Young women who carry a breast cancer gene can reduce their risk of the disease by slimming, a study suggests.
A healthy bodyweight can reduce cancer risk, say experts
An international team of scientists found losing at least 10 pounds between 18 and 30 reduced the risk of cancer by up to 65%.
But they showed gaining 10 pounds increased women's risk of developing cancer before the age of 40.
Experts said the Breast Cancer Research study was the first to link between pre-menopausal weight gain and cancer.
Being overweight after the menopause was already known to increase women's risk of developing the disease.
The researchers, from Canada, the US and Poland, looked at more than 2,000 women who were carrying the faulty BRCA1 or BRCA2 breast cancer genes.
The women were tested for BRCA 1 and 2 and questioned about their weight at ages 18, 30 and 40.
It was found that those women carrying the BRCA1 gene who lost weight saw the greatest benefit.
At 18, they had an average weight of 142.5 pounds. They had lost an average of 18.6 pounds, ranging from 10 to 86 pounds, by the age of 30.
Slimming also reduced the risk for women with another defective breast cancer gene, BRCA 2, but not to a significant degree.
But gaining weight heightened the risk of breast cancer for BRCA 1 mutation carriers who had borne at least two children, increasing their chances of being diagnosed before the age of 40 by 44% if they gained 10 pounds between the ages of 18 and 30.
In their paper, the researchers suggest carrying extra fat around the centre of the body could affect ovarian hormone and glucose metabolism, and promote insulin resistance which are all factors linked to increased breast cancer risk.
The researchers, led by Dr Steven Narod of the University of Toronto in Canada, wrote in Breast Cancer Research: "The results from this study suggest that weight loss in early adult life protects against early-onset BRCA-associated breast cancers.
"Weight gain should also be avoided, particularly among BRCA 1 mutation carriers who elect to have at least two pregnancies."
Dr Kat Arney, cancer information officer at Cancer Research UK, says: "This study highlights the importance of maintaining a healthy bodyweight throughout adult life for women who have inherited an increased risk of breast cancer due to faults in their BRCA1 or BRCA2 genes."
She added: "The results are interesting because previous studies have not found a link between pre-menopausal weight gain and increased cancer risk in the general population.
"However, maintaining a healthy weight is also one of the best ways for everyone to reduce their chances of getting cancer.
"In particular, being overweight after the menopause significantly increases a woman's risk of womb and breast cancer."
Dr Sarah Rawlings, head of policy at the charity Breakthrough Breast Cancer, "The link between weight and breast cancer is complex and we recommend that all women maintain a healthy weight throughout their lives by eating a balanced diet and exercising regularly.
"Women who are concerned about their weight or wish to lose weight should only do so in consultation with their GP."