already known that being obese after the menopause pushes up the chances of developing the disease. But now it appears from work at the National Cancer Institute in Maryland, U.S., that the threat is linked to a woman's weight from the age of 18 onwards.
The pattern was seen in women who did not take hormone replacement therapy, which can increase the likelihood of breast cancer during long-term use. Dr Jiyoung Ahn and colleagues analysed data from 99,039 post-menopausal women.
In 1996, they reported their current body measurements and weight, as well as their weight at ages 18, 35 and 50, and were classified as underweight, normal, overweight or obese.
Throughout 2000, a total of 2,111 of them developed breast cancer.
Periods of weight gain after 18 were each associated with an increased risk of breast cancer when compared with maintaining a stable weight.
Women who were not overweight at 18 but were at 35 and 50 had a 40 per cent increased risk of developing breast cancer in middle age.
Losing weight appeared to offer protection, according to the report published in the Archives of Internal Medicine. Those who lost weight had the same breast cancer risk as those whose weight remained stable.
Source - Daily Mail