Gaining even a moderate amount of weigh after the menopause can increase a woman's risk of breast cancer, a study has concluded.
Being overweight increases cancer risk, researchers say
Putting on 22lbs (9.9kg) increased a woman's risk of developing the disease by 18%, the US team found.
But those who had lost the same amount cut the risk by 57%, the Journal of the American Association study showed.
Cancer experts in the UK said the study showed it was never too late for women to lose weight.
There are around 41,000 cases of breast cancer in the UK each year, and around 13,000 deaths.
Just over 87,000 women, aged 30 to 55 years, were followed for up to 26 years by the team from Brigham and Women's Hospital, and Harvard Medical School.
Researchers noted what their weight had been at age 18, and looked at how it had changed since.
The 50,000 of those who went through the menopause during the course of the study were also assessed for weight change since that point.
It was found that women who gained about 55lb (24.9kg) or more after the age of 18 had a 45% increased risk of developing breast cancer compared with those who maintained their weight.
They also concluded that 15% of the study's breast cancer cases could be attributed to weight gain of 4.4lb or more since the age 18.
But losing weight carried a high degree of protection against breast cancer.
The researchers said weight loss after the menopause lowers levels of oestrogen hormones which are linked to breast cancer, cutting the risk of the disease.
'Age also a factor'
The team, led by Dr Heather Ellassen, said: "These data suggest that weight gain during adult life, specifically since menopause, increases the risk of breast cancer among post-menopausal women, whereas weight loss after menopause is associated with a decreased risk of breast cancer.
"Women should be advised to avoid weight gain both before and after menopause to decrease their post-menopausal breast cancer risk."
Dr Kat Arney, Cancer Research UK science information officer, said: "We know from previous research that the risk of breast cancer goes up if you are overweight after the menopause.
"This study goes further, and tells us that it's never too late to shift some pounds to reduce your risk of breast cancer."
Dr Sarah Rawlings from Breakthrough Breast Cancer added: "Whether you have been through the menopause or not, being overweight is associated with a variety of health problems, including increased risk of heart disease, high blood pressure and several types of cancer."
But Dr Emma Pennery, nurse consultant at the charity Breast Cancer Care said: "Whilst risk rises with any weight gain it only becomes significant with excessive amounts.
"It is important to remember that other risk factors for breast cancer remain, the main factor being increasing age."